Hollow fiber porous membrane applied to ultrafiltration of ultrapure water terminal and preparation method thereof

Abstract

The application discloses a kind of hollow fiber porous membrane applied to ultra-pure water terminal ultrafiltration and its preparation method, by with 10nm colloidal gold as test object, the retention efficiency of porous membrane is quantitatively characterized, after retention test, the part of 10nm colloidal gold retention diameter is mainly located in the outer retention layer, intermediate retention layer and inner retention layer of membrane main body;The outer surface of the outer retention layer is on one side, the intermediate retention layer is located between outer retention layer and inner retention layer;The outer retention layer, the intermediate retention layer and the inner retention layer are spaced distribution, so that the retention efficiency of porous membrane is not less than 95%, with ultra-high retention efficiency;And when carrying out retention test, at least 75% of the colloidal gold of the number of colloidal gold retained is retained in the outer retention layer of porous membrane, the thickness of the outer retention layer is greater than 0.5 μm;The porosity of the porous membrane is not less than 40%, so that the porous membrane also has high flux and longer service life, especially suitable for application terminal filtration of ultra-pure water.

Description

Technical Field

[0001] This invention relates to the field of membrane materials technology, and more specifically to a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water and its preparation method. Background Technology

[0002] Ultrapure water is water produced by the American scientific community using distillation, deionization, reverse osmosis, or other appropriate supercritical fine technologies for the research and development of ultrapure materials (semiconductor components, nano-fine ceramic materials, etc.). This water contains almost no impurities other than water molecules (H2O) (no conductive media, colloidal substances, or other tiny impurities), and is free of bacteria, viruses, dioxins, and other organic matter, as well as essential minerals and trace elements. With the rapid development of the semiconductor industry, the demand for ultrapure water is increasing. Therefore, obtaining high-purity ultrapure water has always been a focus of research and development (in the semiconductor field, even a small amount of impurities in ultrapure water can significantly affect the subsequent processing of wafers and other sensitive components).

[0003] Modern ultrapure water production processes primarily employ pretreatment, electrodialysis, ultraviolet sterilization, reverse osmosis, ion exchange, and ultrafiltration. For example, Chinese patent application CN201480048799.X (Kurita Kogyo Co., Ltd.) titled "Ultrapure Water Manufacturing Apparatus" details each step of ultrapure water production. The final and most crucial step is ultrafiltration, using an ultrafiltration membrane to remove various residual micro-impurities from the water (these micro-impurities can significantly negatively impact the subsequent application of ultrapure water). These micro-impurities typically have particle sizes of only tens of nanometers, making removal difficult, even with ultrafiltration membranes. Even after filtration, a certain amount of minute impurities may still remain in the water. Currently, there are various ultrafiltration membranes on the market for preparing ultrapure water. Although these membranes all claim to have good retention efficiency, their ability to capture various minute impurities in ultrapure water varies greatly in actual applications. Sometimes, a certain number of minute impurities still remain in the ultrapure water after ultrafiltration, resulting in low purity of the produced ultrapure water, which cannot meet the needs of practical applications (especially in the semiconductor field). However, there is currently no good retention standard on the market that can well characterize ultrafiltration membranes suitable for terminal ultrafiltration of ultrapure water, which greatly affects the preparation of ultrapure water and consequently its widespread application.

[0004] Therefore, there is an urgent need in the market for porous membranes that can be quantitatively characterized and have high interception efficiency suitable for terminal filtration of ultrapure water. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water and its preparation method. By using 10nm colloidal gold as a test substance, the retention efficiency of the porous membrane is quantitatively characterized. After the colloidal gold retention test, the porous membrane exhibits ultra-high retention efficiency, which can efficiently capture various tiny impurities in water for a long time, resulting in ultrapure water with high cleanliness. At the same time, the porous membrane also has high flux and long service life.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water, comprising a main body, one side of which is an inner surface facing the inner cavity, and the other side of which is an outer surface. When a 10nm colloidal gold particle is used as an impurity particle for retention testing, the retention efficiency of the porous membrane is not less than 95%.

[0007] The 10nm diameter colloidal gold is mainly retained in the outer retention layer, the middle retention layer, and the inner retention layer of the membrane body; one side of the outer retention layer is the outer surface, and the middle retention layer is located between the outer retention layer and the inner retention layer; the outer retention layer, the middle retention layer, and the inner retention layer are distributed at intervals.

[0008] Furthermore, during the retention test, at least 75% of the retained colloidal gold was retained in the outer retention layer of the porous membrane, the outer retention layer including the outer surface, and the thickness of the outer retention layer being greater than 0.5 μm;

[0009] The porosity of the porous membrane is not less than 40%.

[0010] When using membranes for terminal filtration of ultrapure water, one of the most important performance characteristics is high retention efficiency. The ideal is for the membrane to capture as many tiny impurities as possible in the water to obtain ultrapure water with high purity. However, in current technology, there is no good standard to judge what level of retention efficiency a membrane should achieve to be suitable for terminal ultrafiltration of ultrapure water. Research has shown that when a membrane can achieve a retention efficiency of over 95% for 10nm colloidal gold, it can effectively retain various tiny impurities in the water, producing ultrapure water with extremely high purity, thus enabling its application in the semiconductor field. Colloidal gold is used as the test substance because it is easy to label and detect. Furthermore, colloidal gold tends to aggregate, and the various shapes of these aggregates can vividly characterize tiny impurities of different shapes in the water. The impurities (and their shapes) vary; meanwhile, impurities in water are generally small, typically tens of nanometers. Using 10nm colloidal gold avoids a sudden increase in membrane retention efficiency due to aggregation, ensuring more accurate and realistic retention performance. It also prevents excessively high measurement standards, which could result in overly small pore sizes and extremely low flux in the labeled porous membrane (in nanoscale membranes, flux decreases sharply as pore size decreases), rendering it unusable. After undergoing a 10nm colloidal gold retention test, the porous membrane of this invention exhibits a retention efficiency of no less than 95%, demonstrating its high retention efficiency. This makes it suitable for use in ultrapure water terminal filtration, ensuring high cleanliness of the produced ultrapure water and meeting subsequent practical requirements. Furthermore, this high retention efficiency also reflects the membrane structure; a reasonable membrane structure is essential to guarantee high retention efficiency.

[0011] As is well known, defects of varying degrees are inevitable during membrane fabrication. Even very small defects (invisible to the naked eye) can affect the membrane's retention efficiency, which in turn can impact the production of ultrapure water (i.e., after ultrafiltration, the ultrapure water still contains a small amount of impurity particles). The reason why the porous membrane in this invention can achieve high retention efficiency (producing ultrapure water with high purity) is that the porous membrane of this invention has at least three regions capable of retaining 10nm colloidal gold: an outer retention layer, a middle retention layer, and an inner retention layer (these regions all retain a certain amount of 10nm colloidal gold and can be considered as retention layers capable of capturing tiny impurities). One side of the outer retention layer is the outer surface, and the inner retention layer is the region near the inner surface (which may or may not include the inner surface). The intermediate retention layer is located between the outer and inner retention layers; and the outer, intermediate, and inner retention layers are spaced apart (meaning that the three layers are not continuous, thus achieving multi-stage retention). Through this multi-stage retention, even with minor defects within the membrane, efficient and stable retention is ensured, resulting in ultrapure water with extremely high purity. Even with the same total retention layer thickness, the retention efficiency of the three-layer region is greater than that of a single layer. This is because defects are easily generated during membrane fabrication; with a single retention layer, these defects inevitably affect the retention effect, especially when such high retention efficiency is required. The retention efficiency of the three-layer region is largely unaffected by minor defects, ensuring the membrane's suitability for end-stage filtration of ultrapure water.

[0012] Furthermore, research has revealed that, during retention tests, at least 75% of the colloidal gold in the porous membrane of this invention is retained in the outer retention layer of the membrane. This means that the majority of the colloidal gold is retained in the outer retention layer. Therefore, in actual ultrapure water filtration, most minute impurities will be retained in the outer retention layer (i.e., near the outer surface area), unlike in other applications where most impurities are retained near the inner surface (outlet surface). In this invention, the porous membrane has this retention configuration because, at the final filtration stage, the fluid (the ultrapure water to be purified) already contains relatively few impurities, only a small amount of minute impurities. The porous membrane needs to have a large dirt-holding capacity. When these tiny impurities are almost entirely trapped in the outer retaining layer of the membrane, they will be carried away by the fluid (the ultrapure water to be purified) as filtration continues. In other words, they will not remain in the outer retaining layer indefinitely, thus ensuring that the membrane flux decays very slowly. This allows for the efficient and rapid preparation of ultrapure water over a long period of time, resulting in a long service life (up to 5 years or more). Furthermore, in this invention, the thickness of the outer retaining layer is greater than 0.5 μm, meaning that the outer retaining layer has sufficient thickness (accommodation space) to trap impurity particles in the sample. This ensures that the membrane maintains a good flux and a long service life (slow flux decay).

[0013] Meanwhile, the porosity of the porous membrane is not less than 40%; that is, while ensuring high retention efficiency, it also has high throughput, fast filtration speed, and can quickly prepare high-purity ultrapure water over a long period of time. Commonly used porosity testing methods include mercury intrusion porosimetry, density method, and wet-dry membrane weighing method. Of course, those skilled in the art can also obtain the above parameters through other measurement methods, and the above measurement methods are for reference only.

[0014] Furthermore, the porous membrane of the present invention is integrally formed, meaning that the entire structure of the membrane is made of the same material and is formed directly during the membrane preparation process; in the transition from the membrane thickness direction, there is only a change in the membrane structure, which is an asymmetric membrane; in contrast, for example, composite membranes have a multi-layer structure, which is made by applying a dense layer, which serves as a retention layer, onto a porous layer or porous membrane, often a microporous support layer or support membrane, in a separate process step; the materials constituting the support layer and the retention layer in a composite membrane are often different; the integrally formed porous membrane has higher mechanical strength than the composite membrane and has almost no risk of delamination.

[0015] As a further improvement of the present invention, the outer surface has a plurality of first holes, the SEM average pore size of the first holes being 1-55 nm; the pore density of the first holes being 3-50 holes / 4 μm. 2 .

[0016] In this invention, the outer retention layer of the membrane is the area that retains the most minute impurities in the water. The outer surface is part of the outer retention layer and is also the inlet surface (the area where the outer surface directly contacts the ultrapure water to be purified). Therefore, the properties of the outer surface of the membrane have a significant impact on the membrane's retention efficiency, flux, and pressure resistance. Research has shown that when the average SEM pore size of the first pore on the outer surface is 1-55 nm (preferably 3-50 nm), this pore size can effectively retain various minute impurities, thus further ensuring retention efficiency. Furthermore, with this pore size, most minute impurities are carried away by the fluid as it continues to flow, resulting in a slower flux decay within the membrane, enabling long-term high-efficiency filtration and a longer service life. Additionally, by adjusting the pore density of the first pore to 3-50 pores / 4 μm... 2 With the synergistic effect of such a number and size of membrane pores, on the one hand, a suitable pore area ratio is ensured on the outer surface of the membrane, with minimal impact on the overall membrane flux, allowing the membrane to still maintain a high flux; on the other hand, the outer surface is guaranteed to have high pressure resistance (low pressure resistance will easily cause the membrane pores to deform during ultrafiltration, affecting retention efficiency, or easily collapse, affecting flux). Meanwhile, since ultrapure water is already quite clean during terminal filtration, the number of fine impurities it contains is not large, thus requiring fewer membrane pores. In other words, the pore size and pore density of the first pore on the outer surface of this invention are closely related to ultrapure water applications and are quite unique. This invention, by having a suitable pore size and a suitable number of pores on the porous membrane outer surface, ensures retention efficiency while also possessing good mechanical strength (pressure resistance), with minimal impact on membrane flux, making it particularly suitable for application in ultrapure water terminal filtration.

[0017] In this invention, the SEM average pore size and pore density of the first pore on the outer surface of the porous membrane, and the SEM average pore size and pore density of the second pore on the subsequent inner surface, can all be characterized by scanning electron microscopy (SEM) of the membrane structure, followed by measurement and calculation using computer software (such as Matlab, NIS-Elements, etc.) or manually. During membrane fabrication, the pore size and distribution are generally uniform and consistent in the direction perpendicular to the membrane thickness (planar if the membrane is a flat sheet, and perpendicular to the radius if it is a hollow fiber membrane). Therefore, the overall pore size and quantity on a given plane can be reflected by the pore size and quantity in a specific area. In actual measurement, the inner and outer surfaces of the porous membrane can be characterized using an electron microscope to obtain corresponding SEM images. Since the pore size and distribution on the inner and outer surfaces are generally uniform, a specific area, such as 1 μm, can be selected. 2 (1μm x 1μm) or 100μm2 (10μm x 10μm) or, the specific area size depends on the actual situation. Then, use appropriate computer software or manual measurement to determine the size and number of membrane pores on that area, perform several tests, and take the average value to obtain the SEM average pore size and pore density of the inner and outer surfaces of the porous membrane. Of course, those skilled in the art can also obtain the above parameters through other measurement methods. The above measurement methods are for reference only.

[0018] As a further improvement of the present invention, the inner surface has a plurality of second pores, the SEM average pore diameter of the second pores being 15-300 nm; the pore density of the second pores being 10-80 pores / 4 μm. 2 .

[0019] The ultrapure water filtration of this invention is an external pressure filtration, meaning the fluid (ultrapure water to be purified) first passes through the outer surface of the membrane (inlet surface), then the middle region of the membrane, and finally flows out from the inner surface of the membrane (outlet surface). Therefore, the pores on the inner surface do not need to be very small and can be slightly larger than those on the outer surface. Research has shown that the average SEM pore size of the second pore is 15-300 nm. With such a pore size, on the one hand, the combined effect of thickness and tortuous pathways ensures that the inner retention layer can play a supplementary retention role (retaining tiny impurities in the fluid), ensuring that the membrane has high retention efficiency and the produced ultrapure water has high purity. On the other hand, it ensures the cleanliness of the porous membrane itself, as impurities present in the porous membrane can be easily washed out from the second pores before actual use. In addition, by adjusting the pore density of the second pores on the inner surface to 10-80 pores / 4μm... 2 With the synergistic effect of such a number and size of membrane pores, on the one hand, a suitable pore area ratio (preferably slightly larger than that on the outer surface) is ensured on the inner surface of the membrane, minimizing the impact on the overall membrane flux, and the membrane still maintains a high flux. On the other hand, the inner surface also maintains high pressure resistance, ensuring efficient filtration over a long period. Simultaneously, since the fluid is already relatively clean during terminal filtration, the number of fine impurities inside is small, and most of these micro-impurities are trapped in the outer retention layer, with the inner retention layer only playing a supplementary retention role. Therefore, a large number of membrane pores is not required on the inner surface. The pore size and pore density of the second pores on the inner surface in this invention are closely related to the ultrapure water application and the membrane structure itself, making them quite unique. This invention, by having a suitable pore size and a suitable number of second pores on the inner surface of the porous membrane, not only further ensures retention efficiency but also provides high pressure resistance on the inner surface, ensuring the membrane's own cleanliness, and minimizing the impact on membrane flux (the porous membrane still maintains a high flux), making it particularly suitable for terminal filtration of ultrapure water.

[0020] As a further improvement of the present invention, the portion of colloidal gold with a diameter of 10 nm is mainly located in the outer retention layer, the middle retention layer and the inner retention layer of the main body, and is located in the region from the outer surface at 0%-15%, 45%-70% and 88%-99.9% of the thickness of the main body.

[0021] The outer surface of the main body is taken as the position where the thickness of the main body is 0%, and the inner surface of the main body is taken as the position where the thickness of the main body is 100%.

[0022] By conducting a 10nm colloidal gold retention test on the porous membrane, it was found that there are three regions in the membrane's main structure capable of capturing 10nm colloidal gold: the outer retention layer, the middle retention layer, and the inner retention layer. These three regions are spaced apart and not continuous, allowing for multi-segment retention and ensuring high retention efficiency. The outer retention layer is the first region to retain 10nm colloidal gold, located within 0%-15% of the membrane's thickness. Preferably, the retention of colloidal gold begins from the outer surface (0%) of the membrane (although it can also begin closer to the outer surface). The surface area begins to retain colloidal gold (for example, retention begins at 0.1%). The location of the outer retention layer indicates that it needs sufficient thickness to retain minute impurities, ensuring that most of these impurities are trapped within it. Simultaneously, the outer retention layer's thickness should not be excessive, and it should only retain 10nm of colloidal gold near the outer surface. This ensures that these trapped impurities are carried away by the subsequent filtration fluid and do not remain in the outer retention layer, thus ensuring slow membrane flux decay and a longer lifespan.

[0023] The intermediate retention layer is located within 45%-70% of the main body thickness and plays a supplementary retention role. An intermediate retention layer of appropriate thickness can further ensure the overall retention efficiency of the membrane, while also enabling the membrane to have good flux. In addition, the membrane pores in the intermediate retention layer are relatively small (because it can retain 10nm, its membrane pores will not be very large), thereby further ensuring the mechanical strength and overall pressure resistance of the membrane, and ensuring the stability of the overall membrane flux.

[0024] The inner retention layer is preferably located within 88%-99.9% of the membrane's thickness, not reaching 100% (100% is the inner surface of the membrane, i.e., the liquid outlet). This does not mean that the inner surface cannot retain 10nm colloidal gold, but rather that the 10nm colloidal gold is completely retained before reaching the liquid outlet. If the 10nm colloidal gold is retained only at 100% (the liquid outlet), there is a risk of leakage, meaning that a very small amount of tiny impurities may leak out and appear in the produced ultrapure water. Even such a small amount of tiny impurities can affect the cleanliness of the ultrapure water, causing it to malfunction in high-precision applications such as semiconductors. The appropriate position (thickness) of the inner retention layer further supplements the retention function without affecting the flux, allowing the membrane to still have a large flux. (If the inner retention layer retains too much colloidal gold, the membrane flux will rapidly decline, and the colloidal gold located in the inner retention layer cannot be carried away by the fluid, which will lead to a rapid reduction in the lifespan of the porous membrane and low economic efficiency.)

[0025] After conducting colloidal gold retention experiments using a porous membrane, the distribution of colloidal gold within the membrane can be determined according to the test method in Chinese Patent CN105980038B - "Test Method for Virus Removal Membranes": A slice is cut from the porous membrane after filtering the colloidal gold solution. The brightness distribution of multiple sites in the colloidal gold-stained portion of the slice is measured using an optical microscope. Since colloidal gold absorbs light, the brightness shift depends on the amount of colloidal gold captured. It should be noted that background noise can be removed from the brightness distribution as needed. Then, a graph is prepared with the horizontal axis representing the film thickness and the vertical axis representing the brightness displacement; thereby obtaining the region where colloidal gold of a certain particle size is trapped in the film thickness direction (in this invention, the first porous surface is at 0% of the film thickness, and the second porous surface is at 100% of the film thickness); it should be noted that, in the measurement by optical microscope, when the absolute value of the brightness displacement obtained by subtracting the measured brightness distribution from the constant (255) is less than 10% of the maximum absolute value of the spectrum of the region, from the viewpoint of the virus removal capacity of the virus-removing membrane, the capture of colloidal gold in this region can also be regarded as within the error range.

[0026] Alternatively, it can be understood that although there is a certain amount of colloidal gold in some regions along the film thickness direction, it is very low. Therefore, these regions are not considered as areas where colloidal gold is trapped; they are merely areas where some colloidal gold remains. Thus, in porous membranes, it is preferable to form regions that continuously trap colloidal gold with a diameter of 10 nm along the film thickness direction, which are the regions that truly trap colloidal gold of the corresponding particle size.

[0027] If there is only one region for capturing colloidal gold with a diameter of 10 nm, the following method can be used: In the film thickness direction, measure a first distance 'a' from the first porous surface of the devirtated membrane to the portion of the colloidal gold capturing site closest to the first porous surface; Additionally, in the film thickness direction, measure a second distance 'b' from the first porous surface of the virus-free porous membrane to the portion of the colloidal gold capturing site closest to the second porous surface; then, at multiple sites, calculate the value A (a = a / c) of the first distance 'a' divided by the film thickness 'c' of the virus-free membrane. The first reach is calculated by averaging the values ​​A at multiple sites (expressed as percentages). Additionally, a second distance b is calculated at multiple sites, divided by the membrane thickness c of the virus-free membrane, and expressed as a percentage (ratio of b / c). The average of the values ​​B at multiple sites is then used as the second reach. The sites where 10nm diameter colloidal gold is captured are A10-B10. When there are multiple regions where 10nm diameter colloidal gold is captured, the sites within each region can be processed using the above method. The location of the outer retention layer is: A 外10 -B 外10 The location of the intermediate retaining layer is: A 中10 -B 中10 The location of the inner retaining layer is A. 内10 -B 内10 ;

[0028] It is understandable that, in addition to optical microscopy, those skilled in the art can also characterize the retention area and amount of colloidal gold using known methods. For example, by using energy dispersive spectroscopy (EDS) to analyze the cross-section of a porous membrane containing colloidal gold, a distribution curve of gold in the thickness direction of the porous membrane can be obtained, yielding a similar spectrum.

[0029] As a further improvement of the present invention, the sum of the thicknesses of the outer retention layer, the intermediate retention layer and the inner retention layer is 5-50 μm, and accounts for 3-30% of the total thickness of the membrane.

[0030] The outer, middle, and inner retention layers are the three regions that retain colloidal gold, and the thickness of these three regions affects the membrane's retention efficiency. Studies have shown that when the sum of the thicknesses of these three regions is 5-50 μm, possessing a suitable absolute thickness, the porous membrane exhibits high retention efficiency, a high dirt-holding capacity (this dirt-holding capacity is for ultrapure water containing small amounts of impurities), slow flux decay, and a long service life. Furthermore, this sum of thickness represents a relatively small portion of the overall membrane thickness (generally 3%-30%), resulting in larger pore sizes in most areas of the membrane, thus ensuring high flux and rapid production of highly purified ultrapure water. The thickness of the outer retention layer is: (B 外10 -A 外10)*c; The location of the intermediate retaining layer is: (B 中10 -A 中10 )*c; The location of the inner retaining layer is (B) 内10 -A 内10 )*c; c is the film thickness.

[0031] As a further improvement of the present invention, in the outer retention layer, the capture peak value of 10nm colloidal gold is L. 外 The L 外 The region is located within 0%-5% of the body thickness, starting from the outer surface, and the L... 外 The distance between the part and the outer surface is 0-10 μm, preferably 0-5 μm.

[0032] In the outer retaining layer, the peak value for colloidal gold capture refers to the region within the outer retaining layer along the film thickness direction where the amount of colloidal gold corresponding to the particle size captured is the highest. This is indicated by the largest brightness shift observed in this region (if measured by EDS, the highest gold content can be seen in the spectrum). The amount of colloidal gold captured gradually decreases from this peak value towards both sides. The outer retaining layer is the region within the film where the most colloidal gold is captured, and the peak value within this layer represents the location where the most colloidal gold is captured. In practical applications, this can be considered the location where the most minute impurities are captured. To maintain high flux and a long service life for an extended period, it is desirable for the fluid to carry away the captured minute impurities. Therefore, the distance between the minute impurities and the outer surface should not be too large (if the distance is too large, the impurities are difficult to carry away, which will affect the film flux over time). In other words, the distance between the peak value and the outer surface of the film in the outer retaining layer should not be too large. Research shows that the peak value for capturing 10nm colloidal gold in the outer retaining layer is L. 外 The L 外 The region is located within 0%-5% of the body thickness, starting from the outer surface, and the L... 外 The distance between the part and the outer surface is 0-10μm, preferably 0-5μm; by having the peak value captured in the outer retention layer at a suitable distance from the outer surface, it is ensured that during long-term ultrafiltration, the subsequent fluid can easily carry away various tiny impurities, the membrane flux changes little, and it still has a high flux, thus giving the membrane a long service life.

[0033] As a further improvement of the present invention, the capture peak value of 10nm colloidal gold is retained in the inner retention layer, which is L. 内 The L 内 The region is located within 92%-99.7% of the body thickness, starting from the outer surface, and the L... 内The distance between the part and the inner surface is 0.5-15 μm, preferably 1-10 μm.

[0034] In the inner retaining layer, the peak value for colloidal gold capture refers to the location within the inner retaining layer along the film thickness direction where the amount of colloidal gold corresponding to the particle size is highest (but this does not mean that the highest amount of colloidal gold is captured at this location along the entire film thickness direction). By observing the spectrum, the brightness shift at this location is the largest (if EDS shows the highest gold content in the spectrum), and the amount of colloidal gold captured gradually decreases from this peak value towards both sides. The peak value within the inner retaining layer represents the location with the highest amount of colloidal gold captured, and in practical applications, it can be considered the location where the most minute impurities are captured. If the peak value within the inner retaining layer is too close to the inner surface, there is still a risk of leakage under certain special circumstances (such as a sudden power outage). Research has found that the peak value for capturing 10nm colloidal gold within the inner retaining layer is L. 内 The L 内 The region is located within 92%-99.7% of the body thickness, starting from the outer surface, and the L... 内 The distance between the capture peak and the inner surface is 0.5-15 μm, preferably 1-10 μm; that is, within the inner retention layer, the capture peak is at a suitable distance from the inner surface, further reducing the risk of leakage of minute impurities due to various sudden situations, and further ensuring that high-purity ultrapure water can be produced efficiently over a long period of time. In addition, research has also found that the distance between the capture peak and the inner surface should not be too large, because the capture peak is generally located at a position where the membrane pores are relatively small. If the distance from the inner surface is too large, it is easy to cause a certain reduction in the overall membrane flux. Therefore, in this invention, the capture peak is at a suitable distance from the inner surface, which ensures high membrane flux while maintaining low impurity leakage, which is conducive to the efficient preparation of high-purity ultrapure water.

[0035] As a further improvement of the present invention, the thickness of the intermediate retaining layer is 4-20 μm; the intermediate retaining layer has porous fibers for forming a porous structure, and the SEM average diameter of the porous fibers is 20-300 nm.

[0036] The intermediate retention layer is used to further retain various minute impurities in ultrapure water. Since ultrapure water itself is relatively clean (containing a small number of fine impurities), most of the minute impurities it contains are already retained when the fluid passes through the outer retention layer. Therefore, the intermediate retention layer does not need to be too thick, otherwise it will negatively impact the overall membrane flux. Research has shown that the optimal thickness of the intermediate retention layer is 4-20 μm. At this thickness, the overall membrane retention efficiency is further guaranteed, resulting in a high overall retention efficiency, while also ensuring good flux. Furthermore, because the intermediate retention layer can retain 10 nm colloidal gold, its internal pores are relatively small (relatively dense). The thickness and internal fiber diameter both affect the membrane pore structure between the outer and intermediate retention layers, the membrane pore structure between the inner and intermediate retention layers, and the overall mechanical strength of the membrane. In this invention, the average SEM diameter of the porous fibers is 20nm-300nm. Under the combined effect of the corresponding intermediate retention layer thickness, on the one hand, the stability of the membrane pore structure between the outer and intermediate retention layers and between the inner and intermediate retention layers is ensured (the pores in these two regions are relatively large and prone to collapse), thus meeting the requirements for long-term high-efficiency filtration. On the other hand, it ensures that the membrane as a whole has high mechanical strength, which can meet various processing requirements and terminal filtration of ultrapure water under high pressure.

[0037] In this invention, the SEM average diameter and other characteristics of the porous fibers can be obtained by characterizing the morphology of the membrane cross-section using a scanning electron microscope, then selecting a certain area and measuring it using computer software (such as Matlab, NIS-Elements, etc.) or manually to obtain the diameter of the porous fibers, taking the average value, and then further calculating the corresponding SEM average diameter; of course, it is understood that those skilled in the art can also obtain the above parameters through other measurement methods.

[0038] As a further improvement of the present invention, the distance between the outer retention layer and the intermediate retention layer is a first distance; the distance between the intermediate retention layer and the inner retention layer is a second distance; the lengths of both the first distance and the second distance are not less than 20 μm; and the ratio of the length of the first distance to the length of the second distance is 1.1-2.5:1; the ratio of the sum of the first distance and the second distance to the film thickness is not less than 50%.

[0039] In this invention, the outer retention layer, the middle retention layer, and the inner retention layer are regions used to retain 10nm colloidal gold. To ensure a high overall membrane porosity and consequently a high membrane flux, the regions between the outer and middle retention layers, and between the middle and inner retention layers, are preferably high-porosity structures, preferably finger-like pore structures (the porosity of finger-like pore structures is generally higher than that of sponge-like pore structures). Therefore, to ensure a high overall membrane flux, both the first distance and the second distance (i.e., the thickness of the high-porosity region) need to have a certain length, preferably not less than 20μm. At this length, a high overall membrane porosity (porosity at least not lower than...) is guaranteed. The membrane has a high flux (less than 40%), allowing fluid to pass through it quickly, thus rapidly producing ultrapure water with high purity. Furthermore, the ratio of the first distance length to the second distance length is 1.1-2.5:1; that is, the thickness of the region between the outer and middle retention layers is slightly longer than the thickness of the region between the middle and inner retention layers. This is more conducive to fluid filtration, ensuring the membrane can efficiently retain various fine impurities for a long time, resulting in a long service life. In addition, the ratio of the sum of the first and second distances to the membrane thickness is not less than 50%, further ensuring the porous membrane has high porosity and thus high flux, making it particularly suitable for ultrapure water filtration.

[0040] The location of the outer retention layer is: A 外10 -B 外10 The location of the intermediate retaining layer is: A 中10 -B 中10 The location of the inner retaining layer is A. 内10 -B 内10 Then the length of the first distance is: (A) 中10 -B 外10 )*c; The length of the second distance is (A) 内10 -B 中10 )*c; c is the overall thickness of the membrane;

[0041] As a further improvement of the present invention, the portion of colloidal gold with a diameter of 10 nm further includes a supplementary retention layer, which is located between the intermediate retention layer and the inner retention layer; and the intermediate retention layer, the supplementary retention layer and the inner retention layer are distributed at intervals.

[0042] The supplementary retaining layer is located at 60%-80% of the body thickness from the outer surface;

[0043] The spacing between the intermediate retention layer and the supplementary retention layer is 10-50 μm.

[0044] Ultrapure water contains various fine particulate impurities (such as organic matter, endotoxins, and other inorganic particulate impurities), which can lead to insufficient purity and affect subsequent use. This invention utilizes three layers—an outer retention layer, a middle retention layer, and an inner retention layer—to efficiently capture these fine particulate impurities, ensuring the ultrapure water meets practical application requirements. However, with continuous technological advancements, the requirements for ultrapure water in certain specialized fields will further increase. In such cases, porous membranes with extremely high retention efficiency are needed to guarantee even higher purity ultrapure water. Research has revealed that some porous membranes contain four retention layers, including a supplementary retention layer located between the middle and inner retention layers. This supplementary retention layer extends from the outer surface to 60%-80% of the main body thickness, and the middle, supplementary, and inner retention layers are spaced apart. Through these four retention stages, the supplementary retention layer further supplements the retention process. To further improve retention efficiency, the presence of a supplementary retention layer reduces the amount of colloidal gold retained in the inner retention layer, further reducing the possibility of leakage and ensuring extremely high purity of the produced ultrapure water. Furthermore, it was unexpectedly discovered that the supplementary retention layer improves the membrane's mechanical strength. Since the supplementary retention layer can retain 10nm colloidal gold, its internal pore size is relatively small. To ensure that the presence of the supplementary retention layer has minimal impact on membrane flux, a certain thickness is needed between the intermediate retention layer and the supplementary retention layer to maintain the overall membrane porosity. However, this distance cannot be too large, as an excessively large distance will affect the overall mechanical strength of the membrane, potentially leading to pore collapse in that area due to its relatively large pore size. After adjustment, the distance between the intermediate retention layer and the supplementary retention layer is 10-50μm. This spacing ensures both a high overall membrane flux and a stable overall membrane structure, enabling the stable and efficient production of high-purity ultrapure water during long-term ultrafiltration.

[0045] As a further improvement of the present invention, the thickness of the supplementary retention layer is 0.5-10 μm; the supplementary retention layer has supporting fibers for forming a porous structure, and the SEM average diameter of the supporting fibers is 30 nm-600 nm.

[0046] When the thickness of the supplemental retention layer is too small, it cannot effectively capture fine impurities; when the thickness of the supplemental retention layer is too large, it will greatly reduce the overall membrane flux. Since the ultrapure water terminal filtration is relatively clean with few fine impurities, the supplemental retention layer does not need to be very thick. Research has found that the thickness of the supplemental retention layer is preferably 0.5-10 μm, which can further enhance the retention function while having a small impact on the membrane flux, ensuring that the membrane has a high flux. That is, the thickness of the supplemental retention layer in this invention is synergistic with the application of ultrapure water terminal filtration.

[0047] In addition, the supplementary retention layer contains supporting fibers to form a porous structure. Studies have shown that the average SEM diameter of the supporting fibers is 30-600 nm. With the support of such fine fibers, on the one hand, they can support the membrane pores inside the supplementary retention layer, preventing collapse or shrinkage during filtration. This means that the supplementary retention layer can efficiently retain fine impurities for a long time. On the other hand, the supporting fibers can also support the pore structure in the area between the intermediate retention layer and the supplementary retention layer, thereby ensuring that the membrane has a high flux for a long time and a long service life, generally more than 5 years.

[0048] In this invention, the SEM average diameter and other characteristics of the supporting fibers can be obtained by characterizing the morphology of the membrane cross-section using a scanning electron microscope, then selecting a certain area and measuring it using computer software (such as Matlab, NIS-Elements, etc.) or manually to obtain the diameter of the supporting fibers, taking the average value, and then further calculating the corresponding SEM average diameter; of course, it is understood that those skilled in the art can also obtain the above parameters through other measurement methods.

[0049] As a further improvement of the present invention, the thickness of the porous membrane is 150-250 μm; the porosity of the porous membrane is 50%-90%; and the ratio of the thickness to the inner diameter of the porous membrane is 0.2-0.4.

[0050] The membrane thickness can be determined by characterizing the membrane structure using a scanning electron microscope, followed by calculation using computer software (such as Matlab, NIS-Elements, etc.) or manual measurement. When the membrane thickness is too small, its mechanical strength will be low; simultaneously, the filtration time will be too short, resulting in ineffective filtration. When the porous membrane thickness is too large, the filtration time will be too long, leading to excessive time costs. The porous membrane of this invention has a thickness of 150-250 μm, which is relatively large, ensuring that the membrane not only has high mechanical strength but also effective filtration and high retention efficiency. Commonly used porosity testing methods include mercury intrusion porosimetry, density method, and wet-dry membrane weighing method. Of course, those skilled in the art can also obtain the above parameters through other measurement methods; the above measurement methods are for reference only. Testing has shown that the porosity of this porous membrane is preferably 50%-90%, ensuring both high retention efficiency and high flux, as well as a long service life. When the membrane thickness and membrane... When the ratio of the inner diameter to the membrane diameter is too large (too small), the filtration capacity of the membrane will be greatly reduced, making it impossible to filter a large amount of fluid within a certain time, resulting in low economic efficiency. When the ratio of the membrane thickness to the membrane inner diameter is too small (too large), the mechanical strength of the membrane will be very low, and the pressure resistance of the membrane fibers will be too low to withstand high pressure. In this invention, the ratio of the thickness to the inner diameter of the porous membrane is 0.2-0.4, which ensures that a large amount of fluid can be filtered within a certain time while maintaining the mechanical strength of the membrane fibers, enabling filtration under ideal pressure and ensuring membrane flux.

[0051] As a further improvement of the present invention, the porous membrane has a tensile strength greater than 2 MPa and an elongation of 40%-70%; the TOC precipitation of the porous membrane is ≤0.5 ppb and the metal ion precipitation is ≤10 ppt.

[0052] The water flux of the porous membrane is not less than 25 L*h -1 *m -2 @1psi.

[0053] The tensile strength and elongation at break are important indicators for evaluating the mechanical strength of porous membranes. Under certain conditions, the greater the tensile strength of the membrane, the better its mechanical strength. Tensile strength refers to the membrane's ability to withstand parallel tensile forces. During testing under certain conditions, the membrane sample is subjected to a tensile load until it breaks. Based on the maximum tensile load at the point of failure and the change in the membrane sample's dimensions (length), the tensile strength and elongation at break can be calculated. Both tensile strength and elongation at break can be measured using a universal tensile testing machine. The testing method for tensile strength is well-known in the field; for example, ASTM D790 or ISO 178 details the procedure for tensile strength testing. In this invention, the porous membrane exhibits a tensile strength greater than 2 MPa and an elongation at break of 40%-70%, indicating that the porous membrane possesses good tensile strength and elongation at break, demonstrating good mechanical properties and high industrial applicability. Furthermore, the TOC precipitation of this porous membrane is ≤0.5ppb, and the metal ion precipitation is ≤10ppt, indicating that the membrane fibers themselves have high cleanliness and will not introduce impurities during ultrafiltration. Water flux testing shows that the water flux of the porous membrane of this invention is not less than 25 L*h. -1 *m -2 @1psi, with a relatively large flux, meaning that the porous membrane has a good filtration speed, it can produce a large volume of high-purity ultrapure water in a short time, resulting in high economic benefits.

[0054] Furthermore, this invention also provides a method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water, comprising the following steps:

[0055] Step 1: Preparation of casting solution and core solution:

[0056] The casting solution comprises the following components by weight: 15-25 parts of polysulfone-based substances, 15-40 parts of additives; 1-20 parts of a first organic solvent, and 35-75 parts of a second organic solvent.

[0057] The polysulfone is at least one of polyethersulfone, polysulfone, and polyphenylsulfone, with a weight-average molecular weight of 30,000 to 90,000; the additive is an alcohol with a number-average molecular weight of less than 1,000.

[0058] The first organic solvent is a volatile solvent, and the second organic solvent is a non-volatile solvent;

[0059] The core fluid comprises a second organic solvent and a non-solvent; the content of the non-solvent in the core fluid is 30%-60%; the non-solvent is water;

[0060] Step 2: Spinning: The casting solution and the core solution are extruded together from the die head, and the casting solution forms a molded product with an inner surface and an outer surface in the die head;

[0061] Step 3: Pretreatment: The molded product is pretreated by passing it through an air section, wherein the length of the air section is 1-20cm, the temperature is 30-60℃, and the relative humidity is 10-60%.

[0062] Step 4: Phase Separation: The pretreated molded product is placed in a gel bath for phase separation to form a biological film. The phase separation time is 60-180 seconds. The gel bath is water.

[0063] Step 5: Wash the raw membrane in water and dry it to obtain a hollow fiber porous membrane.

[0064] As a further improvement of the present invention, the first organic solvent is at least one selected from methanol, tetrahydrofuran, acetone, butanone, and ethyl acetate;

[0065] The second organic solvent is at least one selected from dimethyl sulfoxide, dimethylformamide, N-ethylpyrrolidone, dimethylacetamide, N-methylpyrrolidone, and N,N-diethylformamide;

[0066] The additive is at least one of polyvinyl alcohol, ethylene glycol, propylene glycol, isopropanol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, glycerin, ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, and diethylene glycol.

[0067] As a further improvement of the present invention, the temperature of the casting solution is 35-50°C, the temperature of the core solution is 5-15°C lower than the temperature of the casting solution, and the temperature of the gel bath is 30-60°C.

[0068] As a further improvement of the present invention, the die stretching ratio during spinning is 2-6; in step four, the molded article is slightly stretched while it is separating phases in the gel bath, with a stretching ratio of 2-8 times and a stretching rate of 5% / min-20% / min.

[0069] In preparing the hollow fiber porous membrane of the present invention, a casting solution is first prepared. The casting solution includes a film-forming substance, a polysulfone, an organic solvent (for dissolving the polysulfone), and additives. The polysulfone is at least one of polyethersulfone, polysulfone, and polyphenylene sulfone. These polymers possess excellent antioxidant properties, thermal stability, and good mechanical properties, resulting in excellent mechanical properties of the formed membrane, which can meet various processing requirements and has significant industrial value. Simultaneously, they also exhibit good hydrophilicity, facilitating rapid wetting of the membrane by ultrapure water. The molecular weight of the film-forming polymer has a certain influence on the membrane pore structure (pore size and number) and various properties (such as mechanical properties). Research has shown that the weight-average molecular weight of polysulfones is preferably 30,000-90,000, which is beneficial for obtaining an ideal casting solution and thus a porous membrane with an ideal membrane structure. As one of the key aspects of this invention, the additive is an alcohol with a number-average molecular weight of less than 1000 (it can be a polymeric alcohol or a small molecule alcohol). Preferably, the additive is at least one of the following: polyvinyl alcohol, ethylene glycol, propylene glycol, isopropanol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, glycerol, ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, and diethylene glycol. The additive is a single alcohol (or a mixture of multiple alcohols); by adding a certain mass of additive to the casting solution and then performing a suitable phase separation process, it is beneficial to form a porous membrane with at least three retaining layers; at the same time, research has found that the addition of the additive has a certain effect on improving the flux and mechanical strength of the membrane; as one of the key points of this invention, the organic solvent is a mixture, including a first organic solvent and a second organic solvent; wherein the first organic solvent is a volatile solvent (volatile means that the solvent will evaporate at room temperature, thereby increasing the solid content of the film-forming substance and making it easier to form small pores), and the second organic solvent is a non-volatile solvent; This compounding process, combined with subsequent air section and phase separation processes, facilitates the formation of a nearly defect-free and relatively dense outer retention layer (the outer retention layer is the area within the membrane that retains the most impurities), thereby ensuring that the porous membrane has high retention efficiency. By adjusting the ratio of the film-forming polymer, organic solvent, and additives, an ideal casting solution is formed, ensuring that the final porous membrane has an ideal pore structure and pore size for producing ultrapure water with high purity. Furthermore, the core used in the extrusion of the hollow fiber porous membrane in this invention is in liquid form, so a suitable core liquid needs to be selected. The core liquid includes a second organic solvent and a non-solvent, with the non-solvent being water.By selecting a suitable core liquid (the corresponding substances and their proportions), it is possible to ensure that the pressure inside the hollow fiber membrane cavity is balanced with the external pressure, thereby stabilizing the cavity of the hollow fiber membrane and making the wall thickness of the hollow fiber membrane basically the same. On the other hand, the core liquid also affects the pore size and pore size distribution on the inner surface. Under the combined effect of conditions such as the coagulation bath, by controlling the changes in the membrane phase separation process, a porous membrane with an ideal inner surface pore structure can be manufactured, making the porous membrane suitable for ultrapure water terminal filtration.

[0070] The second step is spinning, where the casting solution and the core solution are extruded together from the die head. The casting solution forms a molded product with an inner surface and an outer surface in the die head; the molded product is a hollow fiber membrane; the extruded hollow fiber membrane has a surface facing the cavity, i.e., the inner surface, and a surface opposite to the cavity, i.e., the outer surface.

[0071] In ultrapure water terminal filtration, the most important performance characteristic is the retention efficiency. Defects generated during membrane fabrication can significantly impact this efficiency, so minimizing these defects is crucial. In this invention, the outer retention layer is the key area for retaining minute impurities in ultrapure water. Therefore, during membrane fabrication, the outer retention layer should be virtually free of defects, exhibiting ideal pore size and quantity. The third step involves a pretreatment process on the molded product. The product is pretreated in an air section with a length of 1-20 cm, a temperature of 30-60°C, and a relative humidity of 10-60%. Under these pretreatment conditions, the first organic solvent evaporates to a certain extent, resulting in a higher polymer concentration at the interface between the casting solution and the surrounding atmosphere compared to the bulk concentration. After immersion in the coagulation bath, a relatively dense outer retention layer is formed (ensuring high retention efficiency), and this method produces an outer retention layer with very few defects.

[0072] The fourth step is phase separation: the pretreated molded product is placed in a gel bath for phase separation to form a green membrane, with a phase separation time of 60-180 seconds; the gel bath is water; the desired membrane structure is formed through the combined action of an ideal casting solution formulation, pretreatment process, and phase separation process; finally, the green membrane is washed in water to remove as much organic solvent and other substances as possible from the membrane fibers to ensure the cleanliness of the membrane fibers themselves, and then dried (natural drying or other drying methods can be selected) to finally obtain a hollow fiber porous membrane. This porous membrane has at least 3 retention layers, and its outer surface and outer retention layer are basically free of defects, thereby ensuring that the porous membrane has ultra-high retention efficiency;

[0073] Preferably, the casting solution temperature is 35-50℃, and the core solution temperature is 5-15℃ lower than the casting solution temperature; the gel bath temperature is 30-60℃. Under these conditions, combined with the appropriate casting solution formulation, pretreatment, and phase separation process, ideal membrane pore size and number appear on the inner and outer surfaces. At the same time, it is conducive to the formation of porous fibers and supporting fibers of suitable thickness, which further ensures the membrane's retention efficiency, while also enabling the membrane to have good flux and excellent mechanical strength.

[0074] Preferably, the die stretching ratio during spinning is 2-5. Studies have shown that the die stretching ratio has a certain impact on the uniformity of membrane pores. Under such a stretching ratio, combined with the corresponding casting solution formula and phase separation process, the membrane pores can be made more uniform, and there will be no particularly small or large pores (fewer defects). This ensures that the porous membrane has a good filtration speed while having high retention efficiency.

[0075] Preferably, the molded product is slightly stretched during phase separation in the gel bath, with a stretching ratio of 2-8 times and a stretching rate of 5% / min-20% / min. The stretching can be achieved by the speed difference between the front and rear rollers. Slight stretching of the molded product during phase separation helps to improve the mechanical strength of the membrane fibers, thereby ensuring that the film has high compressive strength and a wider range of applications. At the same time, slight stretching helps to make the pore size of the membrane more uniform, thereby further improving the retention efficiency.

[0076] The beneficial effects of this invention: The hollow fiber porous membrane provided by this invention is suitable for use in ultrapure water terminal filtration. The porous membrane includes a main body, with one side being an inner surface and the other side an outer surface. The main body has three regions capable of retaining 10nm colloidal gold: an outer retention layer, a middle retention layer, and an inner retention layer. One side of the outer retention layer is the outer surface, and the middle retention layer is located between the outer and inner retention layers. The outer retention layer, the middle retention layer, and the inner retention layer are spaced apart. Multi-segment retention is achieved through multiple regions. This porous membrane achieves a retention efficiency of no less than 95% for 10nm colloidal gold, exhibiting high retention efficiency and making it suitable for use in ultrapure water terminal filtration to ensure the high purity of the produced ultrapure water. Simultaneously, most minute impurities are retained in the outer retention layer (i.e., the area near the outer surface), ensuring slow membrane flux decay and enabling long-term, efficient, and rapid ultrapure water production with a long service life (up to 5 years or more). Furthermore, the porous membrane has a porosity of no less than 40%, resulting in high overall membrane flux and the ability to rapidly produce large quantities of ultrapure water. The preparation method provided by this invention allows for the convenient, rapid, and effective preparation of the aforementioned hollow fiber porous membrane. Attached Figure Description

[0077] Figure 1The image shows a scanning electron microscope (SEM) image of the outer surface of the porous membrane prepared in Example 1 after a 10 nm colloidal gold retention test, with a magnification of 50 K×.

[0078] Figure 2 The image shows a scanning electron microscope (SEM) image of the outer layer of the porous membrane obtained in Example 1 after a 10 nm colloidal gold retention test, with a magnification of 20K×.

[0079] Figure 3 The image shows a scanning electron microscope (SEM) image of the trapped layer in the cross section of the porous membrane prepared in Example 1 after a 10 nm colloidal gold trapping test, with a magnification of 50K×.

[0080] Figure 4 The image shows a scanning electron microscope (SEM) image of the middle retention layer of the porous membrane prepared in Example 1 after a 10 nm colloidal gold retention test, with a magnification of 1500×.

[0081] Figure 5 The image shows a scanning electron microscope (SEM) image of the trapped layer in the cross section of the porous membrane prepared in Example 2 after a 10 nm colloidal gold trapping test, with a magnification of 20K×.

[0082] Figure 6 The image shows a scanning electron microscope (SEM) image of the overall cross-section of the porous membrane prepared in Example 3, with a magnification of 50×.

[0083] Figure 7 The image shown is a further magnified scanning electron microscope (SEM) image of the overall cross-section of the porous membrane prepared in Example 3, with a magnification of 700×.

[0084] Figure 8 The image shown is a scanning electron microscope (SEM) image of the inner surface of the cross-section of the porous membrane prepared in Example 3, with a magnification of 1000×.

[0085] Figure 9 The image shown is a scanning electron microscope (SEM) image of the cross-section of the porous membrane prepared in Example 3, near the outer surface, with a magnification of 1000×.

[0086] Figure 10 The image shown is a scanning electron microscope (SEM) image of the outer surface of the porous membrane prepared in Example 3, with a magnification of 10K×.

[0087] Figure 11 The image shown is a scanning electron microscope (SEM) image of the inner surface of the porous membrane prepared in Example 3, with a magnification of 50K×. Detailed Implementation

[0088] To more clearly illustrate the overall concept of this application, detailed descriptions are provided below using examples. Unless otherwise specified, the raw materials and equipment used to prepare the porous membranes in the following examples are commercially available. Specifically, a Hitachi S-5500 scanning electron microscope was used to characterize the structural morphology of the porous membranes.

[0089] Example 1

[0090] A method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water includes the following steps:

[0091] Step 1: Preparation of casting solution and core solution:

[0092] The casting solution comprises the following components by weight: 20 parts of polysulfone, 30 parts of additives; 10 parts of a first organic solvent, and 55 parts of a second organic solvent; the polysulfone is polyethersulfone with a weight-average molecular weight of 60,000; the additive is ethylene glycol; the first organic solvent is ethyl acetate, a volatile solvent; and the second organic solvent is N,N-diethylformamide, a non-volatile solvent.

[0093] The core solution includes a second organic solvent, N,N-diethylformamide, and a non-solvent, water; the content of the non-solvent in the core solution is 45%; the temperature of the casting solution is 42°C, and the core solution temperature is 12°C lower than the casting solution temperature.

[0094] Step 2: Spinning: The casting solution and the core solution are extruded together from the die head, and the casting solution forms a molded product with an inner surface and an outer surface in the die head; the die head stretch ratio is 4 during spinning;

[0095] Step 3: Pretreatment: The molded product is pretreated by passing it through an air section, wherein the length of the air section is 10cm, the temperature is 45℃, and the relative humidity is 35%.

[0096] Step 4: Phase separation: Place the pretreated molded product into a gel bath for phase separation to form a film. The phase separation time is 120 seconds. The gel bath is water. The temperature of the gel bath is 45°C.

[0097] Step 5: Wash the raw membrane in water and dry it to obtain a hollow fiber porous membrane.

[0098] Example 2

[0099] A method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water includes the following steps:

[0100] Step 1: Preparation of casting solution and core solution: The casting solution consists of the following components by weight: 18 parts polysulfone, 25 parts additives; 8 parts first organic solvent, and 50 parts second organic solvent;

[0101] The polysulfone substance is polysulfone with a weight-average molecular weight of 40,000; the additive is polyethylene glycol-200; the first organic solvent is volatile solvent acetone, and the second organic solvent is non-volatile solvent N-ethylpyrrolidone.

[0102] The core solution includes a second organic solvent, N-ethylpyrrolidone, and a non-solvent, water; the content of the non-solvent in the core solution is 40%; the temperature of the casting solution is 40°C, and the core solution temperature is 10°C lower than the casting solution temperature.

[0103] Step 2: Spinning: The casting solution and the core solution are extruded together from the die head, and the casting solution forms a molded product with an inner surface and an outer surface in the die head; the die head stretch ratio is 4 during spinning;

[0104] Step 3: Pretreatment: The molded product is pretreated by passing it through an air section, wherein the length of the air section is 8cm, the temperature is 40℃, and the relative humidity is 30%.

[0105] Step 4: Phase Separation: The pretreated molded product is placed in a gel bath for phase separation to form a biological film. The phase separation time is 140 seconds. The gel bath is water. The temperature of the gel bath is 40°C.

[0106] Step 5: Wash the raw membrane in water and dry it to obtain a hollow fiber porous membrane.

[0107] Example 3

[0108] A method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water includes the following steps:

[0109] Step 1: Preparation of casting solution and core solution: The casting solution comprises the following components by weight: 22 parts of polysulfone, 35 parts of additives; 12 parts of first organic solvent, and 60 parts of second organic solvent; the polysulfone is polyphenylene sulfone with a weight-average molecular weight of 70,000; the additive is glycerol; the first organic solvent is volatile solvent butanone, and the second organic solvent is non-volatile solvent dimethylacetamide;

[0110] The core fluid includes a second organic solvent, dimethylacetamide, and a non-solvent, water; the content of the non-solvent in the core fluid is 50%.

[0111] The temperature of the casting solution is 45°C, and the core solution temperature is 10°C lower than the casting solution temperature.

[0112] Step 2: Spinning: The casting solution and the core solution are extruded together from the die head, and the casting solution forms a molded product with an inner surface and an outer surface in the die head; the die head stretch ratio is 3 during spinning;

[0113] Step 3: Pretreatment: The molded product is pretreated by passing it through an air section, wherein the length of the air section is 12cm, the temperature is 40℃, and the relative humidity is 40%.

[0114] Step 4: Phase Separation: The pretreated molded product is placed in a gel bath for phase separation to form a biological film. The phase separation time is 100 seconds. The gel bath is water. The temperature of the gel bath is 55°C.

[0115] Step 5: Wash the raw membrane in water and dry it to obtain a hollow fiber porous membrane.

[0116] Example 4

[0117] A method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water includes the following steps:

[0118] Step 1: Preparation of casting solution and core solution: The casting solution comprises the following components by weight: 15 parts polysulfone, 15 parts additive; 3 parts first organic solvent, 65 parts second organic solvent; the polysulfone is polyethersulfone with a weight-average molecular weight of 30,000; the additive is ethylene glycol monomethyl ether; the first organic solvent is tetrahydrofuran, a volatile solvent, and the second organic solvent is dimethylformamide, a non-volatile solvent.

[0119] The core solution includes a second organic solvent, dimethylformamide, and a non-solvent, water; the content of the non-solvent in the core solution is 35%; the temperature of the casting solution is 50°C, and the temperature of the core solution is 20°C lower than that of the casting solution.

[0120] Step 2: Spinning: The casting solution and the core solution are extruded together from the die head, and the casting solution forms a molded product with an inner surface and an outer surface in the die head; the die head stretch ratio is 7 during spinning;

[0121] Step 3: Pretreatment: The molded product is pretreated by passing it through an air section, wherein the length of the air section is 4cm, the temperature is 35℃, and the relative humidity is 20%.

[0122] Step 4: Phase Separation: The pretreated molded product is placed in a gel bath for phase separation to form a biological film. The phase separation time is 160 seconds. The gel bath is water. The temperature of the gel bath is 30°C.

[0123] Step 5: Wash the raw membrane in water and dry it to obtain a hollow fiber porous membrane.

[0124] Example 5

[0125] A method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water includes the following steps:

[0126] Step 1: Preparation of casting solution and core solution:

[0127] The casting solution comprises the following components by weight: 23 parts of polysulfone, 28 parts of additives; 14 parts of a first organic solvent, and 40 parts of a second organic solvent; the polysulfone is polyethersulfone with a weight-average molecular weight of 80,000; the additive is propylene glycol; the first organic solvent is ethyl acetate, a volatile solvent; and the second organic solvent is dimethyl sulfoxide, a non-volatile solvent.

[0128] The core solution includes a second organic solvent, dimethyl sulfoxide, and a non-solvent, water; the content of the non-solvent in the core solution is 42%; the temperature of the casting solution is 48°C, and the core solution temperature is 13°C lower than the casting solution temperature.

[0129] Step 2: Spinning: The casting solution and the core solution are extruded together from the die head, and the casting solution forms a molded product with an inner surface and an outer surface in the die head; the die head stretch ratio is 5 during spinning;

[0130] Step 3: Pretreatment: The molded product is pretreated by passing it through an air section, wherein the length of the air section is 15cm, the temperature is 50℃, and the relative humidity is 50%.

[0131] Step 4: Phase separation: The pretreated molded product is placed in a gel bath for phase separation to form a film. The phase separation time is 90s. The gel bath is water. The temperature of the gel bath is 50℃. While the molded product is undergoing phase separation in the gel bath, it is slightly stretched with a stretching ratio of 4 times and a stretching rate of 5% / min.

[0132] Step 5: Wash the raw membrane in water and dry it to obtain a hollow fiber porous membrane.

[0133] Example 6

[0134] A method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water includes the following steps:

[0135] Step 1: Preparation of casting solution and core solution:

[0136] The casting solution comprises the following components by weight: 25 parts of polysulfone, 32 parts of additives; 17 parts of a first organic solvent, and 45 parts of a second organic solvent; the polysulfone is polysulfone with a weight-average molecular weight of 90,000; the additive is polyvinyl alcohol; the first organic solvent is acetone, a volatile solvent, and the second organic solvent is N,N-diethylformamide, a non-volatile solvent.

[0137] The core solution includes a second organic solvent, N,N-diethylformamide, and a non-solvent, water; the content of the non-solvent in the core solution is 48%; the temperature of the casting solution is 44°C, and the core solution temperature is 9°C lower than the casting solution temperature.

[0138] Step 2: Spinning: The casting solution and the core solution are extruded together from the die head, and the casting solution forms a molded product with an inner surface and an outer surface in the die head; the die head stretch ratio is 5 during spinning;

[0139] Step 3: Pretreatment: The molded product is pretreated by passing it through an air section, wherein the length of the air section is 18cm, the temperature is 55℃, and the relative humidity is 45%.

[0140] Step 4: Phase separation: The pretreated molded product is placed in a gel bath for phase separation to form a film. The phase separation time is 80 seconds. The gel bath is water. The temperature of the gel bath is 50°C. While the molded product is undergoing phase separation in the gel bath, it is slightly stretched with a stretching ratio of 5 times and a stretching rate of 10% / min.

[0141] Step 5: Wash the raw membrane in water and dry it to obtain a hollow fiber porous membrane.

[0142] Observations revealed that the porous membranes prepared in Examples 5 and 6 contain four regions for retaining colloidal gold: an outer retention layer, a middle retention layer, a supplementary retention layer, and an inner retention layer. This results in the porous membrane having an extremely high retention efficiency for 10nm colloidal gold, enabling it to efficiently capture various minute impurities in ultrapure water and produce ultrapure water with high purity, which fully meets the needs of practical applications.

[0143] Comparative Example 1

[0144] A method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water includes the following steps:

[0145] Step 1: Preparation of casting solution and core solution:

[0146] The casting solution comprises the following components by weight: 20 parts of polysulfone and 55 parts of a second organic solvent; the polysulfone is polyethersulfone with a weight-average molecular weight of 60,000; the second organic solvent is a non-volatile solvent, N,N-diethylformamide.

[0147] The core solution includes a second organic solvent, N,N-diethylformamide, and a non-solvent, water; the content of the non-solvent in the core solution is 45%; the temperature of the casting solution is 42°C, and the core solution temperature is 12°C lower than the casting solution temperature.

[0148] Step 2: Spinning: The casting solution and the core solution are extruded together from the die head, and the casting solution forms a molded product with an inner surface and an outer surface in the die head; the die head stretch ratio is 4 during spinning;

[0149] Step 3: Pretreatment: The molded product is pretreated by passing it through an air section, wherein the length of the air section is 10cm, the temperature is 45℃, and the relative humidity is 35%.

[0150] Step 4: Phase separation: Place the pretreated molded product into a gel bath for phase separation to form a film. The phase separation time is 120 seconds. The gel bath is water. The temperature of the gel bath is 45°C.

[0151] Step 5: Wash the raw membrane in water and dry it to obtain a hollow fiber porous membrane.

[0152] Because no additives and a first organic solvent were added to the casting solution, the porous membrane did not form three regions capable of retaining 10nm colloidal gold, resulting in a low retention efficiency, far less than 95%, which cannot meet the requirements of practical applications.

[0153] Comparative Example 2

[0154] A method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water includes the following steps:

[0155] Step 1: Preparation of casting solution and core solution:

[0156] The casting solution comprises the following components by weight: 20 parts of polysulfone, 30 parts of additives, and 55 parts of a second organic solvent; the polysulfone is polyethersulfone with a weight-average molecular weight of 60,000; the additive is ethylene glycol; and the second organic solvent is the non-volatile solvent N,N-diethylformamide.

[0157] The core solution includes a second organic solvent, N,N-diethylformamide, and a non-solvent, water; the content of the non-solvent in the core solution is 45%; the temperature of the casting solution is 42°C, and the core solution temperature is 12°C lower than the casting solution temperature.

[0158] Step 2: Spinning: The casting solution and the core solution are extruded together from the die head, and the casting solution forms a molded product with an inner surface and an outer surface in the die head; the die head stretch ratio is 4 during spinning;

[0159] Step 3: Phase separation: Place the molded product in a gel bath for phase separation to form a biological film. The phase separation time is 120 seconds. The gel bath is water. The temperature of the gel bath is 45°C.

[0160] Step 4: Wash the raw membrane in water and dry it to obtain a hollow fiber porous membrane.

[0161] Compared to Example 1, since no first organic solvent was added to the casting solution and no pretreatment was performed, the overall pore size of the porous membrane was too large and the thickness of the outer retention layer was too small, resulting in a very low overall retention efficiency of the membrane. At the same time, due to the small thickness of the outer retention layer, the membrane pores were relatively easy to clog, the flux would decay rapidly, and the service life would be short.

[0162] 1. Structural Characterization

[0163] The morphology of the porous membranes obtained in each embodiment was characterized using scanning electron microscopy, and the required data were then obtained; the specific results are shown in the table below:

[0164] Table 1:

[0165]

[0166] The porous membranes prepared in Examples 1-6 have a suitable number of first pores with appropriate pore size on the first outer surface and a suitable number of second pores with appropriate pore size on the second outer surface, resulting in an ideal inner and outer surface structure. In contrast, the porous membrane prepared in Comparative Example 2 has an excessively large average SEM pore size and a small number of first pores on the first outer surface, and an excessively large average SEM pore size and a small number of second pores on the second outer surface, which affects the retention efficiency and flux.

[0167] Table 2

[0168]

[0169] Table 3

[0170]

[0171] Table 4

[0172]

[0173]

[0174] Table 5

[0175]

[0176] Both the porous membranes prepared in Examples 5 and 6 contain a supplementary retention layer, that is, four regions for retaining tiny impurity particles, which is beneficial to obtaining higher retention efficiency.

[0177] As shown in Tables 1-5, the hollow fiber porous membranes prepared in Examples 1-6 of this invention are all integrally formed without composite processes, making the preparation process simple and suitable for large-scale application. Moreover, the porous membranes all have ideal membrane structures, with appropriate pore size and number on both the inner and outer surfaces, and at least three layers of retention areas, ensuring high-quality retention of various impurities. Their retention efficiency reaches over 95%, making them particularly suitable for terminal filtration of ultrapure water to produce ultrapure water with high purity.

[0178] Performance characteristics

[0179] Membrane flux is calculated as follows: The formula for calculating membrane flux (J) is: J = V / (T × A) Where:

[0180] J -- Membrane flux unit: L*h-1 *m -2

[0181] V -- Sampling volume (L); T -- Sampling time (h); A -- Effective membrane area (m²) 2 )

[0182] The operating conditions used for measuring the membrane separation performance in this invention are as follows: the feed liquid is deionized water, the operating pressure is 1 psi, the operating temperature is 25℃, and the solution pH is 7.

[0183] Water flux tests showed that the water flux of the porous membranes prepared in Examples 1-6 was no less than 25 L*h. -1 *m -2 With a throughput of 1 psi, it can quickly produce ultrapure water with high purity.

[0184] Retention test: The retention efficiency of the porous membranes obtained in each example was tested using a 10 nm colloidal gold aqueous solution.

[0185] Retention efficiency Tensile strength / MPa Elongation at break Porosity Inner diameter / μm Example 1 Greater than 97% 3.0 55％ 70％ 600 Example 2 Greater than 97% 2.8 60％ 75％ 700 Example 3 Greater than 97% 3.2 50％ 65％ 500 Example 4 Greater than 95% 2.5 65％ 72％ 600 Example 5 Greater than 99% 3.4 50％ 60％ 600 Example 6 Greater than 99% 3.6 45％ 55％ 600 Comparative Example 1 52％ 1.4 35％ 65％ 600 Comparative Example 2 64％ 1.7 30％ 60％ 600

[0186] As shown in the table above, the hollow fiber porous membranes prepared in Examples 1-6 of this invention have high flux and high retention efficiency, as well as good mechanical strength, making them particularly suitable for terminal filtration of ultrapure water. In addition, various leaching tests were conducted, and the tests showed that TOC leaching was less than 0.5 ppb and metal ion precipitation was less than 1 ppt. The membrane fibers themselves are very clean and will not introduce new impurities, further ensuring the production of ultrapure water with high purity.

[0187] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A hollow fiber porous membrane for terminal ultrafiltration of ultrapure water, comprising a body, one side of which is an inner surface facing the inner cavity, and the other side of which is an outer surface, characterized in that, When using 10nm colloidal gold as impurity particles for retention testing, the retention efficiency of the porous membrane is not less than 95%. The 10nm diameter colloidal gold is mainly retained in the outer retention layer, the middle retention layer, and the inner retention layer of the membrane body; one side of the outer retention layer is the outer surface, and the middle retention layer is located between the outer retention layer and the inner retention layer; the outer retention layer, the middle retention layer, and the inner retention layer are distributed at intervals. Furthermore, during the retention test, at least 75% of the retained colloidal gold was retained in the outer retention layer of the porous membrane, and the thickness of the outer retention layer was greater than 0.5 μm. The porosity of the porous membrane is not less than 40%; The porous membrane is integrally formed; The outer surface has a plurality of first holes, the average SEM pore size of the first holes being 1-55 nm; the pore density of the first holes is 3-50 holes / 4 μm. 2 ; The areas where 10nm diameter colloidal gold is retained are mainly located in the outer retention layer, middle retention layer and inner retention layer of the main body, and are located in the regions from the outer surface at 0%-15%, 45%-70% and 88%-99.9% of the thickness of the main body. The outer surface of the main body is taken as the position where the thickness of the main body is 0%, and the inner surface of the main body is taken as the position where the thickness of the main body is 100%.

2. The hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 1, characterized in that: The inner surface has a plurality of second pores, the average SEM pore size of the second pores being 15-300 nm; the pore density of the second pores being 10-80 pores / 4 μm. 2 .

3. The hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 1, characterized in that: The sum of the thicknesses of the outer retention layer, the intermediate retention layer, and the inner retention layer is 5-50 μm, and accounts for 3-30% of the total membrane thickness.

4. The hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 1, characterized in that: In the outer trapping layer, the peak capture value for 10nm colloidal gold is L. 外 The L 外 The region is located within 0%-5% of the body thickness, starting from the outer surface, and the L... 外 The distance between the part and the outer surface is 0-10 μm.

5. A hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 4, characterized in that: The L 外 The distance between the part and the outer surface is 0-5 μm.

6. The hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 1, characterized in that: Within the inner retention layer, the peak capture value for 10nm colloidal gold is L. 内 The L 内 The region is located within 92%-99.7% of the body thickness, starting from the outer surface, and the L... 内 The distance between the part and the inner surface is 0.5-15 μm.

7. A hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 6, characterized in that: The L 内 The distance between the part and the inner surface is 1-10 μm.

8. The hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 1, characterized in that: The thickness of the intermediate retaining layer is 4-20 μm; the intermediate retaining layer has porous fibers for forming a porous structure, and the SEM average diameter of the porous fibers is 20-300 nm.

9. A hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 1, characterized in that: The distance between the outer retention layer and the intermediate retention layer is a first distance; the distance between the intermediate retention layer and the inner retention layer is a second distance. The lengths of both the first distance and the second distance are not less than 20 μm; and the ratio of the length of the first distance to the length of the second distance is 1.1-2.5:

1. The ratio of the sum of the first distance and the second distance to the film thickness is not less than 50%.

10. A hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 1, characterized in that: The area for retaining 10nm diameter colloidal gold also includes a supplementary retention layer, which is located between the intermediate retention layer and the inner retention layer; and the intermediate retention layer, the supplementary retention layer, and the inner retention layer are spaced apart. The supplementary retaining layer is located at 60%-80% of the body thickness from the outer surface; The spacing between the intermediate retention layer and the supplementary retention layer is 10-50 μm.

11. A hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 10, characterized in that: The thickness of the supplementary retention layer is 0.5-10 μm; The supplementary retaining layer contains supporting fibers for forming a porous structure, and the SEM average diameter of the supporting fibers is 30nm-600nm.

12. The hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 1, characterized in that: The thickness of the porous membrane is 150-250 μm; the porosity of the porous membrane is 50%-90%; and the ratio of the thickness to the inner diameter of the porous membrane is 0.2-0.

4.

13. A hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 1, characterized in that: The porous membrane has a tensile strength greater than 2 MPa and an elongation of 40%-70%. The porous membrane exhibits TOC precipitation ≤0.5 ppb and metal ion precipitation ≤10 ppt. The water flux of the porous membrane is not less than 25 L*h -1 *m -2 @1psi.

14. A method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to any one of claims 1 to 13, characterized in that: Includes the following steps: Step 1: Preparation of casting solution and core solution: The casting solution comprises the following components by weight: 15-25 parts of polysulfone substances and 15-40 parts of additives. 1-20 parts of the first organic solvent, 35-75 parts of the second organic solvent; The polysulfone is at least one of polyethersulfone, polysulfone, and polyphenylsulfone, with a weight-average molecular weight of 30,000 to 90,000; the additive is an alcohol with a number-average molecular weight of less than 1,000. The first organic solvent is a volatile solvent, and the second organic solvent is a non-volatile solvent; The core fluid includes a second organic solvent and a non-solvent; the content of the non-solvent in the core fluid is 30%-60%; the non-solvent is water; Step 2: Spinning: The casting solution and the core solution are extruded together from the die head, and the casting solution forms a molded product with an inner surface and an outer surface in the die head; Step 3: Pretreatment: The molded product is pretreated by passing it through an air section, wherein the length of the air section is 1-20cm, the temperature is 30-60℃, and the relative humidity is 10-60%. Step 4: Phase Separation: The pretreated molded product is placed in a gel bath for phase separation to form a biological film. The phase separation time is 60-180 seconds. The gel bath is water. Step 5: Wash the raw membrane in water and dry it to obtain a hollow fiber porous membrane.

15. The method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 14, characterized in that, The first organic solvent is at least one selected from methanol, tetrahydrofuran, acetone, butanone, and ethyl acetate; The second organic solvent is at least one selected from dimethyl sulfoxide, dimethylformamide, N-ethylpyrrolidone, dimethylacetamide, N-methylpyrrolidone, and N,N-diethylformamide; The additive is at least one of polyvinyl alcohol, ethylene glycol, propylene glycol, isopropanol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, glycerin, ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, and diethylene glycol.

16. The method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 14, characterized in that, The temperature of the casting solution is 35-50℃, and the temperature of the core solution is 5-15℃ lower than that of the casting solution; the temperature of the gel bath is 30-60℃.

17. The method for preparing a hollow fiber porous membrane for terminal ultrafiltration of ultrapure water according to claim 14, characterized in that, The die stretch ratio during spinning is 2-6; In step four, the molded article is slightly stretched while it is undergoing phase separation in the gel bath. The stretching ratio is 2-8 times and the stretching rate is 5% / min-20% / min.

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