Process for producing asymmetric polyolefin hollow fiber membrane for degassing

Abstract

The application provides a preparation process of an asymmetric polyolefin hollow fiber membrane for degassing, comprising the following steps: S1, spinning, to obtain a semi-finished product; S2, cooling crystallization, to obtain a nascent fiber; S3, primary setting, to obtain a heat-set semi-finished product; S4, stretching to form pores, to obtain a hollow fiber membrane; the infrared absorption spectrum of the heat-set semi-finished product is measured, and the crystallization gradient coefficient X of the heat-set semi-finished product is calculated, wherein the crystallization gradient coefficient X is greater than or equal to 4; the crystallization gradient coefficient X is calculated according to the following formula: X = ABSin / ABSout; in the formula, ABSin is the absorption intensity of the inner surface of the heat-set semi-finished product at a crystal zone band; and ABSout is the absorption intensity of the outer surface of the heat-set semi-finished product at the crystal zone band. As long as the crystallization gradient coefficient of the heat-set semi-finished product meets certain conditions, the prepared hollow fiber membrane can have mechanical properties, degassing effect and service life; and generally, the degassing effect and the service life cannot be achieved simultaneously, and the degassing effect and the mechanical properties cannot be achieved simultaneously.

Description

Technical Field

[0001] This application relates to the field of membrane separation technology, and in particular to a process for preparing an asymmetric polyolefin hollow fiber membrane for degassing. Background Technology

[0002] Over the past few decades, membrane separation technology has been used more and more widely, in addition to filtration, in an increasing number of separation fields, such as distillation, absorption, desorption, extraction, and degassing.

[0003] Among these, membrane separation technology applied to degassing is currently one of the main research directions, with broad application prospects in petrochemical, microelectronics, and food industries. For example, in oilfield production, relevant standards clearly stipulate that water injected into oilfields must undergo deoxygenation treatment; water used in the microelectronics field has high oxygen content requirements, and with the increasing integration of circuits, the requirements for gas content in ultrapure water are also becoming higher; in beer production, deoxygenation treatment of dilution water can solve problems such as poor beer foam and overly concentrated taste.

[0004] There is already a considerable amount of publicly available literature regarding methods for preparing porous membrane materials. Patent publication number US4247498A describes the preparation of porous PP membrane materials using a thermally induced phase separation method; while patent publication number US3801404A describes the preparation of porous PP membrane materials using a melt-stretch method, which is the first time this method has been proposed worldwide. Tsinghua University's patent application publication number CN103464003A describes the preparation of a sponge-like porous PP membrane material by co-extruding a diluent and PP material.

[0005] Compared to flat sheet membranes, hollow fiber membranes have a larger gas-liquid two-phase contact area, making them more suitable for water degassing. Hollow fiber membranes are generally divided into two types: one is a porous hollow fiber membrane with largely interconnected internal and external pores and a certain degree of hydrophobicity; the other is an asymmetric hollow fiber membrane with both dense and porous layers. For example, Chinese invention patent application CN103551046A describes the preparation of a hydrophobic hollow fiber membrane using polypropylene as raw material through a melt stretching method, where the micropores on the inner and outer surfaces completely penetrate the wall thickness of the hollow fiber membrane.

[0006] For degassing efficiency, porous hollow fiber membranes with largely interconnected internal and external pores are still the most commonly used for degassing treatment. However, porous hollow fiber membranes are highly dependent on hydrophobicity. Once the membrane surface is wetted, it will lose its degassing performance. Although the dense layer of asymmetric hollow fiber membranes affects its degassing effect to some extent, it has a significantly improved service life.

[0007] To ensure a long service life and good degassing effect, hollow fiber membranes need to have a dense outer layer and an inner layer with a sufficiently large pore area ratio. In addition, while ensuring that the inner layer has a sufficiently large pore area ratio, due to the different crystallinity of different parts of the membrane fibers, it is necessary to ensure that the hollow fiber membrane does not easily collapse or break down during the stretching and pore-forming process, thereby affecting the mechanical properties of the prepared hollow fiber membrane. Summary of the Invention

[0008] To ensure that the prepared hollow fiber membrane has low damage sensitivity and good degassing effect, and to ensure that the membrane fibers do not easily collapse or break down during the preparation process, thereby affecting its mechanical properties, this application provides a preparation process for asymmetric polyolefin hollow fiber membranes for degassing.

[0009] The preparation process of the asymmetric polyolefin hollow fiber membrane for degassing provided in this application adopts the following technical solution:

[0010] A process for preparing an asymmetric polyolefin hollow fiber membrane for degassing includes the following steps:

[0011] S1. Spinning: The polyolefin material is melt-extruded and formed into a semi-molded product with a hollow inner cavity under the action of a cavity-forming fluid; the polyolefin material is at least one of PP, PE and PMP, the glass transition temperature of the polyolefin material is Tg, and the melting point of the polyolefin material is Tm.

[0012] S2. Cooling and crystallization: The semi-finished product obtained in step S1 is cooled and crystallized to obtain nascent fibers.

[0013] S3. First heat setting: The nascent fibers obtained in step S2 are heat-set and cooled to obtain a heat-set semi-finished product.

[0014] S4. Stretch to form holes. The heat-set semi-finished product obtained in step S3 is subjected to cold stretching to form holes, hot stretching to expand holes, and secondary shaping in sequence to obtain a hollow fiber membrane.

[0015] In step S3, the infrared absorption spectrum of the heat-set semi-finished product is measured, and the crystallization gradient coefficient X of the heat-set semi-finished product is calculated. The crystallization gradient coefficient X ≥ 4. The crystallization gradient coefficient X is calculated by the following formula:

[0016] X = ABS 内 / ABS 外 In the above formula, ABS 内 The absorption intensity at the crystalline region band on the inner surface of the heat-set semi-finished product; ABS 外 The absorption intensity at the crystalline region band on the outer surface of the heat-set semi-finished product.

[0017] Optionally, the crystallization gradient coefficient X satisfies 5 ≤ ​​X ≤ 7.

[0018] By adopting the above technical solution

[0019] The inventors of this application unexpectedly discovered that by measuring the infrared absorption spectra of the inner and outer surfaces of a heat-set semi-finished product using the ATR method, when the internal and external crystallization gradient coefficient X of the heat-set semi-finished product is ≥4, preferably 5-7, the hollow fiber membrane obtained after stretching and pore-forming treatment has a dense outer layer and a multi-microporous inner layer. However, when X < 4, the final hollow fiber membrane either has excessively high porosity in the outer layer, resulting in insufficient density, or excessively low porosity in the inner layer, leading to an uneven pore structure and poor overall performance; furthermore, when X < 4, the mechanical properties of the hollow fiber membrane are often also poor. In other words, the internal and external crystallization gradient coefficient of the heat-set semi-finished product is significantly correlated with the service life, degassing effect, and mechanical properties of the final hollow fiber membrane.

[0020] This is likely because, after the polyolefin material undergoes melt spinning in steps S1-S3 to obtain nascent fibers, the crystallinity, regularity, orientation, and crystal form of the fiber sheath and inner layers are basically fixed after a single shaping treatment. Under relatively reasonable conditions for the hot and cold drawing processes, for the inner layer of the fiber, higher crystallinity, orientation, and regularity result in more and more uniform microporous structures formed during the stretching and pore-forming process. Conversely, for the fiber sheath, lower crystallinity, orientation, and regularity result in fewer microporous structures formed during the stretching and pore-forming process, leading to higher sheath density.

[0021] The inventors of this application have discovered that limiting the crystallization gradient coefficient of the inner and outer surfaces of the heat-set semi-finished product can also ensure that the resulting hollow fiber membrane has good mechanical properties. This may be because when the crystallization gradient coefficient of the inner and outer surfaces of the heat-set semi-finished product is high, the crystallinity of the inner surface of the heat-set semi-finished product is high. Although more microporous structures are generated, the size uniformity and distribution uniformity of the microporous structures are also higher, thus reducing the possibility of stress concentration. Furthermore, a large number of interconnected lamellar crystals can give the hollow fiber membrane better strength and deformation capacity. In addition, a higher crystallization gradient coefficient means that the crystal regularity of the heat-set semi-finished product is higher, reducing the possibility of structural collapse during the stretching and pore-forming stage, resulting in hollow fiber membranes with fewer micro and macro defects. Therefore, degassing effect and mechanical properties can be achieved simultaneously.

[0022] It should be noted that the density mentioned above refers to the fact that, by observing a 10000X SEM image of the cortex, no obvious pore structure is found or the proportion of micropores is ≤5%, which means that the cortex is considered to be dense.

[0023] It should be noted that there is no absolutely accurate method for detecting the crystallinity of polyolefin materials. For example, even for the same polyolefin material, the crystallinity measured by DSC and density method will be different. Therefore, the crystallization gradient coefficient mentioned in this application is not a trend of the absolute value of crystallinity, but it can reflect the gradient of crystallinity to a certain extent.

[0024] The above limitations are unexpected for the following reasons. It is expected that introducing a dense skin structure would inevitably lead to a significant decrease in degassing efficiency. However, the inventors of this application have discovered that, in fact, by ensuring that the crystallization gradient coefficient of the heat-set semi-finished product meets certain conditions, the final hollow fiber membrane can maintain a dense skin while having a high porosity in the inner layer, thereby greatly reducing the impact of the dense skin on its degassing efficiency, thus achieving both service life and degassing efficiency. Furthermore, it is expected that to simultaneously obtain a long service life and good degassing efficiency, sufficiently high porosity must be introduced into the inner layer, which would inevitably affect the mechanical properties of the membrane fibers. However, the inventors of this application have discovered that when the crystallization gradient coefficient of the heat-set semi-finished product meets certain conditions, even with a high porosity introduced into the inner layer of the membrane fibers, the mechanical properties of the membrane fibers remain good, thus achieving both degassing efficiency and mechanical properties. Furthermore, since there is a strong correlation between the crystallization gradient coefficient of the heat-set semi-finished product and the feasibility of the process, this provides a new verification approach for the feasibility verification of the process, which has important reference value.

[0025] Optionally, the polyolefin material is PP. In step S3, the low crystallinity coefficient Y and crystallinity coefficient Z of the heat-set semi-finished product are further calculated. The low crystallinity coefficient Y is calculated by the following formula: Y = ABS 973 / ABS 1460 ABS 973 For heat-set semi-finished products at a wave number of 973 cm⁻¹ -1 Absorption strength at the point; ABS 1460 For heat-set semi-finished products at a wave number of 1460 cm⁻¹ -1 The absorption intensity at the location; the low crystallinity coefficient of the inner surface of the heat-set semi-finished product is Y. 内 The low crystallinity coefficient of the outer surface of the heat-set semi-finished product is Y. 外 The crystallinity coefficient Z is calculated by the following formula: Z = ABS 998 / ABS 1460 ABS 998 For heat-set semi-finished products at a wave number of 998 cm⁻¹ -1 Absorption strength at the point; ABS 1460 For heat-set semi-finished products at a wave number of 1460 cm⁻¹ -1 The absorption intensity at the point; the crystallinity coefficient of the inner surface of the heat-set semi-finished product is Z. 内The crystallinity coefficient of the outer surface of the heat-set semi-finished product is Z. 外 The low crystallinity coefficient Y and crystallinity coefficient Z of the inner surface of the heat-set semi-finished product satisfy at least one of the following conditions: (1) Y 内 / Z 内 ≤2; (2)Y 内 -Z 内 ≤0.2.

[0026] Optionally, the low crystallinity coefficient Y and crystallinity coefficient Z of the inner surface of the heat-set semi-finished product satisfy at least one of the following conditions: (1) 1.5 ≤ Y 内 / Z 内 ≤1.7; (2)Y 内 -Z 内 ≤0.16.

[0027] By adopting the above technical solution, for isotactic PP, 998cm -1 The corresponding coordinated movement of 11-12 repeating units in the crystalline region, 973 cm -1 This corresponds to the amorphous region and the five repeating units in the crystalline chain. 1460cm -1 The vibration at the point is independent of the spatial conformation of PP and is only related to the characteristic vibrational frequencies of the functional groups, therefore it was selected as an internal standard. The low crystallinity coefficient Y can characterize the proportion of amorphous regions on the inner and outer surfaces of the fiber to a certain extent, while the crystallinity coefficient Z can characterize the crystallinity of the inner and outer surfaces of the fiber to a certain extent.

[0028] The inventors of this application have discovered that when the ratio of the low crystallinity coefficient Y to the crystallinity coefficient Z of the inner surface of the fiber is not greater than 2, or when the difference between the low crystallinity coefficient Y and the crystallinity coefficient Z of the inner surface of the fiber is not greater than 0.2 (or even less than 0), the inner layer of the hollow fiber membrane generally has uniform pores, high porosity, and good pore structure, with relatively uniform lamellar thickness. These factors determine that the membrane fiber ultimately possesses both good degassing effect and good mechanical properties; both can be achieved simultaneously.

[0029] It should be noted that although Z is defined as the crystallinity coefficient, it cannot be simply considered a characterization of crystallinity. For example, for isotactic polypropylene, the coefficient 1170 cm⁻¹ reflects the crystallinity state. -1 Location, 998cm -1 841cm -1 Location, but 998cm -1 The peaks at certain points are sharper, thus allowing for more accurate representation. Therefore, the crystallinity coefficient is only positively correlated with crystallinity, and cannot be directly equated with crystallinity.

[0030] Optionally, the low crystallinity coefficient Y and crystallinity coefficient Z of the outer surface of the heat-set semi-finished product satisfy at least one of the following conditions: (1) Y 外 / Z 外 ≥9; (2)Y 外 -Z 外 ≥0.4.

[0031] Optionally, the low crystallinity coefficient Y and crystallinity coefficient Z of the outer surface of the heat-set semi-finished product satisfy at least one of the following conditions: (1) Y 外 / Z 外 ≥14; (2)Y 外 -Z 外 ≥0.55.

[0032] By adopting the above technical solution

[0033] The inventors of this application have also discovered that when the ratio of the low crystallinity coefficient Y of the outer surface of the heat-set semi-finished product to the crystallinity coefficient Z of the outer surface is not less than 9, or when the difference between the low crystallinity coefficient Y of the outer surface of the heat-set semi-finished product and the crystallinity coefficient Z of the outer surface is not less than 0.8, the skin of the hollow fiber membrane obtained in the end generally has a lower porosity (even close to or 0) and a smoother surface, and thus naturally has a longer service life.

[0034] Optionally, step S4 includes the following process steps:

[0035] S41. Cold drawing to form holes: The heat-set semi-finished product obtained in step S3 is subjected to cold drawing to obtain a cold-drawn semi-finished product; wherein, the cold drawing temperature is (Tg+30)~(Tg+80)℃, the cold drawing elongation is 20~40%, and the cold drawing rate is (10~30)% / min.

[0036] S42. Hot drawing and hole expansion: The cold-drawn semi-finished product obtained in step S41 is hot-drawn and hole expanded to obtain a hot-drawn semi-finished product; the hot drawing temperature is (Tm-30)~(Tm-60)℃, the hot drawing elongation is 2 to 6 times that of the cold drawing elongation, and the hot drawing rate is not higher than 15% of the cold drawing rate.

[0037] S43. Secondary heat setting: The hot-drawn semi-finished product obtained in step S42 is subjected to secondary heat setting treatment, and after cooling, hollow fiber membrane is obtained.

[0038] By adopting the above technical solution, based on the limited crystallization gradient coefficients of the inner and outer surfaces of the heat-set semi-finished product, the ability to obtain a dense skin layer, an inner layer with uniform pore size distribution and good pore structure is determined by the cold drawing pore formation stage and the hot drawing pore expansion stage.

[0039] This application specifically employs a cold drawing process with less and faster drawing combined with a hot drawing process with more and slower drawing, resulting in a higher porosity and better pore structure in the inner layer; a lower thickness while maintaining a dense outer layer; and a reduction in the possibility of structural collapse or failure due to stress concentration during the stretching and pore-forming process.

[0040] The appropriate hot and cold drawing process has a significant impact on the thickness and density of the skin layer. To improve the degassing effect of hollow fiber membranes, it is necessary to control not only the porosity of the inner layer but also the thickness of the dense skin layer (the thickness of the dense skin layer is a crucial factor affecting the degassing effect of the membrane fibers). This results in a hollow fiber membrane with a dense and thin skin layer and an inner layer with higher porosity and a better pore structure. The limitations on the hot and cold drawing process parameters in this application not only ensure a good pore structure in the inner layer but also ensure a low skin layer thickness and, based on this, a low porosity. This allows for a balance between service life and degassing effect, as well as between degassing effect and mechanical properties.

[0041] It is important to note that the hot drawing rate must be matched with the cold drawing rate. This is because different cold drawing rates result in different pore structures, different internal residual stress conditions, different degrees of lamellar separation, and different microfiber tethering structures. All of these conditions affect the hot drawing rate during the hot drawing stage. Therefore, the cold drawing rate or the hot drawing rate cannot be adjusted in isolation. The hot drawing elongation rate must also be matched with the cold drawing elongation rate, and the mechanism is roughly the same.

[0042] Optionally, the polyolefin material is PP, and in step S41,

[0043] The cold drawing process for creating holes is performed in 2 to 4 steps.

[0044] The cold drawing draw ratio for each cold drawing is 104.7–118.3%;

[0045] At the end of the cold drawing process, the linear speed of the cold-drawn semi-finished product is 2.5 to 3.5 m / min.

[0046] Optionally, each cold-drawn hole includes 3 to 5 stretching stages.

[0047] By adopting the above technical solution, the inventors unexpectedly discovered that, compared with single cold drawing, multiple cold drawing, especially the cold drawing method of multi-segment stretching in each cold drawing, can significantly improve the pore structure of the inner layer of the hollow fiber membrane, and the porosity and pore structure of the inner layer are improved.

[0048] This is likely because, although the total cold-drawing rate and elongation of multiple cold-drawing stages are the same as those of a single cold-drawing stage, the multiple cold-drawing process is actually a process in which the cold-drawing rate continuously increases. For example, if the cold-drawing rate of a single cold-drawing stage is defined as V0, the total cold-drawing rate of multiple cold-drawing stages should also be V0. Therefore, there must exist cold-drawing stages with elongation rates less than V0 and cold-drawing stages with elongation rates greater than V0. Correspondingly, during multiple cold-drawing stages, the tensile stress also gradually increases.

[0049] Furthermore, further subdividing each cold-drawn hole into 3-5 stretching segments can further improve the fineness of tensile stress growth, thereby further solving the problem of tensile stress concentration caused by excessive tensile stress variation. It should be noted that each cold-drawn hole should not be further subdivided. This is because, although further subdivision can further improve the fineness of tensile stress growth, the tensile stress growth is too slow, giving the molecular chains sufficient time to relax, and the separation effect of lamellar crystals is significantly worse.

[0050] For the cortex, the tensile stress is first increased by a small amount, and then gradually increased. Because the stress is dispersed and the defects are relaxed under the action of tensile stress during multiple cold drawing processes, the possibility of tensile stress concentrating at the defects in the cortex and causing the cortex to form a porous structure is greatly reduced.

[0051] For the inner layer, multiple cold drawing can separate different lamellar crystals in stages and gradually. Lamellar crystals that are easy to separate are separated by small tensile stress, while lamellar crystals that are difficult to separate are separated by larger tensile stress in the future. Since the molecular chains have enough time to relax, the problems of microfiber ties breaking and thick lamellar crystals being difficult to separate are greatly reduced, thereby obtaining greater porosity and better pore structure, so as to obtain better degassing effect and mechanical properties.

[0052] As mentioned above, the increasing cold drawing rate means that even if the cold drawing draw ratio is the same for each cold drawing, as the original fiber length continuously increases (i.e., the base length continuously increases), the absolute value of the cold drawing elongation rate between two cold drawings increases, even if the draw ratio remains unchanged. Correspondingly, the cold drawing rate between two cold drawings also increases. For example, if the first cold drawing stretches the fiber from 100% length to 110% (i.e., the cold drawing draw ratio is 110%), then if the cold drawing draw ratio for the second cold drawing is still 110%, the fiber length after the second cold drawing should be 110% * 110% = 121%. Therefore, the cold drawing rate is actually a continuously increasing process.

[0053] Optionally, in step S42,

[0054] The hot-drawing temperature is 110–130℃;

[0055] The elongation at room temperature is 90–110%.

[0056] The hot-drawing rate is (0.3~1.3)% / min.

[0057] By adopting the above technical solution, for the specific system of this application, the hot stretching temperature and hot stretching elongation greatly affect the final thickness of the skin layer, which in turn greatly affects the degassing efficiency and service life of the final hollow fiber membrane.

[0058] The inventors have discovered that when the present application specifically selects a multiple cold drawing process to cold draw the fiber, the above-mentioned hot drawing parameters can not only obtain a uniform and good pore structure, but also a good microfiber tying structure. In addition, the thickness and density of the cortex are also more suitable.

[0059] Optionally, step S2 includes the following process steps:

[0060] S21. Natural cooling: The semi-molded product obtained in step S1 is cooled by natural cooling to obtain a naturally cooled semi-finished product; the melting point of the polyolefin material is Tm, and the surface temperature of the naturally cooled semi-finished product obtained after natural cooling drops to Tm~(Tm+20)℃.

[0061] S22. Air-cooled crystallization: The naturally cooled semi-finished product obtained in step S21 is subjected to air-cooled crystallization by blowing air, and then wound up to obtain nascent fibers.

[0062] By adopting the above technical solution, the specific two-step cooling method can ensure that the required heat-set semi-finished product is obtained, and correspondingly, it can ensure that the hollow fiber membrane produced in the end has good degassing effect, service life and mechanical properties.

[0063] Optionally, the polyolefin material is PP;

[0064] In step S1, the die temperature during melt extrusion is 180–200°C.

[0065] In step S21, the surface temperature of the naturally cooled semi-finished product obtained after natural cooling is 168-178℃.

[0066] By adopting the above technical solution, limiting the melt extrusion temperature and the fiber surface temperature after natural cooling, it is possible to ensure that PP obtains greater and more uniform tensile stress during melt extrusion and natural cooling, promote the crystallization and lamellar orientation of PP, and improve the crystallinity and crystal orientation of the inner layer of the nascent fiber.

[0067] Optionally, in step S21, the path length for natural cooling is 30 to 1000 mm; the atmosphere for natural cooling is air or nitrogen, and the atmosphere temperature for natural cooling is 40 to 70°C; the cooling rate of the surface of the semi-molded product during natural cooling is (0.1 to 0.6)°C / mm@(100 to 200) m / min.

[0068] By adopting the above technical solution, the path length of natural cooling, the atmosphere temperature, and the cooling rate of the fiber surface during natural cooling all need to be strictly controlled. This is because the cooling behavior of the fiber during natural cooling greatly affects the magnitude and uniformity of the tensile stress on the fiber, and tensile stress is an important influencing factor on lamellar growth and orientation.

[0069] Under the aforementioned natural cooling conditions, the resulting hollow fiber membrane exhibits higher degassing efficiency and a better pore structure in the inner fiber layer. This is likely because, under these conditions, the polyolefin material is cooled to a suitable temperature at an appropriate cooling rate, resulting in fewer defects and greater dimensional stability in the membrane fibers. Furthermore, the fibers are subjected to uniform and appropriately sized tensile stresses, allowing the formation of more lamellar structures in the inner layer to facilitate higher porosity while maintaining high mechanical properties. This ensures that the crystallization gradient coefficient of the heat-set semi-finished product meets the aforementioned limitations, resulting in a hollow fiber membrane with not only excellent service life and degassing performance but also superior mechanical properties.

[0070] Optionally, in step S22, the speed of the cooling airflow is 25-35% of the winding speed; the temperature of the cooling airflow is 40-70°C; and the direction of the cooling airflow is inclined along the conveying direction of the naturally cooled semi-finished product.

[0071] By adopting the above technical solution, the speed and temperature of the cooling airflow need to be matched with the fiber winding speed. The faster the fiber winding speed, the faster the cooling airflow should also be to ensure that the fiber has a reasonable cooling rate and is less prone to defects due to inappropriate cooling speed. In addition, a suitable cooling rate has a significant impact on the thickness of the sheath. A reasonable cooling rate can ensure that the sheath is dense while keeping the thickness as low as possible, so as to obtain good degassing effect and service life at the same time.

[0072] Furthermore, after polyolefin materials are melt-extruded and naturally cooled, their temperature remains above the melting point. If air is blown vertically, the fibers are significantly disturbed by the cooling airflow, making them prone to swaying and reducing dimensional stability. However, setting the direction of the cooling airflow to be the same as the fiber conveying direction and at a certain angle can greatly reduce the disturbance of the cooling airflow on the fibers, thereby improving the dimensional stability of the fibers.

[0073] Optionally, the polyolefin material is characterized by: the isotacticity of PPPP is not less than 95%; the melt index of PP is 1-7 g / min@190℃, 5 kg; the crystallinity of PP is 45-70%; and the wall thickness of the semi-molded product is 0.5-3 mm.

[0074] By adopting the above technical solution, PP is a high-performance polyolefin material, which is very suitable for preparing hollow fiber membranes for degassing. However, not all PP materials are suitable for the preparation process in this application. If the crystallinity of PP is too low, the inner layer of the final hollow fiber membrane will have difficulty forming sufficient porosity during the stretching and pore-forming stage. A dense skin layer and low inner layer porosity mean a poor degassing effect. If the crystallinity of PP is too high, the skin layer will have low density, and the inner layer will have excessively high porosity and large size. Although the degassing effect will be better, the service life and mechanical properties will often be poor.

[0075] Furthermore, when the melt flow index is low, the resistance to molecular chain arrangement increases, the activation energy required for the diffusion of the molecular chain into a crystalline phase increases, resulting in a decrease in the ability of the molecular chains to arrange themselves in a regular pattern, thereby reducing the crystallinity. When the melt flow index is high, the material's plasticity deteriorates, making it difficult to shape.

[0076] In summary, this application includes at least one of the following beneficial technical effects:

[0077] 1. The inventors of this application have unexpectedly discovered that when the crystallization gradient coefficient of the heat-set semi-finished product meets certain conditions, the service life and degassing effect, which are not conventionally known to be simultaneously obtainable, can be simultaneously obtainable; the degassing effect and mechanical properties, which are not conventionally known to be simultaneously obtainable, can be simultaneously obtainable; and the crystallization gradient coefficient has important guiding significance for the verification of process feasibility.

[0078] 2. By limiting the infrared absorption spectrum data of the inner and outer surfaces of the heat-set semi-finished product, it is possible to ensure that the hollow fiber membrane has a dense outer layer and an inner layer with sufficiently high porosity, thereby achieving a balance between degassing effect, mechanical properties and service life; in addition, it provides important theoretical guidance for raw material selection and spinning process adjustment.

[0079] 3. By using a cold drawing process with less and faster stretching combined with a hot drawing process with slower and more stretching, structural defects caused by stress concentration and excessive stretching during the stretching and pore-forming stage of hollow fiber membranes can be further reduced. Attached Figure Description

[0080] Figure 1 This is an SEM image of the outer surface of the hollow fiber membrane prepared in Example 5 of this application, with a magnification of 10000X.

[0081] Figure 2 This is an SEM image of the inner surface of the hollow fiber membrane prepared in Example 5 of this application, with a magnification of 10000X.

[0082] Figure 3 This is a SEM image of the inner surface of the hollow fiber membrane prepared in Comparative Example 3 of this application, with a magnification of 30000X.

[0083] Figure 4 This is a SEM image of the inner surface of the hollow fiber membrane prepared in Comparative Example 3 of this application, with a magnification of 30000X. Detailed Implementation

[0084] The following is in conjunction with the appendix Figure 1-4 This application will be described in further detail.

[0085] Unless otherwise specified, the raw materials and equipment used in the preparation of hollow fiber membranes in the following embodiments are commercially available. The structural morphology of the filter membrane was characterized using a Hitachi S-5500 scanning electron microscope.

[0086] It is important to note that since the hollow fiber membrane prepared in this application is mainly used in the degassing step of ultrapure water preparation (of course, it can also be used for degassing in other fields), due to the characteristics of ultrapure water, the leakage effect of the hollow fiber membrane needs to be strictly controlled. For example, when preparing hollow fiber membranes by methods such as thermally induced phase separation and liquid quenching, solvents are required, and some solvent will inevitably remain in the prepared hollow fiber membrane (even after post-treatment to remove the solvent). This portion of solvent is likely to leak out during the ultrapure water degassing process, leading to ultrapure water contamination. For example, to obtain better mechanical properties, various additives, such as fillers and nucleating agents, are added to polyolefin materials. Similarly, although the above additives can improve the mechanical properties of the prepared hollow fiber membrane to a certain extent, the risk of impurity leakage contradicts the purpose of this application.

[0087] Based on the above, this application discloses a process for preparing an asymmetric polyolefin hollow fiber membrane for degassing. Preferably, the hollow fiber membrane can be used for the degassing treatment of ultrapure water.

[0088] Example 1

[0089] The preparation process of asymmetric polyolefin hollow fiber membranes for degassing includes the following steps:

[0090] In this embodiment, the polyolefin material selected is PP, preferably isotactic homopolymer PP. This PP has a crystallinity of 50% (measured by DSC), an isotacticity of 99.9%, a molecular weight of approximately 80,000, a molecular weight distribution index of 1.5, and a melt index of approximately 2 g / min@(190℃, 5kg). In addition, the melting point Tm of this PP is approximately 168℃, and the glass transition temperature Tg is -10℃.

[0091] The preparation process of hollow fiber membrane for degassing using the above-mentioned PP as raw material includes the following process steps:

[0092] S1. Spinning: The PP is placed in an extruder for melt mixing. Melt mixing includes melting, mixing, and metering in sequence, with temperatures set at 165℃, 200℃, and 205℃ respectively. After melt mixing, the molten material is extruded into fibers using a vertical downward spinning extrusion method. A hollow die is used during extrusion, and a cavity-forming fluid is injected into the hollow cavity of the die. In this embodiment, the cavity-forming fluid is nitrogen (note that the cavity-forming fluid can also be other inert gases, or even liquid fluids). The material outlet wall thickness on the die is 2mm (i.e., the wall thickness is 2mm during fiber extrusion). The die temperature is 190℃, and the die length-to-diameter ratio is 4. Under the action of the cavity-forming fluid, the material is extruded to form a semi-finished product with a hollow cavity. At this time, the semi-finished product is still in a liquid state.

[0093] S2. Cooling and crystallization, which is divided into natural cooling and air cooling crystallization carried out sequentially, i.e., step S2 includes the following process steps:

[0094] S21. Natural cooling: The semi-finished product obtained by extruding the molten material is exposed to air (or an inert gas such as nitrogen) for initial natural cooling. Natural cooling continues until the fiber surface temperature reaches approximately 170°C. The naturally cooled semi-finished product is then obtained. During this process, the path length for natural cooling is controlled to 100mm, and the temperature of the atmosphere surrounding the semi-finished product is controlled to 60°C. The cooling rate of the fiber surface is approximately 0.2°C / mm (150m / min).

[0095] S22. Air-cooled crystallization: After the fiber surface is cooled to near the melting point of the material to obtain a naturally cooled semi-finished product, the fiber is further cooled by blowing air to induce cooling crystallization. In this step, the speed of the cooling airflow is 40 m / min, the temperature of the cooling airflow is 60°C, the direction of the cooling airflow is inclined towards the direction of fiber conveying, and the angle between the direction of the cooling airflow and the axis of the fiber is 50°.

[0096] It is important to note that the air-cooled crystallization process in this step is divided into two sections: a rapid cooling zone and a slow cooling zone. The area where the cooling airflow moves along its direction and directly contacts the fiber is considered the rapid cooling zone. Rapid heat exchange occurs between the cooling airflow and the fiber in this area, resulting in rapid cooling of the fiber surface. The area from the rapid cooling zone to the winding stage is considered the slow cooling zone. After the fiber passes through both the rapid and slow cooling zones, it can be wound up to obtain nascent fiber.

[0097] After passing through the rapid cooling zone, the fiber surface temperature drops to approximately 90°C. It then passes through a slow cooling zone with a path length of 8m for winding at a speed of 150m / min (i.e., spinning speed), with a nozzle draw ratio of 40. The fiber cooling rate in the rapid cooling zone is 200°C / mm@(150m / min).

[0098] It should be noted that the width of the rapid cooling zone varies slightly depending on the angle between the cooling airflow and the fiber. Therefore, the boundary between the rapid cooling zone and the slow cooling zone is determined by the cooling rate of the fiber surface. When the cooling rate of the fiber surface decreases significantly, it is considered that the fiber has moved from the rapid cooling zone to the slow cooling zone.

[0099] S3. First heat setting: The nascent fibers obtained from the winding undergo a first heat setting treatment at a temperature of 130℃ for 30 minutes. During the heat setting process, the nascent fibers are stretched with an elongation rate of 1%. After the first heat setting treatment, the nascent fibers are obtained as a heat-set semi-finished product.

[0100] S4. Drawing to form a hole, which includes sequential cold drawing to form a hole, hot drawing to expand the hole, and secondary shaping. Specifically, step S4 includes the following process steps:

[0101] S41. Cold drawing to create pores: After the first heat setting treatment, the defects within the nascent fiber are largely eliminated. At this point, the fiber can be cold-drawn to initially create a microporous structure in the inner layer where the fiber has a higher degree of crystallinity. In the cold drawing to create pores stage, a single cold drawing process is used, with a cold drawing elongation rate of 30%, a cold drawing rate of 20% / min, and a cold drawing temperature of 30℃. After cold drawing, a cold-drawn semi-finished product is obtained, with a linear speed of 3m / min.

[0102] S42. Hot-drawing and pore expansion: After cold drawing, the inner layer of the fiber separates into lamellar crystals, creating a microporous structure. This can be further expanded by hot drawing. Hot drawing also further separates any unseparated lamellar crystals from the cold drawing process, resulting in a greater number of microporous structures within the fiber's inner layer. In the hot-drawing and pore expansion stage, a single hot-drawing process is used, with a hot-drawing elongation rate of 100%, a hot-drawing rate of 2.4% / min, and a hot-drawing temperature of 120℃. The resulting hot-drawn semi-finished product has a linear velocity of 6 m / min.

[0103] S43. Secondary heat setting: After cold and hot stretching, the microporous structure of the inner layer of the fiber is basically formed. However, once the stretching force is removed, the fiber is prone to shrinkage, which leads to a reduction or even partial closure of the microporous structure. Therefore, a second heat setting treatment is required after the cold and hot stretching process. In the second heat setting stage, the heat setting temperature is 140℃ and the heat setting time is 2 minutes. After the heat setting is completed, the hollow fiber membrane is obtained by natural cooling.

[0104] Example 2

[0105] The main difference between Example 2 and Example 1 is that a secondary cold drawing process is used when cold drawing to create holes. That is, step S41 includes the following process steps:

[0106] S41. Cold drawing to form a hole: In the cold drawing to form a hole stage, a two-stage cold drawing process is adopted. After the two cold drawings, the total cold drawing elongation is still 30%, the average cold drawing rate is still 20% / min, and the cold drawing temperature is still 30℃. After the cold drawing is completed, a cold-drawn semi-finished product is obtained. The cold drawing draw ratio during both cold drawings is 114%, and the linear speed of the cold-drawn semi-finished product at the end of the cold drawing is 3m / min.

[0107] Example 3

[0108] The main difference between Example 3 and Example 2 is that during cold drawing to create holes, each cold drawing includes four stretching stages. As the fiber passes through the two stretching rollers, the fiber is stretched due to the speed difference between the two rollers. That is, when the stretching equipment used for cold drawing has different numbers of stretching rollers, different numbers of stretching stages can be performed in a single cold drawing operation. The equipment in Example 2 has two stretching rollers (one stretching stage), while the equipment in this example has five stretching rollers (four stretching stages).

[0109] Example 4

[0110] The main difference between Example 4 and Example 3 is that a three-stage hot drawing process is used during hot drawing and hole expansion. That is, step S42 includes the following process steps:

[0111] S42. Hot drawing and hole expansion: In the hot drawing and hole expansion stage, a three-stage hot drawing process is adopted. After the three hot drawing stages, the total hot drawing elongation is still 100%, the average hot drawing rate of the three hot drawing stages is still 2.4% / min, and the hot drawing temperature of the three hot drawing stages is 120℃. After hot drawing, a hot-drawn semi-finished product is obtained.

[0112] Example 5

[0113] The main difference between Example 5 and Example 4 is that during hot drawing and hole expansion, each hot drawing includes 30 stretching segments. That is, when the stretching equipment performing the hot drawing operation has different numbers of stretching rollers, different numbers of stretching segments can be performed in one hot drawing operation. The equipment in Example 7 has 2 stretching rollers (1 stretching segment), while the equipment in this example has 31 stretching rollers (30 stretching segments).

[0114] Example 6

[0115] The main difference between Example 6 and Example 5 is that, in step S3, the angle between the direction of the cooling airflow and the axis of the naturally cooled semi-finished product is 90°.

[0116] Examples 7-9

[0117] The main difference between Examples 7-9 and Example 5 is that the process parameters for each step are different, and the process parameters for each step are recorded in the table below:

[0118]

[0119]

[0120] Example 10

[0121] In this embodiment, the polyolefin material is selected as PE, and more preferably, commercially available mLLDPE (metallocene linear low-density polyethylene) is used. This mLLDPE has a crystallinity of 68% (measured by DSC) and a density of 0.93 g / cm³. 3 The molecular weight is 60,000, the molecular weight distribution index is approximately 2.3, and the melt index is 4.5 g / min@(130℃, 5 kg). Furthermore, this mLLDPE has a melting point (Tm) of approximately 119℃ and a glass transition temperature (Tg) of -38℃.

[0122] The preparation process of hollow fiber membrane for degassing using the above-mentioned mLLDPE as raw material includes the following process steps:

[0123] S1. Spinning: The above-mentioned mLLDPE is placed in an extruder for melt mixing. Melt mixing includes melting, mixing, and metering in sequence, with temperatures set at 135℃, 125℃, and 150℃, respectively. After melt mixing, the molten material is extruded into fibers using a vertical downward spinning extrusion method. A hollow die is used during extrusion, and a cavity-forming fluid is injected into the hollow cavity of the die. In this embodiment, the cavity-forming fluid is nitrogen. The material outlet wall thickness on the die is 2mm (i.e., the wall thickness is 2mm during fiber extrusion). The die temperature is 140℃, and the die length-to-diameter ratio is 5. Under the action of the cavity-forming fluid, the material is extruded to form a semi-finished product with a hollow inner cavity. At this time, the semi-finished product is still in a liquid state.

[0124] S2. Cooling and crystallization, which is divided into natural cooling and air cooling crystallization carried out sequentially, i.e., step S2 includes the following process steps:

[0125] S21. Natural cooling: The semi-finished product obtained by extruding the molten material is exposed to air for initial natural cooling until the surface temperature of the fiber reaches approximately 125°C. Natural cooling is then complete, yielding a naturally cooled semi-finished product. During this process, the path length for natural cooling is controlled to be 300 mm, and the temperature of the atmosphere surrounding the semi-finished product is controlled to be 70°C. The cooling rate of the fiber surface is approximately 0.05°C / mm (120 m / min).

[0126] S22. Air-cooled crystallization: After the fiber surface is cooled to near the melting point of the material to obtain a naturally cooled semi-finished product, the fiber is further cooled by blowing air to induce cooling crystallization. In this step, the speed of the cooling airflow is 33 m / min, the temperature of the cooling airflow is 70°C, the direction of the cooling airflow is inclined towards the direction of fiber conveying, and the angle between the direction of the cooling airflow and the axis of the fiber is 60°.

[0127] It is important to note that the air-cooled crystallization in this step is divided into two sections: a rapid cooling zone and a slow cooling zone. After passing through the rapid cooling zone, the fiber surface temperature drops to approximately 80°C. It then passes through a 6m long slow cooling zone for winding at a speed of 120m / min (i.e., spinning speed), with a corresponding nozzle draw ratio of 40. The fiber cooling rate in the rapid cooling zone is 150°C / mm (120m / min).

[0128] It should be noted that the boundary between the rapid cooling zone and the slow cooling zone is determined by the cooling rate of the fiber surface. When the cooling rate of the fiber surface decreases significantly, it is considered that the fiber has moved from the rapid cooling zone to the slow cooling zone.

[0129] S3. First heat setting: The nascent fibers obtained from the winding undergo a first heat setting treatment at a temperature of 95℃ for 30 minutes. During the heat setting process, the nascent fibers are stretched with an elongation rate of 1%. After the first heat setting treatment, the nascent fibers are obtained as a heat-set semi-finished product.

[0130] S4. Cold drawing to form a hole: In the cold drawing to form a hole stage, a single cold drawing process is adopted. The cold drawing elongation rate is 30%, the cold drawing rate is 20% / min, and the cold drawing temperature is 25℃. After cold drawing, a cold-drawn semi-finished product is obtained. At this time, the linear speed of the cold-drawn semi-finished product is 3m / min.

[0131] S5. Hot drawing and hole expansion: In the hot drawing and hole expansion stage, a one-time hot drawing process is adopted. The hot drawing elongation rate is 100%, the hot drawing rate is 2% / min, and the hot drawing temperature is 85℃. After hot drawing, a hot-drawn semi-finished product is obtained. At this time, the linear speed of the hot-drawn semi-finished product is 6m / min.

[0132] S6. Secondary heat setting: The second heat setting stage is carried out at a temperature of 110℃ for 1.5 minutes. After heat setting, the hollow fiber membrane is obtained by natural cooling.

[0133] Example 11

[0134] In this embodiment, the polyolefin material used is PMP, which was purchased from Mitsui, Japan, under the brand name TPX. The crystallinity of this TPX is 55% (measured by DSC), and its density is 0.83 g / cm³. 3 The molecular weight is 98,000, the molecular weight distribution index is 3.2, the Vicat softening temperature is 165℃, and the melt index is 5 g / min@(240℃, 5 kg). Furthermore, the melting point Tm of this TPX is approximately 240℃, and the glass transition temperature Tg is 28℃.

[0135] The preparation process of hollow fiber membrane for degassing using the above-mentioned TPX as raw material includes the following process steps:

[0136] S1. Spinning: The TPX is placed in an extruder for melt mixing. Melt mixing includes melting, mixing, and metering in sequence, with temperatures set at 250℃, 265℃, and 260℃, respectively. After melt mixing, the molten material is extruded into fibers using a vertical downward spinning extrusion method. A hollow die is used during extrusion, and a cavity-forming fluid is injected into the hollow cavity of the die. In this embodiment, the cavity-forming fluid is nitrogen. The material outlet wall thickness on the die is 2mm (i.e., the wall thickness is 2mm during fiber extrusion). The die temperature is 255℃, and the die length-to-diameter ratio is 4. Under the action of the cavity-forming fluid, the material is extruded to form a semi-finished product with a hollow cavity. At this time, the semi-finished product is still in a liquid state.

[0137] S2. Natural cooling: The semi-finished product obtained by extruding the molten material is exposed to air for initial natural cooling until the surface temperature of the fiber reaches approximately 245°C. Natural cooling is then complete, yielding a naturally cooled semi-finished product. During this process, the path length for natural cooling is controlled to 100 mm, and the temperature of the atmosphere surrounding the semi-finished product is controlled to 50°C. The cooling rate of the fiber surface is approximately 0.1°C / mm (150 m / min).

[0138] S3. Air-cooled crystallization: After the fiber surface is cooled to near the melting point of the material to obtain a naturally cooled semi-finished product, the fiber is further cooled by blowing air to induce cooling crystallization. In this step, the speed of the cooling airflow is 52 m / min, the temperature of the cooling airflow is 70°C, the direction of the cooling airflow is inclined towards the direction of fiber conveying, and the angle between the direction of the cooling airflow and the axis of the fiber is 45°.

[0139] It is important to note that the air-cooled crystallization in this step is divided into two sections: a rapid cooling zone and a slow cooling zone. After passing through the rapid cooling zone, the fiber surface temperature drops to approximately 130°C. It then passes through a 7m long slow cooling zone for winding at a speed of 150m / min (i.e., the spinning speed), with a corresponding nozzle draw ratio of 40. The fiber cooling rate in the rapid cooling zone is 230°C / mm@(150m / min).

[0140] It should be noted that the width of the rapid cooling zone varies slightly depending on the angle between the cooling airflow and the fiber. Therefore, the boundary between the rapid cooling zone and the slow cooling zone is determined by the cooling rate of the fiber surface. When the cooling rate of the fiber surface decreases significantly, it is considered that the fiber has moved from the rapid cooling zone to the slow cooling zone.

[0141] S4. Drawing to form a hole, which includes sequential steps of primary shaping, cold drawing to form a hole, hot drawing to expand the hole, and secondary shaping. Specifically, step S4 includes the following process steps:

[0142] S41. First heat setting: The nascent fibers obtained from the winding undergo a first heat setting treatment at a temperature of 180℃ for 25 minutes. During the heat setting process, the nascent fibers are stretched with an elongation rate of 1.5%. After the first heat setting treatment, the nascent fibers are obtained as a heat-set semi-finished product.

[0143] S42. Cold drawing to form holes: In the cold drawing to form holes stage, a one-time cold drawing process is adopted. The cold drawing elongation rate is 25%, the cold drawing rate is 15% / min, and the cold drawing temperature is 35℃. After cold drawing, a cold-drawn semi-finished product is obtained.

[0144] S43. Hot drawing and hole expansion: In the hot drawing and hole expansion stage, a one-time hot drawing process is adopted. The hot drawing elongation rate is 150%, the hot drawing rate is 1.2% / min, and the hot drawing temperature is 180℃. After hot drawing, a hot-drawn semi-finished product is obtained.

[0145] S44. Secondary heat setting: The second heat setting stage is conducted at a temperature of 200℃ for 1 minute. After heat setting, the hollow fiber membrane is obtained by natural cooling.

[0146] Comparative Example

[0147] Comparative Example 1

[0148] The main difference between Comparative Example 1 and Example 5 is that in step S22, the fibers are rapidly cooled by liquid cooling.

[0149] That is, step S22 of Comparative Example 1 includes the following process steps:

[0150] S22, liquid-cooled crystallization, involves cooling the fiber surface to near the material's melting point to obtain a naturally cooled semi-finished product, followed by rapid liquid cooling. In this step, deionized water is used as the cooling medium at 40°C, and the cooling path is 1 meter long. After liquid cooling, the fibers are dried in a 70°C oven and then wound up to obtain nascent fibers.

[0151] Comparative Example 2

[0152] The main difference between Comparative Example 2 and Example 5 is that step S22 of Comparative Example 2 includes the following process steps:

[0153] After the fiber surface is cooled to near the melting point of the material to obtain a naturally cooled semi-finished product, the fiber is further cooled by blowing air to induce cooling crystallization. In this step, the cooling airflow velocity is 20 m / min, the cooling airflow temperature is 70°C, the cooling airflow direction is inclined towards the fiber conveying direction, and the angle between the cooling airflow direction and the fiber axis is 30°.

[0154] After passing through the rapid cooling zone, the fiber surface temperature drops to approximately 120°C. It then passes through a slow cooling zone with a path length of 8m for winding at a speed of 150m / min (i.e., spinning speed). Correspondingly, the nozzle draw ratio is 40. The fiber cooling rate in the rapid cooling zone is approximately 86°C / mm (150m / min).

[0155] Comparative Example 3

[0156] The main difference between Comparative Example 3 and Example 5 is that in step S22, the fibers were not cooled by blowing air, but were cooled by air cooling.

[0157] That is, step S22 of Comparative Example 3 includes the following process steps:

[0158] After the fiber surface is cooled to near the melting point of the material to obtain a naturally cooled semi-finished product, the fiber is further cooled naturally until the surface temperature of the fiber is approximately 80°C. During this process, the path length of the natural cooling is controlled to be 8m, and the temperature of the atmosphere around the semi-finished product is controlled to be 60°C. The cooling rate of the fiber surface is approximately 0.011°C / mm@(150m / min).

[0159] Performance testing methods and performance testing data

[0160] I. Gas Flux Detection

[0161] Using the hollow fiber membranes prepared in the various embodiments or comparative examples as raw materials, membranes with an area of ​​0.1 m² were assembled. 2The component was used as a sample to detect the gas flux.

[0162] Gases of oxygen, nitrogen, and carbon dioxide at a pressure of 0.1 MPa are introduced into the module inlet. The module outlet is connected to a flow meter to record the gas flow rate of the module per unit time.

[0163] Generally speaking, the higher the gas flux, the higher the degassing efficiency of the component, and correspondingly, the higher the degassing efficiency of the hollow fiber membrane.

[0164] II. Degassing efficiency and degradation sensitivity

[0165] 2.1 Deoxygenation efficiency

[0166] Using the hollow fiber membranes prepared in the various embodiments or comparative examples as raw materials, membranes with an area of ​​0.65 m² were assembled. 2 The components are connected to a dissolved oxygen meter, a water circuit, and the components for testing. The water circuit is used to transport the degassed liquid, the components are used to degauge the degassed liquid, and the dissolved oxygen meter is used to detect the oxygen content of the degassed liquid after degauge treatment.

[0167] The membrane is purged with deionized water at a temperature of 25°C on the outer side. Vacuum purging is performed on the inner side of the membrane.

[0168] Step 1: Detect the initial oxygen content of the degassed liquid. Pump the degassed liquid into the water circuit. At this time, turn off the vacuum equipment to maintain atmospheric pressure inside the membrane. After passing through the module (without degasing), the degassed liquid passes through the dissolved oxygen meter, maintaining a flow rate of approximately 1.8 GLH. Observe the changes in the dissolved oxygen reading on the dissolved oxygen meter in real time. Once the dissolved oxygen meter reading stabilizes (the change in the dissolved oxygen meter reading is less than 1% within 5 minutes), read the dissolved oxygen reading. 始 .

[0169] Step 2: Detect the final oxygen content of the degassed liquid. Based on Step 1, turn on the vacuum equipment to perform vacuum purging on the inner layer of the membrane to degauge the liquid. During vacuum purging, maintain the vacuum reading at -0.094 MPa (50 torr). Observe the changes in the dissolved oxygen reading on the dissolved oxygen meter in real time. Once the dissolved oxygen meter reading stabilizes (the change in the dissolved oxygen meter reading is less than 1% within 5 minutes), it is considered that degassed has begun and equilibrium has been reached. Read the dissolved oxygen reading on the dissolved oxygen meter. 终 The deoxygenation efficiency is calculated using the following formula:

[0170]

[0171] 2.2 Sensitivity to acid damage

[0172] Based on test method 2.1, the degassing liquid was replaced with a 5 mol / L hydrochloric acid solution. The module condition was observed every 4 hours. When leakage occurred in the inner layer of the membrane, the module was considered to have been breached and lost its degassing ability. The time when the module was breached was recorded.

[0173] 2.3 Alkali-resistant degradation sensitivity

[0174] Based on test method 2.1, the degassing liquid was replaced with a sodium hydroxide solution with a concentration of 5 mol / L. The condition of the module was observed every 4 hours. When leakage occurred in the inner layer of the membrane, the module was considered to have been breached and lost its degassing ability. The time when the module was breached was recorded.

[0175] 2.4 Sensitivity to wetting damage

[0176] Based on test method 2.1, the degassing liquid was replaced with 100% n-butanol (surface tension of 24.6 dynes). The module condition was observed every 4 hours. When leakage occurred in the inner layer of the membrane, the module was considered to have been breached and lost its degassing ability. The time when the module was breached was recorded.

[0177] III. Mechanical Properties

[0178] The hollow fiber membranes prepared in each embodiment and comparative example were subjected to tensile property tests. The two ends of the hollow fiber membrane were clamped with clamps, and the distance between the two clamps was 5 cm (that is, the actual test length of the hollow fiber membrane was 5 cm). The tensile strength and elongation at break of the hollow fiber membrane were tested.

[0179] 3.1 Tensile Strength

[0180] Tensile strength:

[0181] In the formula: σ—tensile strength; Fb—maximum force at break; So—original cross-sectional area of ​​the specimen; in the instrument, it is represented by "specimen area S".

[0182] 3.2 Elongation at break

[0183] Elongation at break:

[0184] Where: e—elongation at break; ΔL—increment of the length between gauge lengths of the specimen; L—gauge length of the specimen.

[0185] IV. Infrared Absorption Spectroscopy

[0186] The infrared absorption spectra of the inner and outer sides of the heat-set semi-finished product obtained in step S3 were determined by ATR method, and the crystallization gradient coefficient X, low crystallization coefficient Y and crystallization coefficient Z of the heat-set semi-finished product were calculated.

[0187] For hollow fiber membranes made from PP, the crystallization gradient coefficient X, low crystallinity coefficient Y, and crystallinity coefficient Z of the heat-set semi-finished product are calculated using the following formulas:

[0188] X = ABS 998内 / ABS 998外 ;

[0189] ABS 内 The inner surface of the heat-set semi-finished product is at a wave number of 998 cm⁻¹ -1 Absorption strength at the point; ABS 外 The outer surface of the heat-set semi-finished product is at a wave number of 998 cm⁻¹ -1 The absorption intensity at that location.

[0190] Y = ABS 973 / ABS 1460 ;

[0191] ABS 973 For heat-set semi-finished products at a wave number of 973 cm⁻¹ -1 Absorption strength at the point; ABS 1460 For heat-set semi-finished products at a wave number of 1460 cm⁻¹ -1 Absorption intensity at the location;

[0192] The low crystallinity coefficient of the inner surface of the heat-set semi-finished product is Y. 内 The low crystallinity coefficient of the outer surface of the heat-set semi-finished product is Y. 外 .

[0193] Z = ABS 998 / ABS 1460 ;

[0194] ABS 998 For heat-set semi-finished products at a wave number of 998 cm⁻¹ -1 Absorption strength at the point; ABS 1460 For heat-set semi-finished products at a wave number of 1460 cm⁻¹ -1 Absorption intensity at the location;

[0195] The crystallinity coefficient of the inner surface of the heat-set semi-finished product is Z. 内 The crystallinity coefficient of the outer surface of the heat-set semi-finished product is Z. 外。

[0196] Furthermore, to facilitate the characterization of the internal and external infrared absorption spectra of the heat-set semi-finished product, the low crystallinity coefficient Y and the crystallinity coefficient Z are characterized using the following formulas:

[0197] A = Y 内 / Z 内 B = Y 内 -Z 内 C = Y 外 / Z外 D = Y 外 -Z 外 .

[0198] For hollow fiber membranes made from PE, the crystallization gradient coefficient X of the heat-set semi-finished product is calculated using the following formula:

[0199] X = ABS (731+720)内 / ABS (731+720)外 ;

[0200] ABS (731+720)内 For the inner surface of the heat-set semi-finished product at wave numbers 731 and 720 cm⁻¹ -1 The sum of the absorption strength at each point; ABS (731+720)外 For the outer surface of the heat-set semi-finished product at wave numbers 731 and 720 cm⁻¹ -1 The sum of the absorption intensities at each point.

[0201] For hollow fiber membranes made from PMP as raw material, the crystallization gradient coefficient X of the heat-set semi-finished product is calculated by the following formula:

[0202] X = ABS 918内 / ABS 918外 ;

[0203] ABS 内 The inner surface of the heat-set semi-finished product is at a wave number of 918 cm⁻¹ -1 Absorption strength at the point; ABS 外 The outer surface of the heat-set semi-finished product is at a wave number of 918 cm⁻¹ -1 The absorption intensity at that location.

[0204] The morphological features of each embodiment and comparative example are detailed in Table 1:

[0205] Table 1. Morphological characteristics of membrane structures in each embodiment and comparative example.

[0206]

[0207]

[0208] The performance test data for each embodiment and comparative example are detailed in Table 2:

[0209] Table 2 Performance test data of the examples and comparative examples

[0210]

[0211]

[0212] It should be noted that since the differences in Examples 1-5 are mainly due to adjustments in the hot and cold drawing processes, the heat-set semi-finished products are exactly the same, and therefore the data of X, A, B, C, and D of the heat-set semi-finished products are exactly the same.

[0213] in conclusion

[0214] By comparing the technical solutions of Examples 1-5 and the data in Table 1-2, it is easy to see that when the infrared absorption spectrum data of the heat-set semi-finished product meets specific limitations, even with appropriate adjustments to the hot and cold drawing process, the resulting hollow fiber membrane has a long service life and good degassing effect; furthermore, in addition to good degassing effect, it also has good mechanical properties. This means that, contrary to the expectation that service life and degassing effect cannot be simultaneously achieved, or that degassing effect and mechanical properties cannot be simultaneously achieved, in reality, when the infrared absorption spectrum data of the heat-set semi-finished product meets certain conditions, service life, degassing effect, and mechanical properties can be simultaneously achieved. Furthermore, by detecting and calculating the infrared absorption spectrum data of the heat-set semi-finished product, a theoretical basis can be provided for raw material selection and process parameter adjustment, which has important guiding significance.

[0215] By comparing the technical solutions of Examples 5-9 and the data in Tables 1-2, it is easy to see that, based on the determined hot and cold drawing process, even if X, A, B, C, and D of the heat-set semi-finished product are appropriately varied, as long as they remain within the specific range defined in this application, the final hollow fiber membrane exhibits good degassing effect, service life, and mechanical properties. This further demonstrates that defining the infrared absorption spectrum data of the heat-set semi-finished product can indeed ensure that the final hollow fiber membrane has good degassing effect, service life, and mechanical properties. It also provides important guidance for raw material selection and process adjustment.

[0216] By comparing the technical solutions of Examples 10-11 and the data in Tables 1-2, it is easy to see that even for hollow fiber membranes made from PE and PMP as raw materials, as long as the X of the heat-set semi-finished product meets the specific limitations of this application, the hollow fiber membrane finally produced will have good performance.

[0217] By comparing the technical solutions of Example 5 and Comparative Examples 1-3 with the data in Table 1-2, it is easy to see that when X of the heat-set semi-finished product is less than 4, the hollow fiber membrane obtained in the end has defects in degassing effect, service life and mechanical properties, and cannot obtain the hollow fiber membrane with the properties required by this application. Furthermore, A, B, C and D of the heat-set semi-finished product mainly affect the skin morphology or inner layer morphology of the obtained hollow fiber membrane.

[0218] The above are all preferred embodiments of this application and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A process for preparing an asymmetric polyolefin hollow fiber membrane for degassing, characterized in that: The process includes the following steps: S1. Spinning: The polyolefin material is melt-extruded and formed into a semi-molded product with a hollow inner cavity under the action of a cavity-forming fluid; the polyolefin material is at least one of PP, PE and PMP, the glass transition temperature of the polyolefin material is Tg, and the melting point of the polyolefin material is Tm. S2. Cooling and crystallization: The semi-finished product obtained in step S1 is cooled and crystallized to obtain nascent fibers. Step S2 includes the following process steps: S21. Natural cooling: The semi-molded product obtained in step S1 is cooled by natural cooling to obtain a naturally cooled semi-finished product; the melting point of the polyolefin material is Tm, and the surface temperature of the naturally cooled semi-finished product obtained after natural cooling drops to Tm~(Tm+20)℃. S22, Air-cooled crystallization: The naturally cooled semi-finished product obtained in step S21 is subjected to air-cooled crystallization by blowing air, and then wound up to obtain nascent fibers. S3. First heat setting: The nascent fibers obtained in step S2 are heat-set and cooled to obtain a heat-set semi-finished product. S4. Stretch to form holes. The heat-set semi-finished product obtained in step S3 is subjected to cold stretching to form holes, hot stretching to expand holes, and secondary shaping in sequence to obtain a hollow fiber membrane. In step S3, the infrared absorption spectrum of the heat-set semi-finished product is measured, and the crystallization gradient coefficient X of the heat-set semi-finished product is calculated. The crystallization gradient coefficient X ≥ 4. The crystallization gradient coefficient X is calculated by the following formula: X=ABS 内 / ABS 外 ; In the above formula, ABS 内 The absorption intensity of the inner surface of the heat-set semi-finished product at the crystalline region band; ABS 外 The absorption intensity at the crystalline region band on the outer surface of the heat-set semi-finished product.

2. The preparation process of an asymmetric polyolefin hollow fiber membrane for degassing according to claim 1, characterized in that: The polyolefin material is PP. In step S3, the low crystallinity coefficient Y and crystallinity coefficient Z of the heat-set semi-finished product are further calculated. The low crystallinity coefficient Y is calculated by the following formula: Y=ABS 973 / ABS 1460 ; ABS 973 For heat-set semi-finished products at a wave number of 973 cm⁻¹ -1 Absorption intensity at the location; ABS 1460 For heat-set semi-finished products at a wave number of 1460 cm⁻¹ -1 Absorption intensity at the location; The low crystallinity coefficient of the inner surface of the heat-set semi-finished product is Y. 内 The low crystallinity coefficient of the outer surface of the heat-set semi-finished product is Y. 外 , The crystallinity coefficient Z is calculated by the following formula: Z=ABS 998 / ABS 1460 ; ABS 998 For heat-set semi-finished products at a wave number of 998 cm⁻¹ -1 Absorption intensity at the location; ABS 1460 For heat-set semi-finished products at a wave number of 1460 cm⁻¹ -1 Absorption intensity at the location; The crystallinity coefficient of the inner surface of the heat-set semi-finished product is Z. 内 The crystallinity coefficient of the outer surface of the heat-set semi-finished product is Z. 外 ; The low crystallinity coefficient Y and crystallinity coefficient Z of the inner surface of the heat-set semi-finished product satisfy at least one of the following conditions: (1)Y 内 / Z 内 ≤2; (2)Y 内 -Z 内 ≤0.2。 3. The preparation process of an asymmetric polyolefin hollow fiber membrane for degassing according to claim 2, characterized in that: The low crystallinity coefficient Y and crystallinity coefficient Z of the outer surface of the heat-set semi-finished product satisfy at least one of the following conditions: (1)Y 外 / Z 外 ≥9; (2)Y 外 -Z 外 ≥0.4。 4. The preparation process of an asymmetric polyolefin hollow fiber membrane for degassing according to any one of claims 1-3, characterized in that: Step S4 includes the following process steps: S41. Cold drawing to form holes: The heat-set semi-finished product obtained in step S3 is subjected to cold drawing to obtain a cold-drawn semi-finished product; wherein, the cold drawing temperature is (Tg+30)~(Tg+80)℃, the cold drawing elongation is 20~40%, and the cold drawing rate is (10~30)% / min. S42. Hot drawing and hole expansion: The cold-drawn semi-finished product obtained in step S41 is hot-drawn and hole expanded to obtain a hot-drawn semi-finished product; the hot drawing temperature is (Tm-30)~(Tm-60)℃, the hot drawing elongation is 2 to 6 times that of the cold drawing elongation, and the hot drawing rate is not higher than 15% of the cold drawing rate. S43. Secondary heat setting: The hot-drawn semi-finished product obtained in step S42 is subjected to secondary heat setting treatment, and after cooling, hollow fiber membrane is obtained.

5. The preparation process of an asymmetric polyolefin hollow fiber membrane for degassing according to claim 4, characterized in that: The polyolefin material is PP, and in step S41... The cold drawing process for creating holes is performed in 2 to 4 steps. The cold drawing draw ratio for each cold drawing is 104.7–118.3%. At the end of the cold drawing process, the linear speed of the cold-drawn semi-finished product is 2.5 to 3.5 m / min.

6. The preparation process of an asymmetric polyolefin hollow fiber membrane for degassing according to claim 4, characterized in that: In step S42 The hot-drawing temperature is 110–130℃; The elongation at room temperature is 90–110%. The hot-drawing rate is (0.3~1.3)% / min.

7. The preparation process of an asymmetric polyolefin hollow fiber membrane for degassing according to claim 1, characterized in that: The polyolefin material is PP; In step S1, the die temperature during melt extrusion is 180–200°C. In step S21, the surface temperature of the naturally cooled semi-finished product obtained after natural cooling is 168-178℃.

8. The preparation process of an asymmetric polyolefin hollow fiber membrane for degassing according to claim 1, characterized in that: In step S21 The path length for natural cooling is 30–1000 mm; During natural cooling, the atmosphere is air or nitrogen, and the atmosphere temperature is 40–70°C. The cooling rate of the surface of the semi-molded product during natural cooling is (0.1~0.6)℃ / mm@(100~200)m / min.

9. The preparation process of an asymmetric polyolefin hollow fiber membrane for degassing according to claim 1, characterized in that: In step S22 The speed of the cooling airflow is 25-35% of the winding speed; The temperature of the cooling airflow is 40–70°C; The cooling airflow is angled along the conveying direction of the naturally cooled semi-finished product.

10. The preparation process of an asymmetric polyolefin hollow fiber membrane for degassing according to claim 1, characterized in that: The polyolefin material is PP. The isotacticity of PP is not less than 95%; The melt flow index of PP is 1-7 g / min at 190℃, 5 kg; The crystallinity of PP is 45-70%; The wall thickness of the semi-finished product is 0.5 to 3 mm.

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