Asymmetric pp hollow-fiber porous membrane for degassing, and preparation method therefor and use thereof

Abstract

An asymmetric PP hollow-fiber porous membrane for degassing, and a preparation method therefor and the use thereof. The asymmetric PP hollow-fiber porous membrane comprises a main body, wherein a non-directional zigzag channel is arranged in the main body. The average pore diameter in SEM of inner degassing pores is 30-150 nm, and the pore area rate of the inner surface is 10%-40%; the overall porosity of the porous membrane is 20%-65%; and the thickness of the porous membrane is 30-75 μm. A polymer forming the porous membrane at least comprises a hydrophobic additive and a PP, wherein the weight-average molecular weight of the PP is 100,000-600,000, and the molecular weight distribution thereof is 2-7; the hydrophobic additive only comprises two elements, i.e. carbon and hydrogen, and is a pentene substance; the weight-average molecular weight of the hydrophobic additive is 50,000-200,000; and the hydrophobic additive in the polymer forming the porous membrane accounts for 5%-30% of the total mass of the polymer.

Description

Asymmetric PP hollow fiber porous membrane for degassing, preparation method and application thereof Technical Field

[0001] The present invention relates to the technical field of membrane materials, in particular to an asymmetric PP hollow fiber porous membrane for degassing, a preparation method and application thereof. Background Art

[0002] Degassing refers to the process of removing gas from liquids or solids. Currently, commonly used degassing methods include vacuum degassing, membrane degassing, dissolution degassing, and gas displacement degassing. Membrane degassing utilizes the selective permeability of special membrane materials to separate gases (such as oxygen, carbon dioxide, nitrogen, and ammonia) from liquids or solids. Hollow fiber membranes refer to membrane fibers that are fibrous in appearance, self-supporting, and have a hollow inner cavity. Compared with other separation membranes (such as flat membranes), hollow fiber membranes have a smaller diameter and can be tightly arranged in the device, resulting in a larger packing density per unit volume and a larger surface area (for gas exchange). Therefore, hollow fiber membranes are widely used in various degassing fields such as ultrapure water degassing, ink degassing, and wastewater deammoniation.

[0003] Common materials for hollow fiber membranes include poly (4-methylpentene) (PMP) and polypropylene (PP). Membranes made from PMP offer better resistance than PP membranes. However, PMP is less crystalline than PP, so PP membranes generally have higher porosity and air permeability. For example, Chinese patent application CN111346519B, entitled "A Method for Preparing Asymmetric Polyolefin Membranes" (applied by Hangzhou Kebet Filter Equipment Co., Ltd.), discloses hollow fiber membranes made from PP or PMP. These membranes exhibit excellent tensile strength and corrosion resistance, making them suitable for degassing applications such as inks, photoresists, and electroplating solutions.

[0004] In degassing applications such as inks and electroplating solutions, the outer surface of the hollow fiber membrane is usually in long-term contact with liquids such as inks and electroplating solutions, while the inner surface is usually used as the vacuum side for degassing. The driving force for degassing is the pressure difference on both sides of the hollow fiber membrane; however, in some degassing applications where the inner surface of the hollow fiber membrane needs to be in long-term contact with acidic or alkaline liquids, the inner surface is usually in contact with acidic or alkaline absorption liquids. The liquid to be removed moves to the inner surface under the action of the partial pressure difference and gas concentration on both sides and combines with the absorption liquid to achieve removal. For example, in wastewater deammonification applications, the outer surface of the hollow fiber membrane is the ammonia-nitrogen wastewater to be treated, and the inner surface is the acidic absorption liquid (the absorption liquid is dilute sulfuric acid). NH4 +In an alkaline environment, it is converted into free NH3, which diffuses into the interior of the hollow fiber membrane through the concentration boundary layer. Driven by the NH3 partial pressure difference on both sides of the hollow fiber membrane, NH3 moves to the inside of the hollow fiber membrane and combines with dilute sulfuric acid, thereby achieving the removal of NH3. The driving force of degassing is closely related to the NH3 concentration difference on both sides of the hollow fiber membrane. Therefore, the hollow fiber membrane is usually required to have a high NH3 permeability so that NH3 can quickly combine with the acid solution after passing through the hollow fiber membrane to ensure the continuation of the deamination process. Although PMP membrane fibers have good tolerance, their porosity is generally low, resulting in a low permeability of PMP membrane fibers for gases to be removed, such as NH3. As a result, PMP membrane fibers cannot meet the requirements of degassing applications such as wastewater deamination, where the inner surface of the hollow fiber membrane needs to be in long-term contact with acidic, alkaline or other liquids.

[0005] Chinese patent publication number CN103551046A, entitled "A Method for Preparing a Hydrophobic Ammonia-Nitrogen Removal Membrane" (applied by Tianjin Fengyun Water Resources Technology Co., Ltd.), discloses a hollow fiber membrane made of polypropylene, prepared by a melt-stretching method. The hollow fiber membrane has a wall thickness of 65-75 μm, a pore size of 0.3-0.5 μm, and a porosity of 65%-85%. The membrane fibers prepared from polypropylene exhibit a certain degree of hydrophobicity. Although PP membrane fibers have a high NH3 permeability and can better meet the needs of wastewater deammonification applications, their resistance is relatively poor compared to that of polypropylene polymer (PMP). In wastewater deammonification applications, both the inner and outer surfaces of the hollow fiber membrane are in contact with acidic or alkaline liquids for a long time. Therefore, in wastewater deammonification applications, both the inner and outer surfaces of the hollow fiber membrane are susceptible to erosion by the liquid to be degassed or the acidic or alkaline liquid. However, those skilled in the art often focus more on the resistance of the outer surface and ignore the resistance of the inner surface. Normally, in order to ensure the degassing rate during degassing, the inner surface pore size of the PP membrane filament is usually larger and the porosity is usually higher. For example, the pore size of the PP membrane filament in the above-mentioned patent is 300-500nm, and its porosity can reach 65%-85%. However, such PP membrane filaments are prone to inner surface erosion in wastewater deamination applications, resulting in poor long-term tolerance of the membrane filament, which ultimately leads to a reduction in the service life of the PP membrane filament in the field of wastewater deamination. Therefore, there is an urgent need for a hollow fiber membrane with a high degassing rate, good long-term tolerance and suitable for application in the field of wastewater deamination. Summary of the Invention

[0006] The purpose of the present invention is to provide an asymmetric PP hollow fiber porous membrane for degassing, a preparation method and application thereof, and the PP material is hydrophobized by using a hydrophobic additive with an appropriate weight-average molecular weight and proportion, aiming to obtain a hollow fiber porous membrane with a high degassing rate, good long-term tolerance and suitable for application in the field of wastewater deamination.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] An asymmetric PP hollow fiber porous membrane for degassing, comprising a main body, one side of the main body being an inner surface facing an inner cavity, and the other side being an outer surface, the main body having a non-directional tortuous channel, the inner surface having internal degassing pores, the pore area ratio of the outer surface being no greater than 5%, and the pore area ratio of the outer surface being less than the pore area ratio of the inner surface;

[0009] The SEM average pore diameter of the inner degassing pores is 30-150 nm, and the pore area ratio of the inner surface is 10%-40%;

[0010] The overall porosity of the porous membrane is 20%-65%; the thickness of the porous membrane is 30-75 μm;

[0011] The polymer constituting the porous membrane includes at least a hydrophobic additive and PP, wherein the weight average molecular weight of the PP is 100,000 to 600,000 and the molecular weight distribution is 2 to 7;

[0012] The hydrophobic additive only contains two elements, carbon and hydrogen, and is a pentene-type substance; the weight average molecular weight of the hydrophobic additive is 50,000-200,000;

[0013] The hydrophobic additive in the polymer constituting the porous membrane accounts for 5% to 30% of the total mass of the polymer.

[0014] As we all know, in various degassing application fields (such as ultrapure water, ink and electroplating liquid degassing fields), the current tolerance of hollow fiber membranes is mainly reflected in the structure and performance related to the outer surface of the hollow fiber membrane. The reason is that in degassing fields such as ultrapure water, ink and electroplating liquid, the outer surface of the hollow fiber membrane is the main side in contact with the liquid to be removed, while the inner surface of the hollow fiber membrane is usually used as the vacuum side. Therefore, under normal circumstances, when technical personnel in this field consider the overall tolerance of the hollow fiber membrane, they usually mainly consider the tolerance of the outer surface of the hollow fiber membrane, and generally rarely consider the tolerance of the inner surface of the hollow fiber membrane.

[0015] Unlike the common degassing applications mentioned above, in the wastewater deamination application field, the outer surface of the hollow fiber membrane is the ammonia-nitrogen wastewater to be treated, and the inner surface is the acidic absorption liquid (the absorption liquid is dilute sulfuric acid). That is, during the degassing process of the ammonia-nitrogen wastewater, both the inner and outer surfaces of the hollow fiber membrane are in contact with alkaline and acidic liquids. At this time, the overall tolerance of the hollow fiber membrane depends on the relevant structure and performance of the inner and outer surfaces. It is well known that the performance of PP membrane filaments is relatively active compared to PMP membrane filaments, and the tolerance of PP membrane filaments is poorer than that of PMP membrane filaments. In addition, the crystallization performance of PP membrane filaments is relatively good. Therefore, when PP membrane filaments are prepared by the melt stretching method, the pore size and overall porosity of the inner surface of the PP membrane filaments are generally relatively large. When such PP membrane filaments are used in the wastewater deamination field, their inner surface will be more susceptible to erosion by solutions such as dilute sulfuric acid due to the pore size and overall porosity, resulting in a decrease in the overall tolerance of the PP membrane filaments.

[0016] As one of the innovations of the present invention, the present invention further improves the overall hydrophobicity of the hollow fiber porous membrane by adding a hydrophobic additive to the polymer constituting the hollow fiber porous membrane, which can reduce the surface energy of the hollow fiber porous membrane to a certain extent, making the hollow fiber porous membrane relatively hydrophobic. Therefore, in application fields such as wastewater deamination, the inner surface of the hollow porous membrane can have better resistance to erosion by solutions such as dilute sulfuric acid, thereby giving the inner surface of the hollow fiber porous membrane better tolerance.

[0017] However, under normal circumstances, when using conventional hydrophobic additives to hydrophobize PP materials, it is easy to cause the following problems: the compatibility of hydrophobic additives and PP materials is poor, which may cause the hydrophobic additives and PP materials to be unable to mix evenly, thereby failing to effectively improve the overall hydrophobic properties of the PP membrane; in addition, hydrophobic additives are usually non-crystalline substances, and their crystallization properties are relatively poor compared to PP materials. Therefore, after adding hydrophobic additives, the crystallization properties of PP materials will be affected to a certain extent, especially when using the melt stretching method to prepare PP membranes, PP materials The crystallization performance of the PP membrane is affected, which may lead to a decrease in the crystallinity of the PP membrane filaments, and ultimately affect the pore structure of the PP membrane filaments, that is, the porosity and pore size of the PP membrane filaments may be reduced, resulting in a decrease in the degassing rate and degassing efficiency of the PP membrane filaments. In severe cases, the strength of the PP membrane filaments may also decrease, affecting the application of the PP membrane filaments in degassing; in the wastewater deammonification application, we hope that the degassing rate of the hollow fiber membrane is higher, thereby ensuring that the NH3 generated on the wastewater side can quickly pass through the hollow fiber membrane and combine with dilute sulfuric acid, prompting a good concentration gradient difference to be maintained on both sides of the hollow fiber membrane.

[0018] As one of the key innovations of the present invention, the present invention selects a suitable hydrophobic additive and a suitable PP material, that is, uses a pentene-type substance containing only hydrocarbon elements as a hydrophobic additive to perform a hydrophobic treatment on the PP material. We surprisingly found that when the weight average molecular weight of the PP material is controlled within the range of 100,000-600,000, the molecular weight distribution is controlled within the range of 2-7, and the weight average molecular weight of the hydrophobic additive is controlled within the range of 50,000-200,000, combined with the combined effect of the appropriate added mass ratio of the hydrophobic additive and the PP material (the hydrophobic additive accounts for 5%-30% of the total mass of the polymer, and the total mass of the polymer refers to the sum of the total mass of the hydrophobic additive and the PP material), the compatibility of the PP material and the hydrophobic additive is good (because the hydrophobic additive contains only hydrocarbon elements and is a pentene-type substance, and the PP material also contains only carbon). Hydrogen and two elements and both are olefin substances, that is, the properties of the two are similar, so that the hydrophobic additive and the PP material can be well blended during the melting process), so that the hydrophobic additive can be mixed with the PP material more evenly, which can significantly improve the hydrophobic properties of the inner and outer surfaces of the PP membrane; at the same time, under such a suitable ratio of hydrophobic additive and PP material, we found that the crystallinity of the PP membrane did not decrease significantly as expected, but was still at a better level. Then, when the membrane was stretched into holes, the appropriate membrane pore size and then the ideal membrane pore structure could be obtained, and the PP membrane with a relatively suitable overall porosity (20%-65%) and a suitable inner surface pore structure (the SEM average pore diameter of the inner degassing pores is 30-150nm, and the pore area rate of the inner surface is 10%-40%) can still be formed.

[0019] It is generally believed that the degassing rate and tolerance of hollow fiber membranes are relatively incompatible. This is because, when the degassing rate of the hollow fiber membrane is high, it often means that the pore size and porosity of the hollow fiber membrane are high, so that the hollow fiber membrane can have a higher gas exchange area and more gas flow channels during the degassing process, which enables the gas to be removed to penetrate through the hollow fiber membrane relatively quickly, which is reflected in the hollow fiber membrane having a higher degassing rate; and the tolerance of the hollow fiber membrane considers the ability of the hollow fiber membrane to withstand erosion by corrosive liquids such as the liquid to be removed. When the pore size and porosity of the hollow fiber membrane are maintained at a high level, the corrosive liquid such as the liquid to be removed will relatively easily enter the interior of the hollow fiber membrane through the pore size and pore structure of the hollow fiber membrane and erode it, resulting in insufficient tolerance of the hollow fiber membrane with such pore size and porosity.

[0020] However, we were pleasantly surprised to find that when a hydrophobic additive with a suitable weight-average molecular weight and proportion is used to modify the PP material, the PP membrane can have a suitable pore size (the SEM average pore size of the internal degassing pores is 30-150nm), a pore area ratio of the inner surface (10%-40%) and a suitable overall porosity (20%-65%). Under the action of such a suitable pore size, inner surface pore area and suitable overall porosity, the hollow fiber membrane of the present invention can have a higher degassing rate and degassing efficiency; at the same time, the hollow fiber membrane with such a membrane pore structure can have a high degassing rate and degassing efficiency under the action of a suitable hydrophobic additive, combined with the surface Under the combined effect of the pore area of ​​the surface being no more than 5%, the overall thickness of the appropriate porous membrane (30-75 μm) and the non-directional tortuous path, the inner and outer surfaces of the hollow fiber membrane of the present invention have high tolerance, which is reflected in the fact that the hollow fiber porous membrane of the present invention has good long-term tolerance while also having a high degassing rate and degassing efficiency, which can better meet the needs of the application fields where the inner and outer surfaces of the hollow fiber membrane need to be in contact with active liquids such as acid and alkaline for a long time during degassing applications such as wastewater (such as landfill leachate, industrial wastewater and wastewater from the chemical industry) deamination and carbon dioxide capture, and can generate high economic value. In addition, after research, it was found that the hollow fiber membrane of the present invention has a good effect in the application of some degassing fields where water vapor is easily formed and condensed to form a liquid film, can significantly alleviate the phenomenon of liquid film formation in the hollow fiber membrane, and can better reduce the loss of water vapor condensed to form a liquid film. For example, in the application field of ultrapure water, it can significantly reduce the loss of ultrapure water in the degassing process and improve the utilization rate of ultrapure water.

[0021] The non-directional tortuous pathways in the present invention refer to randomly oriented groove structures and / or discretely distributed pore structures, and each non-directional tortuous pathway is interconnected; and the fibers forming the porous structure of the membrane are continuous. It can be understood that "continuous" means that basically all fibers are connected to each other as a whole and are formed as one body without the need to use additional adhesives to connect them to each other. Unless torn by external force, the network-like fibers cannot be separated from each other, that is, the porous membrane of the present invention is a single-layer membrane structure, not a composite membrane structure.

[0022] The pore area ratio of the outer surface is no more than 5% in the present invention means that when the outer surface of the hollow fiber membrane is observed by SEM electron microscope at a magnification of 50,000 times, no pore structure can be observed or only a very small number of pore structures can be observed.

[0023] The pore size, pore area ratio and other characteristics of the inner surface of the hollow fiber membrane of the present invention can be characterized by using a scanning electron microscope to characterize the membrane structure, and then measured using computer software (such as Matlab, NIS-Elements, etc.) or manually, and corresponding calculations can be performed; in the preparation process of the membrane, in the direction perpendicular to the membrane thickness (if the membrane is in the form of a flat membrane, this direction is the planar direction; if the membrane is in the form of a hollow fiber membrane, this direction is perpendicular to the radial direction), its various characteristic membrane pore sizes and membrane pore distribution are roughly uniform and basically consistent; so the pore area ratio on the plane can be reflected by the size of the membrane pore area ratio of a part of the area on the corresponding plane; when actually measuring, the inner surface of the membrane filament can be characterized by an electron microscope to obtain a corresponding SEM image, and since the membrane pore size and membrane pore distribution on the inner surface of the membrane are roughly uniform, a certain area can be selected, such as 1μm 2 (1 μm multiplied by 1 μm) or 100 μm 2 (10 μm multiplied by 10 μm) or, the specific area size depends on the actual situation, and then use the corresponding computer software or manually measure the pore area ratio and pore size on the area, conduct several tests, and take the average value to obtain the pore area ratio and pore size on the inner surface of the membrane; of course, those skilled in the art can also obtain the above parameters by other measurement methods, and the above measurement methods are for reference only).

[0024] The internal degassing pores in the present invention may have two morphologies when observed through SEM electron microscopy, one is a relatively regular circular hole, and the other is an elliptical hole. The SEM average pore size of the internal degassing pores claimed in the present invention specifically refers to the diameter length of the relatively regular circular hole and the length in the short diameter direction of the elliptical hole.

[0025] The thickness of the hollow fiber membrane in the present invention can be determined by characterizing the cross-section of the hollow fiber membrane using a SEM electron microscope, then measuring using computer software (such as Matlab, NIS-Elements, etc.) or manually, and performing corresponding calculations to obtain the thickness of the hollow fiber membrane.

[0026] The weight average molecular weight of PP and the hydrophobic additive in the present invention refers to the statistical average molecular weight by mass, which is the molecular weight averaged over a unit weight. The weight average molecular weight can be obtained by GPC (gel permeation chromatography) measurement.

[0027] Common porosity testing methods include mercury intrusion method, density method and dry and wet membrane weighing method. The overall porosity of the hollow fiber membrane of the present invention can be obtained by any of the above testing methods.

[0028] Furthermore, the infrared absorption spectra of the inner and outer surfaces of the porous membrane are tested by the ATR method, and the hydrophobic additive doping coefficient X of the porous membrane is calculated;

[0029] X=ABS 720 / ABS 1460 ;

[0030] ABS 720 For porous membranes, the wave number is 720 cm -1 the absorption intensity nearby;

[0031] ABS 1460 For porous membranes, the wave number is 1460 cm -1 the absorption intensity nearby;

[0032] Hydrophobic additive doping coefficient X 内 =ABS 内表面720 / ABS 内表面1460 ;

[0033] where X 内 The value range is 0.0005-0.15; preferably 0.001-0.08;

[0034] ABS 内表面720 The outer surface of the porous membrane is at a wave number of 720 cm -1 the absorption intensity nearby;

[0035] ABS 内表面1460 The outer surface of the porous membrane is at a wave number of 1460 cm -1 The absorption intensity near

[0036] The present invention selects the same hollow fiber membrane to perform infrared spectrum test using a Fourier infrared spectrometer, including but not limited to, and controls the test pressure, selects the area of ​​the hollow fiber membrane test and other factors to be consistent. In the infrared spectrum obtained by the test, after eliminating errors (such as baseline calibration, etc.), the wave number is 720cm -1 Nearby (usually 710-730cm -1 The absorption intensity of ) (the present invention uses peak area to characterize the absorption intensity, which is more accurate and objective) is the absorption characteristic peak used to characterize pentene. Its value is related to the amount of hydrophobic additive (the hydrophobic additive is a pentene-like substance containing only hydrocarbons). Generally, the higher the amount of hydrophobic additive, the greater the absorption intensity (peak area) at this location. In order to ensure that the doping coefficient of the hydrophobic additive can be reflected more objectively, the wave number of 1460 cm is used in the present invention. -1 Nearby (usually 1450-1470cm -1 ) is used as the standard peak, which is due to the wave number of 1460 cm -1 The peak near the wave number is the bending vibration peak used to characterize the CH structure. Its value is relatively stable and changes little. Finally, after research, it was determined that the wave number was 720cm-1 The absorption intensity (peak area) near the wave number is 1460 cm -1 The ratio of the absorption intensities (peak areas) near the inner and outer surfaces can be used to more accurately reflect the distribution degree of the hydrophobic additive on different surfaces of the hollow fiber membrane, that is, the doping coefficient of the hydrophobic additive on the inner and outer surfaces.

[0037] Since the inner surface of some hollow fiber membranes needs to be in contact with active liquids such as acidic and alkaline for a long time and in the field of degassing (such as wastewater deamination and carbon dioxide capture), usually in order to ensure the degassing rate and degassing efficiency of the hollow fiber membrane, the pore structure of the inner surface is relatively more than that of the outer surface, which will cause the pore structure of the inner surface to be more easily corroded by active liquids such as acidic and alkaline, thereby causing the tolerance of the hollow fiber membrane to deteriorate, thereby affecting the service life of the hollow fiber membrane in these application fields and greatly reducing the economic value of the hollow fiber membrane. The present invention controls the doping coefficient X of the hydrophobic additive on the inner surface. 内 Within the range of 0.0005-0.15, preferably 0.001-0.08, it can reflect the distribution degree of the hydrophobic additive on the inner surface of the hollow fiber membrane to a certain extent, that is, it reflects the hydrophobic degree of the inner surface of the hollow fiber membrane, and embodies that the inner surface of the hollow fiber membrane has good hydrophobic properties. On the one hand, it can make the inner surface of the hollow fiber membrane have better resistance to erosion and breakthrough of active liquids such as acid and alkaline liquids; on the other hand, it can also make the inner surface of the hollow fiber membrane have better resistance to the formation of liquid film, thereby ensuring the long-term tolerance of the hollow fiber membrane in the above-mentioned application fields, improving the service life of the hollow fiber membrane, and thus generating higher economic value.

[0038] If X 内 If the value is too large, it means that the distribution of the hydrophobic additive on the inner surface of the hollow fiber membrane is relatively high. Since the hydrophobic additive is usually a non-crystalline substance, it may cause some pore structures on the inner surface of the hollow fiber membrane to be affected to a certain extent, which may have a certain impact on the stability of the inner surface of the hollow fiber membrane during degassing, and may cause a certain impact on the degassing efficiency of the hollow fiber membrane; although the overall addition ratio of the hydrophobic additive is in a relatively appropriate range, when X 内 When it is too large, it means that the distribution of the hydrophobic additive in the hollow fiber membrane is relatively uneven, which may also affect the tensile strength and elongation at break of the hollow fiber membrane. 内If it is too small, it means that the distribution degree of the hydrophobic additive on the inner surface of the hollow fiber membrane is relatively low. Although the inner surface of the hollow fiber membrane still has good hydrophobic properties and tolerance, after being corroded for a long time by active solutions such as acid and alkaline, the stability and compressive strength of the pore structure on the inner surface of the hollow fiber membrane during degassing may be affected and decreased to a certain extent, which may cause a certain impact on the degassing efficiency of the hollow fiber membrane.

[0039] Furthermore, the hydrophobic additive doping coefficient X 外 =ABS 外表面720 / ABS 外表面1460 ;

[0040] where X 外 The value range is 0.001-0.3; preferably 0.005-0.1;

[0041] ABS 外表面720 The inner surface of the porous membrane at a wave number of 720 cm -1 the absorption intensity nearby;

[0042] ABS 外表面1460 The inner surface of the porous membrane at a wave number of 1460 cm -1 The absorption intensity near

[0043] As is known to all, the outer surface of the hollow fiber membrane is usually the side that is in long-term contact with various liquids to be degassed, and it is also the side that those skilled in the art are currently more concerned about in terms of tolerance. In addition, in the field of wastewater deamination applications, the ammonia-nitrogen-containing wastewater that the outer surface is in long-term contact with is compared to the dilute sulfuric acid that the inner surface is in long-term contact with. The erosion effect of ammonia-nitrogen-containing wastewater on the outer surface is usually greater than the erosion effect of dilute sulfuric acid and other solutions on the inner surface. After research, it was found that the doping coefficient of the hydrophobic additive on the outer surface can be appropriately increased, which can better make the outer surface have excellent hydrophobic properties, thereby improving the long-term tolerance of the hollow fiber membrane. However, it should be noted that the doping coefficient of the hydrophobic additive on the outer surface cannot be too high. The reason is that once the doping coefficient of the hydrophobic additive is too large, it means that the distribution degree of the hydrophobic additive on the outer surface is high. Since the hydrophobic additive is a non-crystalline substance, it may cause the compressive strength of the outer surface of the hollow fiber membrane and the stability during degassing to be reduced, and it will also have a certain impact on the tensile strength and elongation at break of the hollow fiber membrane as a whole. Therefore, the present invention controls X 外 In the range of 0.001-0.3, preferably 0.005-0.1, the study found that after the outer surface of the hollow fiber membrane has a suitable hydrophobic additive doping coefficient, the tolerance of the membrane fiber is significantly improved. At the same time, after being subjected to long-term erosion by the liquid to be removed, the overall degassing efficiency, compressive strength, tensile strength and elongation at break of the membrane fiber can still maintain a good level.

[0044] Furthermore, the porous membrane was tested by XRD method, and the XRD crystallinity of the porous membrane was 60%-90%; the overall porosity of the porous membrane was 25%-60%, and the thickness of the porous membrane was 40-65 μm.

[0045] The present invention tests the crystallinity of the porous membrane by the XRD method. We are pleasantly surprised to find that by selecting a hydrophobic additive containing only hydrocarbon pentene substances, although the hydrophobic additive is usually used as a non-crystalline substance and polypropylene (PP) is used as a crystalline substance, the XRD crystallinity of the final porous membrane did not decrease significantly as expected, but was still at a better level (60%-90%). To a certain extent, it also reflects that the present invention greatly reduces the influence of the addition of amorphous substances on the crystallization properties of the porous membrane by selecting a suitable hydrophobic additive. At the same time, under the effect of the suitable XRD crystallinity of the present invention, the hollow fiber porous membrane has good degassing stability and mechanical strength. Combined with the porous membrane of the present invention having a suitable overall porosity (25%-60%) and thickness (40-65μm), the hollow fiber porous membrane is given a higher degassing rate and degassing efficiency. In addition, under the effect of such overall porosity and thickness of the porous membrane, combined with the hydrophobic treatment of the hollow fiber porous membrane using hydrophobic additives, the hollow fiber porous membrane can have excellent tolerance in degassing applications such as wastewater deamination and carbon dioxide capture, where both the inner and outer surfaces of the hollow fiber membrane need to be in contact with the liquid to be degassed or acidic or alkaline active liquids.

[0046] The XRD crystallinity of the porous membrane in the present invention refers to the XRD diffraction pattern of the porous membrane obtained by X-ray diffraction, and the diffraction peak area representing the crystalline form and the sum of the diffraction peak area representing the crystalline form and the diffraction peak area of ​​the amorphous form are calculated respectively. The XRD crystallinity value of the porous membrane is obtained by the diffraction peak area of ​​the crystalline form: the sum of the diffraction peak area of ​​the crystalline form and the diffraction peak area of ​​the amorphous form.

[0047] Furthermore, the half-peak width of the porous membrane at a 2θ angle of about 14.1° is 0.2-0.7, and the grain size is not less than 150 angstroms.

[0048] The present invention tests the porous membrane by the XRD method and obtains the X-ray diffraction spectrum of the porous membrane. The diffraction peak at a 2θ angle of 14.1° contributes the most to the overall spectrum. Therefore, the performance related to the diffraction peak at a 2θ angle of 14.1° is used to characterize the quality of the crystallization-related performance of the porous membrane. The half-peak width refers to the half-peak width of each diffraction peak in the XRD diffraction spectrum, which is usually expressed as an angle or 2θ value. The half-peak width can be used to measure the change in lattice parameters in the crystal structure. Generally, the smaller the half-peak width of the diffraction peak, the more ordered the arrangement of atoms in the crystal and the fewer crystal defects; the larger the half-peak width, the more disordered the arrangement of atoms in the crystal and the more crystal defects. Therefore, the half-peak width in the XRD spectrum can be used to evaluate the quality and degree of defects of the crystal. The half-maximum width of the diffraction peak at a 2θ angle of 14.1° in the present invention is within a suitable numerical range (0.2-0.7), and the grain size is not less than 150 angstroms, which means that the more ordered the atomic arrangement and the fewer crystal defects in the porous membrane crystal of the present invention, the easier it is to form pores during the stretching process and the more stable the formed pore structure, so that the porous membrane can have better degassing stability during degassing; at the same time, the more ordered the atomic arrangement and the fewer crystal defects in the crystal, the better the mechanical strength and heat resistance of the porous membrane; in addition, the more ordered the atomic arrangement and the fewer crystal defects in the crystal, the hollow fiber membrane may still maintain good degassing stability and mechanical strength after being subjected to long-term erosion by the liquid to be degassed and / or active liquids such as acidic and alkaline liquids, so that the hollow fiber membrane can still have a higher degassing rate and degassing efficiency.

[0049] Furthermore, the water contact angle of the inner surface is 95°-120°, and the roughness Ra of the inner surface is 20-60 μm; and / or;

[0050] The water contact angle of the outer surface is 100°-130°, and the roughness Ra of the outer surface is 15-70 μm.

[0051] The size of the water contact angle depends on the properties of the material itself and the properties of the material surface. The water contact angle of the outer surface of the degassing membrane of the present invention is 100°-130°. Under normal circumstances, the larger the water contact angle, the better the hydrophobicity of the substance, which means that the outer surface of the degassing membrane of the present invention has good hydrophobicity. The degassing membrane can have good tolerance and service life when used in various applications of degassing active liquids such as acidic and alkaline liquids; at the same time, the outer surface has a relatively suitable roughness (15-70μm), and the roughness can reflect the degree of contact between the outer surface and the gas to be removed to a certain extent, so that the hollow fiber membrane has a suitable gas permeability rate, and ultimately the hollow fiber membrane has a high degassing rate and degassing efficiency while having good tolerance. In addition, due to the presence of a relatively suitable surface roughness and a relatively hydrophobic outer surface, water vapor will encounter greater resistance when penetrating into the hollow fiber membrane through the outer surface, thereby being able to better curb the water vapor permeation rate, so that the hollow fiber membrane of the present invention can significantly reduce the loss of water vapor evaporation and condensation when used in degassing fields where water vapor is easily formed and condensed to form a liquid film.

[0052] The water contact angle of the inner surface of the hollow fiber membrane of the present invention is 95°-120°, and the roughness of the inner surface is controlled to be within the range of 20-60 μm, so that the inner surface of the hollow fiber membrane also has good hydrophobicity, so that when the hollow fiber membrane is applied to some hollow fiber membranes, the inner surface needs to be in contact with active liquids such as acidic and alkaline for a long time and degassed (such as wastewater deammonification, carbon dioxide capture and other fields), and when it is applied to some degassing fields where water vapor is easily formed and condensed to form a liquid film, the inner surface of the hollow fiber membrane can have good tolerance and can better inhibit the formation of the liquid film, thereby reducing the amount of water vapor evaporation and condensation loss. Wherein, when carbon dioxide capture is applied, the inner surface of the hollow fiber membrane is usually an alkali solution for reacting with carbon dioxide and capturing it.

[0053] The water contact angles of the inner and outer surfaces of the hollow fiber membrane of the present invention are measured and obtained by Krues Scientific Instruments (Shanghai) Co., Ltd. using the William dynamic contact angle method using a K100 surface tension meter. When testing the water contact angle, it is necessary to use tools such as glass slides to flatten the degassing membrane filaments and form thin sheets, and then perform water contact angle tests on the inner and outer surfaces.

[0054] The roughness of the inner and outer surfaces of the hollow fiber membrane of the present invention is measured and obtained using a Keyence 3D scanning microscope.

[0055] Furthermore, some adjacent inner degassing pores are separated by supporting fibers, the SEM average length of the supporting fibers is 50-500 nm, and the ratio of the SEM average length of the supporting fibers to the SEM average width of the supporting fibers is 1.5-15:1;

[0056] The length direction of the supporting fibers is consistent with the circumferential direction of the porous membrane, and the width direction of the supporting fibers is consistent with the length direction of the porous membrane.

[0057] In some degassing applications such as ink, electroplating solution and ultrapure water, the inner surface of the hollow fiber membrane will be subjected to relatively large gas pressure during the degassing process. The present invention forms support fibers between some adjacent inner degassing pores and controls the SEM average length of the support fibers to be 50-500 nm and the ratio of the SEM average length to the SEM average width of the support fibers to be within the range of 1.5-15:1, thereby enhancing the stability of the inner degassing pores during degassing, so that the inner surface has better compressive strength and stability during degassing; at the same time, the support fibers with a suitable length-to-width ratio can support and separate the inner degassing pores, on the one hand, making the inner degassing area more stable during degassing, and on the other hand, combined with the joint action of the hydrophobic additive, it can also hinder and resist the erosion of the inner degassing pore structure by active liquids such as acidic and alkaline liquids that the inner surface contacts, thereby improving the tolerance of the hollow fiber membrane.

[0058] If the ratio of the SEM average length to the SEM average width of the support fibers is too large, the support fibers may become long and thin, failing to effectively improve the compressive strength and degassing stability of the inner surface. The fibers may even break due to gas pressure or be corroded and broken by reactive liquids such as acids and alkalis, thereby affecting the compressive strength and degassing stability of the hollow fiber membrane inner surface. More seriously, if the support fibers break, adjacent internal degassing pores may merge, resulting in excessively large pore diameters, which may also affect the tolerance of the hollow fiber membrane inner surface. If the ratio of the SEM average length to the SEM average width of the support fibers is too small, meaning that the SEM average length of the support fibers is too small, the presence of the support fibers may affect the permeation rate of gases such as NH3 in wastewater deamination, O2 in ultrapure water applications, and CO2 in carbon dioxide capture, thereby affecting the degassing rate and degassing efficiency of the hollow fiber membrane and preventing efficient and rapid degassing.

[0059] Furthermore, the inner degassing pores are stacked and arranged to form an inner degassing zone, and some adjacent inner degassing zones are separated by a dense zone, the SEM average width of the inner degassing zone is 80-1200 nm, and the SEM average length of the inner degassing zone is at least 1.5 times the SEM average width of the inner degassing zone;

[0060] The length direction of the inner degassing zone is consistent with the circumferential direction of the porous membrane, and the width direction of the inner degassing zone is consistent with the length direction of the porous membrane.

[0061] In the present invention, the degassing pores on the inner surface are not evenly distributed, but are relatively concentrated in a part of the inner surface, which is called the inner degassing zone; that is, there are several inner degassing zones on the inner surface, and the inner degassing pores in these degassing zones are stacked and arranged with each other. At the same time, under the joint action of the inner degassing pores having a suitable pore size (30-150nm), it is ensured that the inner degassing zone has a suitable pore area ratio, thereby ensuring that the hollow fiber membrane as a whole has a high degassing rate, which is convenient for efficient degassing, and is particularly suitable for applications such as wastewater deamination, which requires NH3 to pass through the hollow fiber membrane quickly and combine with dilute sulfuric acid, and other applications that require a high degassing rate; the inner degassing zone is an area on the inner surface of the hollow fiber membrane with a relatively high pore area ratio, which needs to have a certain area (if its area is too small, it will affect the degassing rate of the hollow fiber membrane and then affect the degassing efficiency; if its area is too large, it will affect the compressive strength of the inner surface of the hollow fiber membrane, and may even affect The tensile strength and elongation at break of the hollow fiber membrane affect the tensile strength and elongation at break of the hollow fiber membrane, thereby affecting the stability of the internal degassing pores during degassing). After regulation, the SEM average width of the internal degassing zone is controlled to be 80-1200 nm, and the SEM average length of the internal degassing zone is at least 1.5 times the SEM average width of the internal degassing zone. At this time, the area occupied by the internal degassing zone is relatively appropriate, which can not only ensure that the inner surface of the hollow fiber membrane has a high degassing rate and thus a high degassing efficiency, but also has little effect on the compressive strength of the inner surface of the hollow fiber membrane, so that the internal degassing pores can degas efficiently for a long time; in addition, under the combined effect of the appropriate area of ​​the internal degassing zone and the appropriate pore size of the internal degassing pores, combined with the improvement of the hydrophobicity of the inner surface brought about by the hydrophobic additive, the inner surface of the hollow fiber membrane has better resistance to erosion by active liquids such as acid and alkaline, and the inner surface can also better inhibit the formation of a liquid film after water vapor condensation.

[0062] Furthermore, the inner degassing pores include a plurality of elliptical pores, the SEM average major diameter of the pores is 100-600 nm, the ratio of the SEM average major diameter of the pores to the SEM average pore diameter of the pores is 1.5-10:1, and the ratio of the SEM average pore diameter of the pores to the SEM average width of the supporting fibers is 1.5-12:1;

[0063] The long diameter direction of the air pores is consistent with the length direction of the porous membrane.

[0064] The internal degassing holes in the present invention also include a number of elliptical air holes. Compared with the internal degassing holes in the shape of circular holes, the area of ​​the elliptical air holes will be larger than the area of ​​the internal degassing holes in the shape of circular holes, under the premise that the short diameter length of the elliptical air holes is the same as the diameter of the circular holes. Therefore, when the pore size of the internal degassing holes is relatively small, the elliptical air holes can still better pass the gas to be removed, so that the hollow fiber membrane has a better degassing rate; at the same time, the tolerance of the hollow fiber membrane will also be enhanced to a certain extent as the internal degassing holes are reduced. The SEM average major diameter of the air pores in the present invention is controlled within the range of 100-600 nm. Combined with the internal degassing pores with suitable pore size and suitable pore area ratio on the inner surface and the hydrophobic additive, the hollow fiber membrane can have a higher degassing rate and degassing efficiency. At the same time, the hollow fiber membrane can also have good tolerance and can be well applied to some fields where the inner surface of the hollow fiber membrane needs to be in contact with active liquids such as acid and alkaline for a long time and degassed (such as wastewater deammonification, carbon dioxide capture and other fields), and can also be applied to some degassing fields where water vapor is easily formed and condensed to form a liquid film.

[0065] At the same time, the ratio of the SEM average long diameter of the pores to the SEM average pore diameter of the pores of the present invention (the SEM average pore diameter of the pores here is the SEM average short diameter of the pores, which is defined as the ratio of the long and short diameters of the pores) is controlled within the range of 1.5-10:1. Although the ventilation area of ​​the elliptical pores is effectively increased, the morphology of the pores is still relatively narrow and long, that is, the presence of the elliptical pores does not have much effect on the pressure resistance of the inner surface and the stability during degassing, that is, the inner surface of the degassing membrane still has good pressure resistance and degassing stability. By controlling the ratio of the SEM average pore diameter of the pores to the SEM average width of the supporting fibers within a suitable range, the present invention can further enhance the mechanical strength of the inner surface of the degassing membrane and improve the stability of the inner degassing pores during degassing.

[0066] Furthermore, the SEM average width of the dense region is 60-800 nm, and the SEM average width of the supporting fibers: the SEM average width of the dense region is 1:4-10;

[0067] The width direction of the dense area is consistent with the length direction of the porous membrane.

[0068] The dense area can reflect the spacing between adjacent inner degassing zones in the length direction of the porous membrane to a certain extent, and the dense area is also the basis for the inner surface to have good tolerance; the SEM average width of the dense area of ​​the hollow fiber membrane of the present invention is controlled within the range of 60-800nm, reflecting that there is a more appropriate spacing between adjacent inner degassing zones on the inner surface, ensuring that the hollow fiber membrane has a faster degassing rate while avoiding the distribution between the inner degassing zones as much as possible, which leads to the influence of the inner surface compressive strength and stability of the hollow fiber membrane during degassing. In addition, the ratio of the SEM average width of the supporting fiber of the present invention to the SEM average width of the dense area is controlled within the range of 1:4-10. The supporting fiber, as part of the entity in the inner degassing zone, can also reflect the tolerance and erosion resistance of the inner degassing zone to a certain extent. After research, it was found that when the ratio of the two is controlled within the above range, the hollow fiber membrane can have better tolerance. At the same time, after being subjected to the erosion of active liquids such as acidic and alkaline liquids for a long time, the hollow fiber membrane can still maintain a high degassing efficiency and mechanical strength.

[0069] The dense area in the present invention refers to a dense area on the inner surface where no pore structure or only a very small number of pores can be observed when photographed by a scanning electron microscope at 50,000 times, and the pore area ratio of the dense area is less than 1%.

[0070] Furthermore, the main body includes a cortex and a support layer, one side of the support layer is the inner surface, one side of the cortex is the outer surface, and the other side of the cortex and the other side of the support layer are transitioned by continuous fibers; the support layer has porous fibers for forming a porous structure, the SEM average diameter of the porous fibers is 30-200nm, and the density of the porous fibers is 10-300 / 1μm 2 .

[0071] The hollow fiber membrane of the present invention is controlled by the skin layer and the support layer, combined with the appropriate diameter (30-200nm) and number (10-300 / 1μm 2)'s porous fibers, the hollow fiber membrane has better tolerance, and at the same time has a higher degassing rate and higher degassing efficiency. This is due to the relatively low porosity of the hollow fiber membrane skin layer. Combined with the effect of the hydrophobic additive, the resistance to the degassed liquid penetrating the skin layer is greatly increased, thus giving the hollow fiber membrane good tolerance and ensuring the service life of the degassing membrane in various degassing applications such as ultrapure water and wastewater ammonia removal. In addition, the porous fibers with such a large pore size and density in the support layer give the hollow fiber membrane good compressive strength and degassing stability of the membrane pore structure within the support layer. The combined effect of the porous fibers with such a large diameter and number reflects the presence of an appropriate number of non-directional tortuous pathways within the support layer, thereby achieving a good degassing rate for the degassing membrane. Although the porosity within the support layer is relatively large, the effect of the appropriate hydrophobic additive still slows the water vapor permeation rate within the support layer. This is due to the combined effect of the porous fibers (hydrophobicity) and the tortuous pathways within the hollow fiber membrane. Therefore, the hollow fiber membrane is well suited for degassing applications where water vapor is easily formed and condensed to form a liquid film.

[0072] The hollow fiber membrane body of the present invention includes a cortex and a support layer. Specifically, by performing SEM electron microscope observation on the cross-sectional structure of the hollow fiber membrane, it is found that the main structure of the hollow fiber membrane mainly includes two regions, wherein the cortex refers to a certain area (the area near the outer surface), and its membrane pore diameter is very small and the porosity is low; the support layer refers to a certain area (the area near the inner surface), and its membrane pore diameter is relatively large and the porosity is relatively large.

[0073] Furthermore, the SEM average diameter of the porous fibers varies in a gradient of 0.1-1.2 nm / μm;

[0074] The gradient of the change in the SEM average diameter of the porous fibers = (the SEM average diameter of the porous fibers close to the inner surface - the SEM average diameter of the porous fibers far from the inner surface) / the thickness of the support layer.

[0075] The diameter of the porous fibers in the support layer of the present invention is not constant. In some membrane filaments, the porous fibers near the inner surface will be relatively thicker, while the porous fibers away from the inner surface (close to the cortex) will be relatively thinner. In the present invention, the change in the diameter of the porous fibers in the membrane support layer is reflected by the gradient of the change in the SEM average diameter of the porous fibers. Therefore, this value is obtained by first subtracting the SEM average diameter of the porous fibers close to the inner surface from the SEM average diameter of the porous fibers away from the inner surface, and the difference is divided by the thickness of the support layer; the area away from the inner surface of the present invention refers to the area with the interface between the cortex and the support layer as the starting line, and the distance from the cross-section support layer to the starting line is 1 μm; the area close to the inner surface of the present invention refers to the area with the inner surface as the starting line, and the distance from the cross-section support layer to the starting line (inner surface) is 1 μm.

[0076] The limitation of the gradient of the average diameter change of the porous fibers in the present invention directly reflects that the SEM average diameter of the porous fibers tends to gradually increase along the thickness direction of the hollow fiber membrane (from the outer surface to the inner surface of the hollow fiber membrane). This is because both the inner and outer surfaces of the hollow fiber membrane need to have a certain compressive strength to maintain stability during degassing, and the relatively dense cortical structure near the outer surface makes the outer surface have better compressive strength and better tolerance. The porous fibers near the inner surface have a relatively large diameter, which can better support the inner surface. On the other hand, when the hollow fiber membrane is used in some fields where the inner surface of the hollow fiber membrane needs to be in contact with active liquids such as acid and alkaline for a long time and degassed (such as wastewater deammonification, carbon dioxide capture and other fields), the distribution of porous fibers with relatively large diameters near the inner surface and the combined effect of hydrophobic additives make the inner surface of the hollow fiber membrane have better tolerance. At the same time, when the inner surface is subjected to long-term erosion by active liquids such as acid and alkaline, the inner surface of the hollow fiber membrane can still have a good degassing rate and compressive strength.

[0077] Furthermore, the outer surface is a dense surface, the hole area ratio of the outer surface is less than 1%, the skin thickness is 0.1-3 μm, and the thickness of the support layer is at least 29 μm greater than the skin thickness.

[0078] The outer surface of the hollow fiber membrane of the present invention is a dense surface, and the pore area ratio of the outer surface is less than 1%. Combined with the skin thickness being controlled within the range of 0.1-3 μm, the outer surface of the hollow fiber membrane has good tolerance, so that the hollow fiber membrane can be used in various applications such as degassing of active solutions; at the same time, the outer surface of the hollow fiber membrane has a relatively low gas permeability, which makes it relatively difficult for water vapor to penetrate into the hollow fiber membrane under the action of the dense surface and hydrophobic additives when the hollow fiber membrane is undergoing a process such as ultrapure water degassing, thereby reducing the water vapor penetration into the hollow fiber membrane. The amount of water vapor in the hollow fiber membrane is greatly reduced, and the probability of water vapor penetrating into the hollow fiber membrane and condensing to form a liquid film is greatly reduced; on this basis, combined with the inner surface of the present invention having internal degassing holes with suitable pore size and suitable pore area ratio, and the support having porous fibers with suitable diameter and density (suitable support layer thickness) under the joint action, the degassing rate of the hollow fiber membrane is not significantly reduced due to the density of the outer surface, but the degassing rate of the hollow fiber membrane is still within a relatively suitable numerical range, and then the hollow fiber membrane still has a suitable degassing rate and a higher degassing efficiency.

[0079] The dense surface in the present invention specifically refers to the outer surface of the degassing membrane photographed at 50,000 times magnification by a scanning electron microscope, where no pore structure or only a very small number of pores can be observed, and the pore area ratio of the outer surface is less than 1%.

[0080] Furthermore, the outer surface has an outer degassing zone and an outer dense zone, the pore area ratio of the outer dense zone is less than 1%, the outer degassing zone has a plurality of outer degassing pores, the SEM average pore diameter of the outer degassing pores in the outer degassing zone is 10-90nm, and the density of the outer degassing pores is 10-80 / 1μm 2 .

[0081] The outer surface of the hollow fiber membrane of the present invention has an outer degassing zone and an outer dense zone. The outer degassing pores are concentrated in the outer degassing zone, that is, the outer degassing pores are unevenly distributed on the outer surface. Combined with the appropriate SEM average pore diameter (10-90nm) of the outer degassing pores in the outer degassing zone and the appropriate density of the outer degassing pores in the outer degassing zone (10-80 / 1μm 2), and at the same time, under the joint action of the appropriate internal degassing pore diameter (30-150nm) and the appropriate pore area ratio (10%-40%) on the inner surface of the hollow fiber membrane, the hollow fiber membrane has an extremely high degassing rate and degassing efficiency. This is probably because the concentrated distribution of the external degassing pores in the external degassing zone can make the airflow in the external degassing zone more concentrated, which makes the oxygen and other gas molecules more inclined to collision between molecules during penetration to a certain extent, which can reduce the resistance of oxygen and other gases to penetrate through the external degassing pores, and can reduce the influence of airflow scattering and diffusion, thereby increasing the permeation rate of gases such as NH3 and oxygen; at the same time, due to the existence of the external dense zone (relatively dense, with almost no pore structure) and the action of the hydrophobic additive, the outer surface of the hollow fiber membrane has better compressive strength and better tolerance. In short, under the combined effects of the outer surface membrane pore structure, inner surface membrane pore structure and hydrophobic additives, the hollow fiber membrane has an extremely high degassing rate and degassing efficiency, while also having good compressive strength and tolerance. It is suitable for use in degassing fields such as wastewater deamination, where NH3 (gas to be removed) needs to be removed quickly and both the inner and outer surfaces of the hollow fiber membrane need to have good tolerance.

[0082] The outer dense zone and the outer degassing zone in the present invention specifically refer to the observation of the outer surface of the hollow fiber membrane through SEM electron microscope, which can clearly observe that the outer surface of the hollow fiber membrane is divided into a pore structure area with a smaller pore size and a relatively dense area; wherein the pore area rate of the outer dense zone is less than 1%, specifically refers to the observation by scanning electron microscope at 50,000 times, in which no pore structure can be observed or a very small number of pores can be observed in the outer dense zone.

[0083] Furthermore, the outer surface has a plurality of evenly distributed external degassing pores, the SEM average pore diameter of the external degassing pores is 10-90nm, and the density of the external degassing pores is 1-30 / 10μm. 2 .

[0084] The outer surface of the hollow fiber membrane of the present invention has uniformly distributed external degassing pores. Compared with the outer surface of the hollow fiber membrane with unevenly distributed external degassing pores, the outer degassing pores with uniform distribution and appropriate pore size (10-90nm) combined with appropriate external degassing pore density (1-30 / 10μm) are more uniform. 2) and hydrophobic additives, the outer surface of the hollow fiber membrane can have a relatively lower water vapor permeation rate. This is because the density of the external degassing pores on the outer surface of the hollow fiber membrane is relatively low, which means that the spacing between adjacent external degassing pores is relatively large. Combined with the relatively small pore size of the external degassing pores (10-90nm) and the combined effect of the hydrophobic additives, the rate at which water vapor enters the hollow fiber membrane through the external degassing pores is greatly reduced, thereby making the hollow fiber membrane have a high degassing rate and high degassing efficiency while having good tolerance. At the same time, it can also avoid water vapor from penetrating into the hollow fiber membrane and condensing to form a liquid film layer as much as possible. It is particularly suitable for use in degassing fields such as ultrapure water, which are prone to generate water vapor and condense to form a liquid film in the hollow fiber membrane.

[0085] The uniformly distributed external degassing pores in the present invention specifically refer to multiple observations of the outer surface of the hollow fiber membrane through SEM electron microscopy, which clearly show that there is basically no relatively dense area on the outer surface of the hollow fiber membrane, and the membrane pores are basically uniformly distributed on the outer surface.

[0086] Furthermore, the deammoniation efficiency of the porous membrane is at least greater than 80%;

[0087] The permeation rate of O2 of the porous membrane is not less than 30L / (min·bar·0.1m 2 );

[0088] The water evaporation rate of the porous membrane is not higher than 0.1 ml / m 2 / min;

[0089] The deoxidation efficiency of the porous membrane is greater than or equal to 90%.

[0090] The hollow fiber membrane of the present invention has good tensile strength and elongation at break. It also has a high deamination efficiency, exceeding 80%, capable of removing as much NH3 as possible from ammonia-nitrogen-containing wastewater. It also has an extremely high deoxygenation efficiency, exceeding 90%, capable of removing as much as possible from gases such as dissolved oxygen in ultrapure water. Furthermore, the hollow fiber membrane has a high degassing rate, characterized by the O2 permeation rate. Furthermore, the hollow fiber membrane has a low water evaporation rate, making it suitable for degassing applications in areas such as ultrapure water, where water vapor is easily generated and condenses to form a liquid film within the hollow fiber membrane.

[0091] The deoxygenation efficiency of the present invention can be achieved by using the hollow fiber membrane of the present invention as a raw material to assemble a membrane assembly, and connecting the assembly to the process section and polishing section of the ultrapure water preparation process. Under the rated flow rate, the water inlet temperature is tested to be 25°C, the inlet oxygen concentration should respectively meet the following conditions: the inlet oxygen concentration of the deoxygenation membrane assembly in the process section is 6-8 mg / L and the inlet oxygen concentration of the deoxygenation membrane assembly in the polishing section is ≤10 μg / L, the absolute vacuum is 0.006 MPa, the nitrogen purge volume is 0.012 times the rated inlet flow rate, and the deoxygenation efficiency of the deoxygenation membrane assembly is tested respectively. In the present invention, deoxygenation efficiency is used to represent degassing efficiency, mainly because the oxygen content in ultrapure water will have a greater impact on the application field of semiconductors, and the oxygen content in ultrapure water is generally relatively high.

[0092] The deamination efficiency of the present invention can be measured by using the hollow fiber membrane of the present invention as a raw material, assembling a membrane assembly, and using the membrane assembly to degas wastewater containing ammonia nitrogen, wherein the concentration of the ammonia nitrogen wastewater is 1000 ppm, the pH value is 11, the temperature is controlled at 35°C, the flow rate of the ammonia nitrogen wastewater is 100 L / hr, and the flow ratio of acid solution (dilute sulfuric acid with a pH value of 1.5): ammonia nitrogen-containing wastewater is 4:1. The concentrations of ammonia nitrogen in the wastewater before and after degassing are measured using the ammonia nitrogen determination method in HJ 535-2009, and the deamination efficiency is finally calculated.

[0093] The O2 permeation rate of the present invention can be measured by the following test method: under the conditions of a temperature of 25°C, a pressure of 0.1 bar, and a membrane sample area of ​​0.1 square meters, one side of the membrane sample is subjected to the gas to be tested (oxygen / carbon dioxide); the gas to be tested (oxygen / carbon dioxide) is supplied into the inner cavity of the hollow fiber membrane; the volume flow rate of the gas passing through the sample membrane wall is measured using a flow meter (Japan KOFLOC / 4800); the test is performed three times from the inside of the membrane to the outside of the membrane, and three times from the outside of the membrane to the inside of the membrane, and then the average value is taken. The average value is the oxygen / carbon dioxide permeation rate of the membrane, and the unit of oxygen / carbon dioxide permeation rate is ml / (min·bar·0.1m 2 ).

[0094] The water evaporation amount of the present invention can be measured by the following test method: the hollow fiber membrane of the present invention is used as a raw material to assemble into a membrane assembly, and the water channel and the assembly are connected for testing so that the water fills the outer surface of the membrane fiber. The pressure is controlled at 0.4 MPa. After 12 hours, the amount of condensed water after condensation is collected on the vacuum side of the hollow fiber membrane.

[0095] Furthermore, the present invention discloses a process for preparing an asymmetric PP hollow fiber porous membrane for degassing, comprising the following steps:

[0096] S1, spinning, blending PP material and hydrophobic additives into a melt, and extruding through a die to form a molded product with a hollow inner cavity, wherein the extrusion screw is a special single screw, the compression ratio of the feeding section is 1.05-1.4, the compression ratio of the mixing section is 1.4-2.7, the mixing section includes a mixing thread, the length of the metering section is 5-12 / screw diameter is 5-12, and the die extrusion temperature is 165-180°C; wherein the melting process includes melting, mixing and metering steps, the metering temperature is 10-20°C higher than the die extrusion temperature, and the die length-diameter ratio is 2-8; the isotacticity of the PP is not less than 95%; the melt index of the PP raw material is 1-7g / min@(190°C, 5kg);

[0097] S2, pre-crystallization, placing the molded product in the air section for pre-crystallization;

[0098] S3, air cooling crystallization, cooling the molded product obtained in step S2 under air atmosphere to obtain spun fibers;

[0099] S4, primary setting, subjecting the spun fibers to a first heat setting treatment and stretching the spun fibers, wherein the first heat setting temperature is 80-110° C. and the tensile elongation is 0.5-2%, to obtain a heat-set semi-finished product;

[0100] S5, cold drawing to form a hole, wherein the heat-set semi-finished product is cold drawn twice, wherein the ratio of the temperature of the first cold drawing to the first heat setting temperature is 1:(2-5), and the cold drawing rate of the two cold drawing processes is not higher than 20% / min, and the cold drawing rate of the first cold drawing is lower than the cold drawing rate of the second cold drawing, to obtain a cold drawn semi-finished product;

[0101] The elongation of the first cold drawing is 5%-20%, and the elongation of the second cold drawing is 10%-30%;

[0102] S6, hot drawing and hole expansion, wherein the cold-drawn semi-finished product is subjected to hot drawing and hole expansion twice, wherein the temperature of the two hot drawing times is 100-120° C., and the hot drawing rate of the two hot drawing times is not higher than 5% / min, and the hot drawing rate of the first hot drawing time is lower than the hot drawing rate of the second hot drawing time, to obtain a hot-drawn semi-finished product;

[0103] S7, secondary shaping, subjecting the hot-drawn semi-finished product to a second heat-setting treatment to obtain a hollow fiber porous membrane.

[0104] Furthermore, in step S2, the temperature of the air section is 40-70°C, and the residence time of the air section is 0.01-0.1s;

[0105] In step S3, the blowing temperature is 30-70° C., and the blowing speed is 20-80 m / min.

[0106] Furthermore, in step S4, the first heat setting time is 20-50 minutes;

[0107] In step S7, the temperature of the second heat setting is 110-140° C., and the time of the second heat setting is 2-5 minutes.

[0108] Furthermore, in step S5, the cold drawing rate of the first cold drawing is 5-10% / min, the cold drawing rate of the second cold drawing is 15%-20%, and the temperature of the second cold drawing is 0-5°C higher than the temperature of the first cold drawing;

[0109] In step S6, the first hot drawing rate is 0.2-0.8% / min, the second hot drawing rate is 1.5-2%, the first hot drawing elongation is 20%-80%, and the second hot drawing elongation is 40%-100%.

[0110] The present invention prepares a PP hollow fiber porous membrane using a melt-stretching method. This method involves crystallizing a polymer melt during extrusion under tensile stress and cooling, forming a parallel crystalline structure. After heat treatment, the semi-finished product separates between crystals after stretching, forming micropores, which are then heat-set to produce a microporous membrane. Typically, a hydrophobic additive is a non-crystalline substance, while polypropylene (PP) is a crystalline substance. While hydrophobicizing PP with a hydrophobic additive can improve the hydrophobic properties of the membrane filaments made from the PP material, it also affects the crystallization properties of the PP material. This is particularly true during the melt-stretching process for preparing PP membrane filaments, which can lead to a reduction in the number of crystal nuclei generated in the PP membrane filaments, subsequently affecting parameters such as the porosity and pore size of the PP membrane filaments. Furthermore, there may be poor compatibility between the hydrophobic additive and the PP material, which can easily result in the hydrophobic additive not being properly mixed with the PP material during the melt-stretching process, thereby failing to effectively improve the overall hydrophobic properties of the PP membrane filaments.

[0111] Taking the above factors into consideration, the hydrophobic additive of the present invention is selected from pentene substances (pentene substances refer to olefin substances containing pentene structure) that only include two elements, carbon and hydrogen. The pentene hydrophobic additive and polypropylene (PP) both include only two elements, carbon and hydrogen, and the properties of the two are similar, so that the pentene hydrophobic additive and the PP material can have better compatibility during the melting process, thereby making the hydrophobic additive and the PP material more evenly mixed, and then significantly improving the hydrophobic properties of the PP membrane fiber, so that the inner and outer surfaces of the PP membrane fiber can have lower surface energy and stronger hydrophobicity.

[0112] It can be expected that the relevant properties of hydrophobic additives and PP can affect and determine the surface properties and microporous structure of hollow fiber membranes to a certain extent. After research, it was found that the weight-average molecular weight of the hydrophobic additive, the weight-average molecular weight of PP and its molecular weight fraction index and other parameters have a great influence on the hollow fiber membrane. Isotacticity refers to the distribution of methyl groups on the polypropylene molecular chain. The isotacticity of the polypropylene raw material of the present invention is not less than 95%, which means that the more regularly the methyl groups on the molecular chain are arranged, the more complete the crystal structure is formed during the melt stretching process, and the easier it is to obtain an ideal membrane structure. The melt index of the PP material is controlled at 1-7g / min@(190℃, 5kg), which means that the PP material has good fluidity after melting. The molecular weight distribution index refers to the index of the degree of dispersion of the molecular weight distribution. The molecular weight distribution index of the PP material of the present invention is controlled within the range of 2-7, which means that the molecular weight of the PP material of the present invention is relatively uniform, and its processing stability, mechanical properties, etc. are all good. Combined with the weight average molecular weight of the PP material being controlled within the range of 100,000-600,000 and the weight average molecular weight of the hydrophobic additive being controlled within the range of 50,000-200,000, the PP material and the hydrophobic additive can be mixed more fully and evenly during the blending and melting process, which is beneficial to the preparation of PP membrane filaments with good hydrophobic properties. On this basis, the present invention controls the hydrophobic additive and the PP material within a suitable addition ratio (the mass ratio of the hydrophobic additive to the PP is 1:10-3:7) so that the mixed molten material can have good fluidity. At the same time, unexpectedly, the crystallinity of the prepared PP membrane filaments does not decrease significantly as expected, but still has relatively good crystallinity, and can still form PP membrane filaments with suitable porosity and membrane pore structure and good hydrophobic properties.

[0113] In the present invention, step S1 is to melt-extrude the PP material and the hydrophobic additive after blending, and form a molded product with an inner surface and an outer surface under the action of an inert gas as an inner core liquid. First, the PP material and the hydrophobic additive are melted (mainly for conveying, pushing and preheating the hydrophobic additive and the PP material, so that the hydrophobic additive and the PP material gradually melt and finally are in a completely molten state to achieve blending), mixing (mainly for mixing, compressing and pressurizing and exhausting the hydrophobic additive and the PP material) and metering steps (mainly for mixing, melting, conveying, providing sufficient pressure, maintaining the uniform temperature of the mixed raw materials and a stable melt flow rate); the present invention controls the metering temperature to be 10-20°C higher than the extrusion temperature of the die head. When the molten mixed material is extruded at the die head, the molten mixed material will undergo heat conduction inside, causing its temperature to drop. , and in the process of cooling, the temperature of the inner and outer surfaces is more uniform, ensuring that the crystallization process is relatively sufficient, and then ensuring that the pore structure of the prepared membrane is relatively stable, which is more conducive to the formation of a suitable membrane pore structure; as one of the important conditions of the process steps of the present invention, during the melt extrusion process, the die extrusion temperature is controlled at 165-180°C. The reason is that the hydrophobic additive is a non-crystalline substance with a relatively low glass transition temperature. Controlling the die extrusion temperature relatively low can make the hydrophobic additive and the PP material blend more evenly and have relatively suitable fluidity after extrusion, which is conducive to the preparation of an ideal membrane structure; if the die extrusion temperature is too high, the fluidity of the hydrophobic additive and the mixing uniformity with the PP may be affected to a certain extent, and then a hollow fiber membrane structure with relatively hydrophobic and uniform inner and outer surfaces cannot be obtained.

[0114] Furthermore, by controlling the die's aspect ratio to 2-8, the shear forces acting on the macromolecular chains of the molten mixture during its flow create orientation, allowing the PP material in the mixture to easily form crystal nuclei. This results in the final film filament possessing an appropriate degree of crystallinity, resulting in a more stable pore structure, greater tensile strength, and elongation at break. Furthermore, the use of inert gas as a core liquid during the melt extrusion process ensures a relatively uniform thickness of the finished degassed film.

[0115] In this process, the present invention controls the extrusion screw to be a special single-screw screw, controls the compression ratio of the feeding section to 1.05-1.4, and controls the compression ratio of the mixing section to 1.4-2.7. It should be noted that the "compression ratio" in the present invention refers to the inlet screw groove depth / exit screw groove depth of the section. Generally speaking, the greater the compression ratio, the greater the average tensile stress given to the melt by the screw, and the stronger the plastic deformation ability of the melt, thereby reducing the stress deformation rate of the molten material during the extrusion process; secondly, the greater the compression ratio, the smaller the volume of the screw spiral groove, which increases the braking heating effect of the stirring force on the molten material, thereby increasing the temperature of the molten material. The present invention reasonably controls the compression ratio of the feeding section and the mixing section, so that the hydrophobic additive and the PP material can be mixed more evenly; in addition, the mixing section includes a mixing thread, and the setting of the mixing thread enables the screw to have additional shear force on the molten material, thereby promoting a more uniform mixing between the hydrophobic additive and the PP material; the metering section length / screw diameter can affect the melting and conveying process of the material. The present invention controls the metering section length / screw diameter within the range of 5-12, so that the residence time of the material in the screw is more appropriate, which is conducive to the full melting and mixing of the hydrophobic additive and the PP material. The present invention reasonably controls the compression ratio of the feeding section and the mixing section, reasonably selects the metering section length / screw diameter, and combines the action of the mixing thread to ensure that the hydrophobic additive and the PP material can be distributed more evenly during the blending and melting process, so that the inner and outer surfaces of the prepared hollow fiber membrane have good hydrophobicity. The hollow fiber membrane can be well applied to degassing fields such as wastewater deamination and carbon dioxide capture, where both the inner and outer surfaces of the hollow fiber membrane need to be in long-term contact with the liquid to be degassed or the active acidic or alkaline liquid.

[0116] On this basis, step S2 of the present invention pre-crystallizes the molded product after extrusion through the die head through an air segment. The temperature and residence time of the air segment mainly affect the pore structure and cortical structure acting on the outer surface of the hollow fiber membrane (when the temperature of the air segment is relatively high and the residence time of the air segment is relatively long, the outer surface of the degassing membrane also has relatively sufficient time to form crystal nuclei of appropriate size and number, and a certain pore structure is formed on the outer surface after stretching into holes, and the thickness of the cortical structure will also be relatively small at this time; and as the temperature of the air segment and the pre-crystallization time decrease, the number of crystal nuclei generated on the outer surface of the hollow fiber membrane gradually decreases, and even crystal nuclei of appropriate size cannot be formed. After stretching into holes, a relatively dense membrane pore structure on the outer surface is formed, and the thickness of the cortical structure will also be relatively increased at this time).

[0117] Next, the molded product that has passed through the air section is subjected to air-cooling crystallization. After the pre-crystallization process in the air section, a certain number and size of crystal nuclei are formed inside the molded product. The present invention controls the appropriate blowing temperature (30-70°C) and blowing speed (20-80m / min) to form a suitable number and size of crystal nuclei in the molded product, thereby preparing membrane filaments with an ideal membrane structure (that is, by controlling the blowing temperature and blowing speed, and combining the pre-crystallization process in the air section, membrane filaments with different outer surface membrane pore structures can be formed).

[0118] Next, the spun fibers undergo a first heat setting process. As one of the key steps in the present invention, a stretching process is performed during the first heat setting process, with a tensile elongation of 0.5-2%. By controlling the tensile elongation within the range of 0.5-2% during the first heat setting process, a certain amount of stretching is achieved within the heat-set semi-finished product, thereby forming a certain number of tiny cracks or rudiments of a pore structure within the heat-set semi-finished product. This allows the subsequent cold-drawing process to better stretch the crystal nuclei apart and form a more uniform pore structure. If the first heat-setting stretching elongation is too high, it may cause relatively excessive stretching in the heat-setting semi-finished product, which may result in the formation of tiny cracks or hole structure prototypes with a large number and area, resulting in the membrane filaments formed after the subsequent cold drawing and hot drawing processes having excessive pore size and porosity, which may affect the overall tolerance of the membrane filament; if the first heat-setting stretching elongation is too low, it may result in the inability to form tiny cracks or hole structure prototypes of the appropriate number and area in the heat-setting semi-finished product, so that the final membrane filament cannot obtain the appropriate porosity and pore size.

[0119] It should be noted that the tensile elongation during the first heat setting stretching treatment also needs to be coordinated with the temperature and time of the first heat setting. In the present invention, the temperature of the first heat setting is controlled at 80-110°C, and the time of the first heat setting is controlled at 20-50 minutes. This is because during the first heat setting stretching treatment, what we hope for the stretching treatment is that the heat-set semi-finished product can be properly stretched to form a certain number and area of ​​tiny cracks or pore structure prototypes. However, if the temperature of the first heat setting is too high or the time of the first heat setting is too long, it may cause the cracks or pore structure prototypes formed by the stretching treatment to diffuse (the number and area are too large), which may cause the pore structure formed by the heat-set semi-finished product after the cold drawing and hot drawing processes to be too large in pore size or adjacent pore structures to merge, which is undesirable.

[0120] After the first heat setting, the heat-set semi-finished product is subjected to a cold drawing step. The cold drawing process mainly serves to pull apart the crystal nuclei within the heat-set semi-finished product and form a porous structure. This is also one of the more important steps in the present invention. The present invention cold draws the heat-set semi-finished product twice, wherein the ratio of the temperature of the first cold drawing to the first heat setting temperature is controlled within the range of 1:(2-5), and the cold drawing rate of both cold drawing steps is no more than 20%. The cold drawing rate of the first cold drawing is less than the cold drawing rate of the second cold drawing. The elongation of the first cold drawing is 5%-20%, and the elongation of the second cold drawing is 10%-30%. Preferably, the cold drawing rate of the first cold drawing is 5-10% / min, and the cold drawing rate of the second cold drawing is 15-20% / min. The temperature of the second cold drawing is 0-5°C higher than the temperature of the first cold drawing.

[0121] First, the present invention adopts two cold drawing treatments. The reason is that after the stretching treatment in the first heat setting, a certain number of tiny cracks and pore structure prototypes will be formed in the heat-set semi-finished product. The present invention uses two cold drawing treatments. On the one hand, these tiny cracks and pore structure prototypes can be better stretched to form a pore structure of appropriate size. On the other hand, the area with a suitable number and size of crystal nuclei in the heat-set semi-finished product (this area may not form tiny cracks or pore structure prototypes after the first heat setting stretching treatment) can be stretched and form a pore structure of appropriate size, which is exactly what a single cold drawing process cannot achieve.

[0122] The first cold drawing process of the present invention adopts less drawing and slow drawing (the first cold drawing rate is controlled to be 5-10% / min, and the first cold drawing elongation is 5-20%. It should be noted that "slow drawing" and "fast drawing" here refer to the comparison between the cold drawing rates during the first cold drawing and the second cold drawing, and have nothing to do with the absolute value of the cold drawing rates during the two cold drawing. The cold drawing rates of the two cold drawing cannot be higher than 20% / min; "less drawing" and "more drawing" here refer to the comparison between the cold drawing elongation during the first cold drawing and the second cold drawing, and have nothing to do with the absolute value of the cold drawing rates during the two cold drawing. The absolute value of the cold drawing elongation has nothing to do with it, and the ratio of the first cold drawing temperature to the first heat setting temperature is in the range of 1:(2-5). We found that under such a temperature ratio and the effect of "less drawing" and "slow drawing" in the first cold drawing treatment, the first cold drawing treatment can better stretch the tiny cracks and the prototype of the pore structure formed during the first heat setting stretching treatment (with a relatively small elongation) to form a suitable pore structure, and to a certain extent can reduce the formation of defective pores (such as the situation where the distance between adjacent pore structures is too close or partial merging occurs).

[0123] After the first cold drawing, the crack structure or pore structure prototype in the heat-set semi-finished product can basically be well stretched and form a relatively stable pore structure, but there are still some areas of the crystal nuclei in the heat-set semi-finished product that have not been properly stretched and formed a pore structure; we found that the second cold drawing treatment adopts more pulling and faster pulling, and the second cold drawing temperature is 0-5°C higher than the first cold drawing temperature, which can better pull apart the crystal nucleus areas that have not been stretched to form a pore structure during the first cold drawing process, so that a more suitable pore structure can be formed in these areas, and then the final membrane fiber has a suitable porosity and membrane pore structure, so as to have a relatively high degassing rate and degassing efficiency and meet the needs of degassing fields such as wastewater deammoniation that require rapid gas removal.

[0124] On this basis, the cold drawing rate during the two cold drawing processes cannot be higher than 20%. This may be because the present invention adds a pentene hydrophobic additive, which is usually a non-crystalline substance. Compared with PP materials, the toughness of the heat-set semi-finished product will be affected to a certain extent and decrease after the addition of the pentene hydrophobic additive. If the cold drawing rate of the two cold drawing processes is too high (higher than 20%), tiny cracks or pore structure prototypes may appear in the first heat-set stretching treatment, and adjacent pore structures may merge in the process of the crystal nuclei in the heat-set semi-finished product being pulled apart to form a pore structure.

[0125] After cold drawing to form a hole, the cold-drawn semi-finished product is subjected to hot drawing to expand the hole. In the present invention, the hot drawing temperature is controlled to be 100-120° C., and the hot drawing rates of the two hot drawing processes are both no higher than 5% / min, and the hot drawing rate of the first hot drawing process is lower than that of the second hot drawing process. Preferably, the hot drawing rate of the first hot drawing process is 0.2-0.8% / min, and the hot drawing rate of the second hot drawing process is 1.5-2%. The elongation of the first hot drawing process is 20%-80%, and the elongation of the second hot drawing process is 40%-100%.

[0126] Similar to the cold drawing process, the present invention also adopts a two-step hot drawing process for stretching treatment. The first hot drawing treatment adopts less pulling and slower pulling, and the second hot drawing treatment adopts more pulling and faster pulling, and the hot drawing rate of the two hot drawing treatments cannot be higher than 5%; this is because in the process of cold drawing to form a pore structure, the formation of the pore structure is roughly divided into two categories. One is the pore structure formed after cold drawing of the tiny cracks or pore structure prototypes formed in the first heat-setting stretching process, and the other is the pore structure formed by pulling apart the difficult-to-stretch crystal core structure in the heat-setting semi-finished product in the second cold drawing process; if only one hot drawing process is performed, the pore structures in the above two may not be able to obtain better pore expansion treatment, that is, a more suitable membrane pore structure cannot be formed, which may cause defects such as low porosity and small membrane pores, which is not conducive to the application of membrane filaments in degassing applications such as wastewater deamination that require a faster degassing rate.

[0127] The present invention adopts a first hot drawing process with less stretching and slower stretching, and a second hot drawing process with more stretching and faster stretching, combined with the effect of a suitable hot drawing temperature (100-120°C). Its purpose is to first subject the pore structure formed by the tiny cracks or pore structure prototype to a relatively suitable stretching treatment, so that such pore structure can be relatively stably present in the hot-drawn semi-finished product, and then expand the pore structure formed by the latter through the second hot drawing process. In this way, the probability of forming an oversized pore structure or merging adjacent pore structures can be reduced to a certain extent. It should be noted that the cold drawing elongation and hot drawing elongation in the present invention are calculated in a superimposed manner. For example, if the elongation of the cold-drawn semi-finished product after the first hot drawing process is 20% and the elongation after the second hot drawing process is 40%, then the hot drawing elongation of the cold-drawn semi-finished product after the two hot drawing processes is 20% + 40% = 60%.

[0128] Finally, the heat-drawn semi-finished product is subjected to a second heat setting process. Preferably, the temperature of the second heat setting is controlled to 110-140°C, and the time of the second heat setting is controlled to 2-5 minutes. Under the action of such heat setting temperature and time, the residual stress in the stretching and pore-forming stage can be better eliminated. At the same time, it can also avoid as much as possible the impact of the second heat setting temperature on the internal pore structure and the stability of the pore structure of the membrane fiber due to the excessively high temperature. As a result, the final membrane fiber has excellent tolerance, high degassing rate and degassing efficiency, and has good tensile strength and elongation at break. It is suitable for use in some fields where the inner surface of some hollow fiber membranes needs to be in long-term contact with active liquids such as acid and alkaline liquids and degassed (such as wastewater deammoniation, carbon dioxide capture and other fields). At the same time, in some degassing fields where water vapor is easily formed and condensed to form a liquid film, it can also better curb the formation of liquid film and reduce the amount of water vapor lost after condensation.

[0129] Furthermore, the present invention also provides a use of an asymmetric PP hollow fiber porous membrane for degassing, wherein the porous membrane is used in degassing fields such as ultrapure water degassing and / or wastewater deamination.

[0130] It should be noted that the wastewater claimed in the present invention can come from landfill leachate, industrial wastewater, wastewater from chemical products, etc. The hollow fiber membrane of the present invention can be well applied in fields where the inner surface of the hollow fiber membrane needs to be in contact with active liquids such as acidic and alkaline liquids for a long time and degassing (such as wastewater deammoniation and carbon dioxide capture). It can also be well applied in fields where water vapor is easily formed and condensed to form a liquid film (such as ultrapure water degassing).

[0131] In summary, the present invention includes at least one of the following beneficial technical effects:

[0132] 1. The present invention adds a pentene hydrophobic additive to make the hollow fiber porous membrane made of PP material have better hydrophobicity, so that the hollow fiber porous membrane can better resist the erosion of the inner and outer surfaces of the hollow fiber membrane that need to be in contact with the degassed liquid, acidic or alkaline active solution, and endow the hollow fiber porous membrane with better long-term tolerance; at the same time, the hollow fiber porous membrane of the present invention can also be well used in the degassing field where water vapor is easily formed and condensed to form a liquid film, and can significantly reduce the loss of water vapor condensed to form a liquid film.

[0133] 2. The present invention selects hydrophobic additives and PP materials with appropriate weight-average molecular weight and addition ratio, so that the prepared hollow fiber porous membrane still has a high degree of crystallinity, thereby giving the hollow fiber porous membrane a good degassing rate and high degassing efficiency. In the wastewater deamination application, the deamination efficiency can reach more than 80%, and the deoxygenation efficiency can reach more than 90%. BRIEF DESCRIPTION OF THE DRAWINGS

[0134] The present invention will be further described below in conjunction with the accompanying drawings:

[0135] FIG1 is a scanning electron microscope image of the inner surface of the hollow fiber porous membrane prepared in Example 1, wherein the magnification is 20K×;

[0136] FIG2 is a scanning electron microscope image of the outer surface of the hollow fiber porous membrane prepared in Example 1, wherein the magnification is 50K×;

[0137] FIG3 is a scanning electron microscope image of the inner surface of the hollow fiber porous membrane prepared in Example 2, wherein the magnification is 5K×;

[0138] FIG4 is a scanning electron microscope image of the outer surface of the hollow fiber porous membrane prepared in Example 5, wherein the magnification is 50K×. DETAILED DESCRIPTION

[0139] The present invention is further described in detail below with reference to the examples. Unless otherwise specified, in the following examples, the raw materials and equipment used to prepare the hollow fiber membranes can be purchased through commercial channels.

[0140] Example 1

[0141] A process for preparing an asymmetric PP hollow fiber porous membrane for degassing comprises the following steps:

[0142] S1, spinning, blending PP material and a hydrophobic additive into a melt, and extruding through a die to form a molded product with a hollow inner cavity, wherein the extrusion screw is a special single screw, the compression ratio of the feeding section is 1.2, the compression ratio of the mixing section is 2, the mixing section includes a mixing thread, the length of the metering section is / screw diameter is 9, and the die extrusion temperature is 170°C; wherein the melting process includes melting, mixing and metering steps, the metering temperature is 180°C, and the die length-diameter ratio is 6; the isotacticity of the PP material is not less than 95%; the melt index of the PP raw material is 5g / min@(190°C, 5kg);

[0143] The weight average molecular weight of the PP material is 150,000, the molecular weight distribution of the PP material is 7, the weight average molecular weight of the hydrophobic additive is 50,000, and the hydrophobic additive accounts for 20% of the total mass of the polymer (referring to the total mass of the hydrophobic additive and the PP material);

[0144] S2, pre-crystallization, placing the molded product in the air section for pre-crystallization, where the temperature of the air section is 50°C and the residence time in the air section is 0.1s;

[0145] S3, air cooling crystallization, cooling the molded product obtained in step S2 by blowing air in an air atmosphere, wherein the blowing temperature is 55° C. and the blowing speed is 40 m / min, to obtain spun fibers;

[0146] S4, primary setting, subjecting the spun fibers to a first heat setting treatment and stretching the spun fibers, wherein the first heat setting temperature is 100° C., the first heat setting time is 30 minutes, and the tensile elongation is 0.8%, to obtain a heat-set semi-finished product;

[0147] S5, cold drawing to form a hole, cold drawing the heat-set semi-finished product twice, wherein the first cold drawing temperature is 50° C., the cold drawing rate of the first cold drawing is 8.5% / min, and the elongation of the first cold drawing is 15%, and the second cold drawing temperature is 53° C., the cold drawing rate of the second cold drawing is 18% / min, and the elongation of the second cold drawing is 25%, to obtain a cold-drawn semi-finished product;

[0148] S6, hot drawing and hole expansion, wherein the cold-drawn semi-finished product is subjected to hot drawing and hole expansion twice, wherein the temperature of the two hot drawing times is 115° C., the hot drawing rate of the first hot drawing time is 0.6% / min, and the elongation of the first hot drawing time is 60%, and the hot drawing rate of the second hot drawing time is 1.8% / min, and the elongation of the second hot drawing time is 80%, thereby obtaining a hot-drawn semi-finished product;

[0149] S7, secondary shaping, subjecting the hot-drawn semi-finished product to a second heat-setting treatment, the second heat-setting temperature is 130° C., the second heat-setting time is 3.5 minutes, and a hollow fiber porous membrane is obtained.

[0150] Examples 2-7

[0151] The difference between Example 2-7 and Example 1 lies in the different process parameters. The specific differences are shown in Table 1-1, Table 1-2 and Table 1-3.

[0152] Comparative Example 1

[0153] The difference between Comparative Example 1 and Example 1 is that no hydrophobic additive is added. Other differences are shown in Table 1-1, Table 1-2 and Table 1-3.

[0154] Comparative Example 2

[0155] The difference between Comparative Example 2 and Example 1 is that a hydrophobic additive is added, wherein the hydrophobic additive is BASF's silicone resin Silres 80. Other differences are shown in Table 1-1, Table 1-2 and Table 1-3.

[0156] Comparative Example 3

[0157] The difference between Comparative Example 3 and Example 1 is that the amount of the hydrophobic additive added is 50% of the total mass of the polymer. Other differences are shown in Table 1-1, Table 1-2 and Table 1-3.

[0158] Comparative Example 4

[0159] The difference between Comparative Example 4 and Example 1 is that the amount of the hydrophobic additive added is 1% of the total mass of the polymer. Other differences are shown in Table 1-1, Table 1-2 and Table 1-3.

[0160] Comparative Example 5

[0161] The difference between Comparative Example 5 and Example 1 lies in the different process parameters, and the specific differences are shown in Table 1-1, Table 1-2 and Table 1-3.

[0162] The hydrophobic additive used in Examples 1-2 is cis-1,4-polyisoprene rubber, the hydrophobic additive used in Examples 3-4 is styrene-isoprene-styrene block copolymer SIS polymer, the hydrophobic additive used in Examples 5-7 is Mitsui's tafmer propylene / pentene copolymer, the hydrophobic additive used in Comparative Example 2 is BASF's silicone resin Silres 80, and the hydrophobic additive used in Comparative Examples 3-5 is cis-1,4-polyisoprene rubber.

[0163] Table 1-1

[0164] Table 1-2

[0165] Table 1-3

[0166] Membrane structure parameter detection

[0167] The PP hollow fiber porous membranes prepared in Examples 1-7 and Comparative Examples 1-5 were morphologically characterized using a scanning electron microscope (Hitachi S-5500). The outer surface, inner surface, and cross section of the PP hollow fiber porous membranes were selected as observation objects. The specific detection and measurement results are shown in Tables 2-1 and 2-2.

[0168] Table 2-1

[0169] Table 2-2

[0170] The thickness of the skin layer of Examples 1-4 was measured, and the test results are shown in Table 2-3.

[0171] Table 2-3

[0172] The pore size and density of the external degassing pores on the outer surfaces of Examples 5-7 were measured, and the test results are shown in Tables 2-4.

[0173] Table 2-4

[0174] Membrane performance parameter testing

[0175] 1.1 Tensile strength and elongation at break test

[0176] After testing, the tensile strength of the PP hollow fiber porous membranes prepared in Examples 1-7 was not less than 200CN, and the elongation at break was 50-500%, that is, they all had large tensile strength and elongation at break, and could meet industrial requirements.

[0177] 1.2 Deamination efficiency test; 1.3 Oxygen permeation rate test; 1.4 Water evaporation test; 1.5 Deoxygenation efficiency test; 1.6 XRD crystallinity test; 1.7 Water contact angle test; 1.8 Roughness test.

[0178] The test results are shown in Table 3, where the water evaporation amount refers to the amount of water lost during the degassing process as the gas is continuously removed. The water in the liquid system is converted into water vapor due to factors such as evaporation and condenses along with the gas to be removed through the hollow fiber porous membrane.

[0179] Table 3

[0180] As can be seen from the above, the hollow fiber porous membranes prepared in Examples 1-7 of the present invention can significantly improve the hydrophobic properties of the hollow fiber porous membranes after adding the pentene hydrophobic additives in the present invention, so that the internal and external water contact angles of the hollow fiber porous membranes are both high. On this basis, after adding the pentene hydrophobic additives in the present invention, the crystallinity of the hollow fiber porous membranes is almost unaffected, but still has a high level, so that the hollow fiber porous membranes of the present invention can be preferably applied to the fields (such as wastewater deamination, carbon dioxide capture and other fields) where the inner surface of some hollow fiber membranes needs to be in contact with active liquids such as acidic and alkaline for a long time and degassed. When the hollow fiber porous membranes of the present invention are applied to the field of wastewater deamination, the deamination efficiency of the hollow fiber porous membranes can reach more than 80% (the deamination effect of the samples prepared in Examples 5-7 is particularly good), which can produce higher economic value. In addition, the hollow fiber porous membrane of the present invention also has lower water evaporation amount, and has higher degassing efficiency and good degassing rate concurrently, thus making the hollow fiber porous membrane of the present invention also can be preferably applied to some easily form water vapor and condense to form the degassing field (such as the fields such as ultrapure water degassing) of liquid film, when being especially applied to ultrapure water degassing, can remove the dissolved oxygen content in ultrapure water as much as possible, reduce the loss amount of ultrapure water, can have higher production efficiency and produce higher economic value. Finally the hollow fiber porous membrane of the present invention also has good tolerance, through the long-term test of wastewater deamination process, the hollow fiber porous membrane of the present invention can be after using 3 months, and the deamination efficiency of hollow fiber porous membrane can still remain substantially unchanged, and the hollow fiber porous membrane obtained by comparative example 1-5 is very poor due to tolerance, and after testing 24 hours, the deamination efficiency of hollow fiber porous membrane can be further greatly reduced.

[0181] However, the water evaporation rate of the hollow fiber porous membranes prepared in Examples 1-5 is relatively high, and it is difficult to be well applied to applications such as ultrapure water degassing. At the same time, the long-term tolerance of the hollow fiber porous membranes prepared in Examples 1-5 is relatively poor, and it is difficult to meet the degassing applications in some fields where the inner surface of the hollow fiber membrane needs to be in contact with active liquids such as acid and alkaline for a long time and degassed. At the same time, this will also cause the degassing efficiency and degassing rate of the hollow fiber porous membrane to be affected and decreased, making it difficult to meet the needs of actual applications.

[0182] While the preferred embodiments of the present invention have been described in detail above, it should be understood that, after reading the above teachings of the present invention, those skilled in the art may make various changes or modifications to the present invention. Such equivalents also fall within the scope of the appended claims.

Claims

1. An asymmetric PP hollow fiber porous membrane for degassing, comprising a main body. One side of the main body is an inner surface facing the inner cavity, and the other side is an outer surface. The main body has non-directional tortuous channels. The inner surface has inner degassing holes. The hole area ratio of the outer surface is not more than 5%, and the hole area ratio of the outer surface is less than that of the inner surface. It is characterized in that the average SEM pore diameter of the inner degassing holes is 30 - 150 nm, and the hole area ratio of the inner surface is 10% - 40%; the overall porosity of the porous membrane is 20% - 65%; the thickness of the porous membrane is 30 - 75 μm; the polymer constituting the porous membrane includes at least a hydrophobic additive and PP. The weight-average molecular weight of the PP is 100,000 - 600,000, and the molecular weight distribution is 2 - 7; the hydrophobic additive only contains carbon and hydrogen and is a pentene substance; the weight-average molecular weight of the hydrophobic additive is 50,000 - 200,000; the hydrophobic additive in the polymer constituting the porous membrane accounts for 5% - 30% of the total mass of the polymer.

2. An asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that, The infrared absorption spectra of the inner and outer surfaces of the porous membrane are tested by the ATR method, and the doping coefficient X of the hydrophobic additive of the porous membrane is calculated; X = ABS 720 / ABS 1460 ; Among them, ABS 720 is the absorption intensity of the porous membrane at a wavenumber of 720 cm -1 nearby; ABS 1460 is the absorption intensity of the porous membrane at a wavenumber of 1460 cm -1 nearby; Hydrophobic additive doping coefficient X 内 = ABS 内表面720 / ABS 内表面1460 ; wherein X 内 has a value range of 0.0005 - 0.15; preferably 0.001 - 0.08; ABS 内表面720 is the absorption intensity at the wavenumber of 720 cm -1 near the outer surface of the porous membrane; ABS 内表面1460 is the absorption intensity of the outer surface of the porous membrane at a wavenumber near 1460 cm -1 -1.

3. An asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that, Hydrophobic additive doping coefficient X 外 = ABS 外表面720 / ABS 外表面1460 ; wherein X 外 has a value range of 0.001 - 0.3; preferably 0.005 - 0.1; ABS 外表面720 is the absorption intensity of the inner surface of the porous membrane at a wavenumber near 720 cm -1 ; ABS 外表面1460 is the absorption intensity at the inner surface of the porous membrane at a wavenumber near 1460 cm -1 -1.

4. An asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that, The porous membrane is tested by the XRD method. The XRD crystallinity of the porous membrane is 60% - 90%; the overall porosity of the porous membrane is 25% - 60%, and the thickness of the porous membrane is 40 - 65 μm.

5. An asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that, The full width at half maximum of the porous membrane at around 2θ angle of 14.1° is 0.2 - 0.7, and the crystal grains are not less than 150 Å.

6. An asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that, The water contact angle of the inner surface is 95° - 120°, and the surface roughness Ra of the inner surface is 20 - 60 μm; and / or; The water contact angle of the outer surface is 100° - 130°, and the surface roughness Ra of the outer surface is 15 - 70 μm.

7. An asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that, Some adjacent inner degassing holes are separated by support fibers. The average SEM length of the support fibers is 50 - 500 nm, and the ratio of the average SEM length of the support fibers to the average SEM width of the support fibers is 1.5 - 15:1; wherein the length direction of the support fibers is consistent with the circumferential direction of the porous membrane, and the width direction of the support fibers is consistent with the length direction of the porous membrane.

8. An asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that, The inner degassing holes are stacked and arranged to form an inner degassing area. Some adjacent inner degassing areas are separated by a dense area. The average SEM width of the inner degassing area is 80 - 1200 nm, and the average SEM length of the inner degassing area is at least 1.5 times the average SEM width of the inner degassing area; wherein the length direction of the inner degassing area is consistent with the circumferential direction of the porous membrane, and the width direction of the inner degassing area is consistent with the length direction of the porous membrane.

9. An asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that The inner degassing holes include a number of elliptical ventilation holes. The average SEM major axis of the ventilation holes is 100 - 600 nm. The ratio of the average SEM major axis of the ventilation holes to the average SEM pore diameter of the ventilation holes is 1.5 - 10:

1. The ratio of the average SEM pore diameter of the ventilation holes to the average SEM width of the support fibers is 1.5 - 12:1; wherein the major axis direction of the ventilation holes is consistent with the length direction of the porous membrane.

10. An asymmetric PP hollow fiber porous membrane for degassing according to claim 8, characterized in that, The average SEM width of the dense region is 60 - 800 nm, and the ratio of the average SEM width of the support fibers to that of the dense region is 1:4 - 10; Among them, the width direction of the dense region is consistent with the length direction of the porous membrane.

11. An asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that, The main body includes a cortex layer and a support layer. One side of the support layer is the inner surface, and one side of the cortex layer is the outer surface. The other side of the cortex layer and the other side of the support layer are transitioned by continuous fibers. The support layer has porous fibers for forming a porous structure. The average SEM diameter of the porous fibers is 30 - 200 nm, and the density of the porous fibers is 10 - 300 per 1 μm 2 .

12. An asymmetric PP hollow fiber porous membrane for degassing according to claim 11, characterized in that, The gradient of the average SEM diameter of the porous fibers is 0.1 - 1.2 nm / μm; The gradient of the average SEM diameter of the porous fibers = (the average SEM diameter of the porous fibers near the inner surface - the average SEM diameter of the porous fibers far from the inner surface) / the thickness of the support layer.

13. The asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that, The outer surface is a dense surface, the hole area ratio of the outer surface is less than 1%, the cortex thickness is 0.1 - 3 μm, and the thickness of the support layer is at least 29 μm greater than the cortex thickness.

14. An asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that, The outer surface has an outer degassing area and an outer dense area. The porosity area ratio of the outer dense area is less than 1%. There are a number of outer degassing holes in the outer degassing area. The SEM average pore diameter of the outer degassing holes in the outer degassing area is 10 - 90 nm, and the density of the outer degassing holes is 10 - 80 holes / 1μm 2 .

15. An asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that, The outer surface has a number of outer degassing holes evenly distributed thereon. The average SEM pore diameter of the outer degassing holes is 10 - 90 nm, and the density of the outer degassing holes is 1 - 30 holes per 10 μm 2 .

16. An asymmetric PP hollow fiber porous membrane for degassing according to claim 1, characterized in that, The deamination efficiency of the porous membrane is at least greater than 80%; The permeation rate of O2 through the porous membrane is not less than 30 L / (min·bar·0.1 m 2 ); The water evaporation rate of the porous membrane is not higher than 0.10 ml / m 2 / min; The deoxygenation efficiency of the porous membrane is greater than or equal to 90%.

17. The preparation process of an asymmetric PP hollow fiber porous membrane for degassing according to any one of claims 1-16, characterized in that, It includes the following process steps: S1, Spinning: Blend and melt the PP material and the hydrophobic additive, and extrude through a die head to form a molded product with a hollow inner cavity. Among them, the extrusion screw is a special single-screw, the compression ratio of the feeding section is 1.05 - 1.4; the compression ratio of the mixing section is 1.4 - 2.7, and the mixing section includes mixing threads; the ratio of the length of the metering section to the screw diameter is 5 - 12, and the die head extrusion temperature is 165 - 180 °C; among them, the melting process includes melting, mixing, and metering steps, the metering temperature is 10 - 20 °C higher than the die head extrusion temperature, and the die head length-diameter ratio is 2 - 8; the isotacticity of the PP is not less than 95%; the melt index of the PP raw material is 1 - 7 g / min@(190 °C, 5 kg); S2, Pre-crystallization: Place the molded product in the air section for pre-crystallization; S3, Air-cooling crystallization: Blow and cool the molded product obtained in step S2 in an air atmosphere to obtain primary fibers; S4, First shaping: Conduct the first heat setting treatment on the primary fibers and stretch the primary fibers. Among them, the first heat setting temperature is 80 - 110 °C, and the stretching elongation rate is 0.5 - 2%, to obtain a heat-set semi-finished product; S5, Cold drawing to form pores: Conduct two cold drawing treatments on the heat-set semi-finished product. Among them, the ratio of the temperature of the first cold drawing to the first heat setting temperature is 1:(2 - 5), and the cold drawing rates of both cold drawing treatments are not higher than 20% / min, and the cold drawing rate of the first cold drawing is less than that of the second cold drawing, to obtain a cold-drawn semi-finished product; The elongation rate of the first cold drawing is 5% - 20%, and the elongation rate of the second cold drawing is 10% - 30%; S6, Heat drawing and expanding pores: Conduct two heat drawing and expanding pore treatments on the cold-drawn semi-finished product. Among them, the temperatures of the two heat drawings are 100 - 120 °C, and the heat drawing rates of both heat drawings are not higher than 5% / min, and the heat drawing rate of the first heat drawing is less than that of the second heat drawing, to obtain a heat-drawn semi-finished product; S7, Second shaping: Conduct the second heat setting treatment on the heat-drawn semi-finished product to produce a hollow fiber porous membrane.

18. The preparation process of an asymmetric PP hollow fiber porous membrane for degassing according to claim 17, characterized in that, In step S2, the temperature of the air section is 40 - 70 °C, and the residence time in the air section is 0.01 - 0.1 s; In step S3, the blowing temperature is 30 - 70 °C, and the blowing speed is 20 - 80 m / min.

19. The preparation process of an asymmetric PP hollow fiber porous membrane for degassing according to claim 17, characterized in that, In step S4, the time for the first heat setting is 20 - 50 min; In step S7, the temperature for the second heat setting is 110 - 140 °C, and the time for the second heat setting is 2 - 5 min.

20. The preparation process of an asymmetric PP hollow fiber porous membrane for degassing according to claim 17, characterized in that, In step S5, the cold drawing rate for the first cold drawing is 5 - 10% / min, the cold drawing rate for the second cold drawing is 15 - 20% / min, and the temperature of the second cold drawing is 0 - 5 °C higher than that of the first cold drawing; In step S6, the hot drawing rate for the first hot drawing is 0.2 - 0.8% / min, the hot drawing rate for the second hot drawing is 1.5 - 2% / min, the elongation rate of the first hot drawing is 20% - 80%, and the elongation rate of the second hot drawing is 40% - 100%.

21. Application of an asymmetric PP hollow fiber porous membrane for degassing according to any one of claims 1-16, characterized in that, The porous membrane is applied to the degassing of ultrapure water and / or the deammoniation of wastewater.

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