Hollow fiber membranes with asymmetric structures, their preparation methods, and applications

Abstract

This invention provides a hollow fiber membrane with an asymmetric structure, its preparation method, and its applications, relating to the technical field of polymer separation membrane materials. The method includes the following steps: melt spinning a masterbatch followed by sequential air cooling, spray cooling, and a second air cooling to obtain virgin hollow fibers; then, isothermal heat treatment of the virgin hollow fibers to obtain heat-treated hollow fibers; subsequently, cold stretching to expand pores and hot stretching to create pores are performed on the heat-treated hollow fibers, followed by isothermal stress relief to obtain a hollow fiber membrane with an asymmetric structure. This invention solves the technical problem of existing methods for preparing asymmetric hollow fiber membranes relying on toxic organic solvents, which are harmful to the health of workers and the environment. It achieves the technical advantages of a solvent-free preparation process, simple technology, green sustainability, and a membrane with high flux, high rejection rate, and high antifouling performance.

Description

Technical Field

[0001] This invention relates to the technical field of polymer separation membrane materials, and in particular to a hollow fiber membrane with an asymmetric structure, its preparation method, and its applications. Background Technology

[0002] High-performance separation membrane materials, as the core of membrane separation technology, play an important role in water treatment, gas separation, chemical and pharmaceutical industries, hemodialysis, and textile processing. As a commonly used separation membrane material, hollow fiber membranes possess characteristics such as self-supporting structure, good flexibility, and high specific surface area, thus finding wide application in wastewater treatment, seawater desalination, biomedicine, and gas separation.

[0003] However, traditional methods for preparing asymmetric hollow fiber membranes with dense skin and porous support layers (such as non-solvent-induced phase separation and thermal phase separation) rely on toxic organic solvents, thus making it impossible to prepare asymmetric hollow fiber membranes in an environmentally friendly manner. In addition, they also generate a large amount of wastewater containing organic solvents, which poses risks to the health of practitioners and the environment, and does not conform to the concept of green and sustainable development.

[0004] Melt spinning-stretching is a solvent-free, green membrane fabrication method based on the separation of oriented lamellar clusters in a polymer to form micropores. The hollow fiber membranes prepared by this method have a symmetrical structure and good mechanical properties. However, melt spinning-stretching suffers from two drawbacks: firstly, it results in the absence of a dense skin layer on the outer surface, leading to poor antifouling performance; secondly, it leaves many inseparable lamellar clusters within the membrane wall, resulting in low porosity and poor permeability. Melt spinning-stretching cannot construct asymmetric membranes with a skin structure, thus limiting the application range and resulting in a very low market share.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] One of the objectives of this invention is to provide a method for preparing hollow fiber membranes with an asymmetric structure, which can solve the problem that existing membrane manufacturing processes rely on toxic organic solvents, resulting in health risks to workers and environmental problems. This method achieves the effects of solvent-free preparation, simple process, and green sustainability.

[0007] The second objective of this invention is to provide a hollow fiber membrane with an asymmetric structure, which has high flux, high rejection rate and high antifouling performance.

[0008] The third objective of this invention is to provide an application of hollow fiber membranes with asymmetric structures, which has outstanding performance in the field of membrane separation.

[0009] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0010] In a first aspect, a method for preparing a hollow fiber membrane with an asymmetric structure includes the following steps:

[0011] (a) After the masterbatch is melt-spun, it is sequentially subjected to first air cooling, spray cooling and second air cooling to obtain initial spun hollow fiber;

[0012] The film-forming components of the masterbatch include a polymer matrix and a hydrophilic second phase;

[0013] The conditions for the first air cooling include: air cooling temperature of 5–30℃ and air cooling length of 0.1–4m;

[0014] The conditions for spray cooling include: water temperature of spray cooling 0-90℃, and spray cooling length 0.1-2m;

[0015] (b) The initial spun hollow fibers obtained in step (a) are subjected to isothermal heat treatment to obtain heat-treated hollow fibers.

[0016] The isothermal heat treatment temperature is 90–150°C, and the isothermal heat treatment time is 20–120 min;

[0017] (c) The heat-treated hollow fibers obtained in step (b) are subjected to cold stretching to expand the pores and hot stretching to create pores in sequence, and then the thermal stress is eliminated isothermally to obtain the hollow fiber membrane with the asymmetric structure.

[0018] The conditions for cold drawing and expanding include: cold drawing ratio of 10-50% and cold drawing temperature of 5-30℃.

[0019] The conditions for hot stretching to create pores include: a hot stretching ratio of 60-290% and a hot stretching temperature of 80-150°C.

[0020] Furthermore, the conditions for the second air cooling include: an air temperature range of 5–30°C and an air cooling length of 0.5–2 m.

[0021] Furthermore, the masterbatch comprises the following film-forming components in parts by weight:

[0022] 60–95 parts of polymer matrix and 5–40 parts of hydrophilic second phase;

[0023] The polymer matrix includes at least one of homopolymer polypropylene, copolymer polypropylene, high-density polyethylene, linear low-density polyethylene, and polymethylpentene;

[0024] The melt index of the polymer matrix is ​​0.3 to 100 g / 10 min;

[0025] The hydrophilic second phase includes at least one of ethylene-vinyl alcohol copolymer, nylon 6, nylon 66, surface-hydroxylated silica, surface-aminated silica, surface-hydroxylated titanium dioxide, and surface-aminated titanium dioxide.

[0026] Furthermore, the film-forming components of the masterbatch also include a compatibilizer;

[0027] The compatibilizer is present in a mass fraction of less than 10 parts;

[0028] The compatibilizer includes at least one of polypropylene grafted with maleic anhydride, polypropylene grafted with acrylic acid, polypropylene grafted with glycidyl methacrylate, polyethylene grafted with maleic anhydride, polyethylene grafted with acrylic acid, and polyethylene grafted with glycidyl methacrylate.

[0029] Secondly, a hollow fiber membrane with an asymmetric structure prepared by any of the above-described preparation methods, wherein the hollow fiber membrane has a dense skin layer and a bimodal pore support layer.

[0030] Furthermore, the bimodal porous support layer is composed of micropores formed by lamellar separation and macropores formed by the separation of two-phase interfaces;

[0031] The hollow fiber membrane has hydrophilic properties on its inner surface, outer surface, and wall.

[0032] Furthermore, the pore size of the nanopores in the dense skin layer is below 100 nm.

[0033] Furthermore, the pore size of the micropores formed by the separation of the lamellar crystals is less than 1 μm and greater than 100 nm, and the pore size of the macropores formed by the separation of the two-phase interface is less than 10 μm and greater than 1 μm.

[0034] Thirdly, the application of any of the above-mentioned hollow fiber membranes in the field of membrane separation.

[0035] Furthermore, the membrane separation field includes at least one of the fields of wastewater treatment, seawater desalination, biomedicine, and gas separation.

[0036] Compared with the prior art, the present invention has at least the following beneficial effects:

[0037] The present invention provides a method for preparing hollow fiber membranes with asymmetric structures. After melt spinning of the masterbatch, a dense outer skin layer is first constructed using a spray cooling process. The principle is to rapidly freeze the molecular chains on the fiber surface through air cooling and spray cooling, thereby constructing a dense outer skin layer structure. During this process, the outer skin layer structure can be controlled by adjusting the cooling process, such as the length and temperature of the spray cooling. Subsequently, small micropores are formed through oriented lamellar separation, and large micropores are formed through two-phase interface separation. The principle is to co-mix a small amount of a hydrophilic second phase with a strength and melting point higher than the polymer matrix into the polymer matrix, which is incompatible with the polymer matrix, to form a typical Sea-Island structure in the composite material. This is achieved by controlling the compatibility of the two-phase interface and the membrane-forming process, combined with lamellar separation. The pores are separated from the phase to form pores, thus constructing a novel bimodal pore support layer with two different levels and independent pore size distributions. The small micropores provide support, while the large micropores increase the membrane porosity. In addition, the hydrophilic second phase forms a uniformly dispersed island structure within the hollow fiber membrane wall, which can improve the hydrophilicity of the system by combining directional migration and uniform dispersion of polar compatibilizers. The preparation method of this invention, through the synergistic coordination of each step and its process parameters, achieves the goal of constructing an asymmetric membrane structure with a dense skin and a bimodal pore support layer in a green and solvent-free manner, and prepares a hydrophilic hollow fiber membrane with high porosity, good permeability, good retention performance, and antifouling properties. At the same time, the preparation method provided by this invention has the advantages of being simple, easy to implement, and low in cost, and is easy to promote for large-scale production.

[0038] The hollow fiber membrane with an asymmetric structure provided by the present invention is a hollow fiber membrane with an asymmetric multi-scale structure having a dense skin layer and a bimodal pore support layer, which has high flux, high rejection rate and high antifouling performance.

[0039] The hollow fiber membrane with an asymmetric structure provided by this invention can be specifically applied in the fields of wastewater treatment, seawater desalination, biomedicine, and gas separation, achieving outstanding application results. Attached Figure Description

[0040] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0041] Figure 1 The hollow fiber membrane provided in Embodiment 1 of the present invention is shown in (a) the outer surface and (b) the longitudinal section structural morphology diagram.

[0042] Figure 2The hollow fiber membrane provided in Comparative Example 1 of the present invention is shown in (a) the outer surface and (b) the longitudinal section structural morphology.

[0043] Figure 3 The images show the structural morphology of the hollow fiber membrane (a) on its outer surface and (b) in its longitudinal section, as provided in Comparative Example 2 of this invention. Detailed Implementation

[0044] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] According to a first aspect of the present invention, a method for preparing a hollow fiber membrane having an asymmetric structure is provided, comprising the following steps:

[0046] (a) After the masterbatch is melt-spun, it is sequentially subjected to first air cooling, spray cooling and second air cooling to obtain initial spun hollow fiber;

[0047] The film-forming components of the masterbatch include, but are not limited to, a polymer matrix and a hydrophilic second phase;

[0048] The preparation method of the present invention combines air cooling and spray cooling processes. The air cooling stage is used for the formation and growth of lamellar crystals in the inner wall of the fiber, and the spray cooling stage is used to increase the supercooling degree and reduce the crystallization of surface molecular chains, thereby forming a dense outer skin layer.

[0049] In this invention, the conditions for the first air cooling include, but are not limited to: an air cooling temperature of 5 to 30°C, with typical but non-limiting temperatures such as 5°C, 10°C, 15°C, 20°C, 25°C, and 30°C; and an air cooling length of 0.1 to 4 m, with typical but non-limiting lengths such as 0.1 m, 0.5 m, 1 m, 1.5 m, 2 m, 2.5 m, 3 m, 3.5 m, and 4 m.

[0050] The conditions for spray cooling include, but are not limited to: spray cooling water temperature of 0–90°C, with typical but non-limiting spray cooling water temperatures such as 0°C, 5°C, 10°C, 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 60°C, 70°C, 80°C, and 90°C; and spray cooling length of 0.1–2 m, with typical but non-limiting lengths such as 0.1 m, 0.5 m, 1 m, 1.5 m, and 2 m.

[0051] (b) The initial spun hollow fibers obtained in step (a) are subjected to isothermal heat treatment to obtain heat-treated hollow fibers.

[0052] Isothermal heat treatment can promote the secondary formation and growth of lamellar crystals in the inner wall of the fiber, and can also reduce the thickness of the dense outer skin layer on the fiber surface.

[0053] In this invention, the isothermal heat treatment temperature can be 90–150°C, and typical but non-limiting temperatures include, for example, 90°C, 95°C, 100°C, 110°C, 120°C, 130°C, 140°C, and 150°C; the isothermal heat treatment time can be 20–120 min, and typical but non-limiting times include, for example, 20 min, 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, and 120 min.

[0054] (c) The heat-treated hollow fibers obtained in step (b) are subjected to cold stretching to expand the pores and hot stretching to create pores in sequence. Then, the thermal stress is eliminated isothermally to obtain a hollow fiber membrane with an asymmetric structure.

[0055] In this invention, the conditions for cold drawing and expanding include, but are not limited to: a cold drawing ratio of 10 to 50%, typically but not limited to 10%, 20%, 30%, 40%, and 50%; and a cold drawing temperature of 5 to 30°C, typically but not limited to 5°C, 15°C, 20°C, 25°C, and 30°C.

[0056] The conditions for hot stretching to induce pores include, but are not limited to: a hot stretching ratio of 60% to 290%, with typical but non-limiting stretching ratios such as 60%, 70%, 80%, 90%, 100%, 120%, 140%, 160%, 180%, 200%, 230%, 260%, and 290%; and a hot stretching temperature of 80% to 150°C, with typical but non-limiting temperatures such as 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, and 150°C.

[0057] The present invention provides a method for preparing hollow fiber membranes with asymmetric structures. After melt spinning of the masterbatch, a dense outer skin layer is first constructed using a spray cooling process. The principle is to rapidly freeze the molecular chains on the fiber surface through air cooling and spray cooling, thereby constructing a dense outer skin layer structure. During this process, the outer skin layer structure can be controlled by adjusting the cooling process, such as the length and temperature of the spray cooling. Subsequently, small micropores are formed through oriented lamellar separation, and large micropores are formed through two-phase interface separation. The principle is to co-mix a small amount of polymer with strength and melting point higher than that of the polymer matrix. The composite matrix is ​​constructed with an incompatible hydrophilic second phase to form a typical Sea-Island structure. By controlling the interfacial compatibility of the two phases and the film-forming process, and combining lamellar separation pore formation and phase separation pore formation, a novel bimodal pore support layer with two different levels and independent pore size distributions is constructed. The small micropores provide support, while the large micropores increase the membrane porosity. In addition, the hydrophilic second phase forms a uniformly dispersed island structure within the hollow fiber membrane wall, and the hydrophilicity of the system can be improved by combining a polar compatibilizer that migrates in a directional manner and disperses uniformly.

[0058] The hollow fiber membrane preparation method provided by this invention, through the synergistic cooperation of each step and its process parameters, achieves the goal of constructing an asymmetric membrane structure with a dense skin layer and a bimodal pore support layer in a green and solvent-free manner, and prepares a hydrophilic hollow fiber membrane with high porosity, good permeability, good retention performance and antifouling performance. At the same time, the preparation method provided by this invention has the advantages of being simple, easy to implement and low in cost, and is easy to promote for large-scale production.

[0059] In a preferred embodiment, the conditions for the second air cooling in this invention include, but are not limited to: an air temperature range of 5 to 30°C, with typical but non-limiting temperatures such as 5°C, 10°C, 15°C, 20°C, 25°C, and 30°C; and an air cooling length of 0.5 to 2 m, with typical but non-limiting lengths such as 0.5 m, 1 m, 1.5 m, and 2 m.

[0060] In a preferred embodiment, the masterbatch of the present invention comprises the following film-forming components in parts by mass: 60-95 parts of polymer matrix and 5-40 parts of hydrophilic second phase;

[0061] The typical but non-limiting mass fractions of the polymer matrix are, for example, 60, 65, 70, 75, 80, 85, 90, and 95 parts; the typical but non-limiting mass fractions of the hydrophilic second phase are, for example, 5, 10, 15, 20, 25, 30, 35, and 40 parts.

[0062] In this invention, the polymer matrix refers to a semi-crystalline polymer that can form an oriented lamellar structure after melt spinning. It can be one or more of the following: homopolymer polypropylene, copolymer polypropylene, a mixture of homopolymer polypropylene and copolymer polypropylene in proportion, high-density polyethylene, linear low-density polyethylene, and polymethylpentene, but is not limited thereto. Furthermore, the melt index of the polymer matrix can be 0.3 to 100 g / 10 min, and typical but non-limiting melt indices are, for example, 0.3 g / 10 min, 0.5 g / 10 min, 1 g / 10 min, 5 g / 10 min, 10 g / 10 min, 20 g / 10 min, 30 g / 10 min, 40 g / 10 min, 50 g / 10 min, 60 g / 10 min, 70 g / 10 min, 80 g / 10 min, 90 g / 10 min, and 100 g / 10 min.

[0063] In this invention, the hydrophilic second phase refers to a polymer or inorganic powder with a melting point and strength greater than that of the polymer matrix, and can be one or more of the following: ethylene-vinyl alcohol copolymer (ethylene content ≤48mol%), nylon 6, nylon 66, micron-sized surface-hydroxylated or amination-treated silica (hydroxylation or amination rate can be 1-5wt.%), and micron-sized surface-hydroxylated or amination-treated titanium dioxide (hydroxylation or amination rate can be 1-5wt.%).

[0064] In a preferred embodiment, the film-forming component of the masterbatch further includes a compatibilizer; wherein the mass fraction of the compatibilizer may be less than 10 parts.

[0065] In this invention, the compatibilizer may be one or more of the following: polypropylene grafted with maleic anhydride (grafting rate may be 1-5 wt.%), polypropylene grafted with acrylic acid (grafting rate may be 1-5 wt.%), polypropylene grafted with glycidyl methacrylate (grafting rate may be 1-5 wt.%), polyethylene grafted with maleic anhydride (grafting rate may be 1-5 wt.%), polyethylene grafted with acrylic acid (grafting rate may be 1-5 wt.%), and polyethylene grafted with glycidyl methacrylate (grafting rate may be 1-5 wt.%), but is not limited thereto.

[0066] A typical method for preparing a hollow fiber membrane with an asymmetric structure includes the following steps:

[0067] S1: Take the film-forming components according to the mass fraction, dry them and mix them thoroughly, add them to a twin-screw extruder for extrusion granulation, and dry them to obtain composite masterbatch;

[0068] The film-forming components are formulated as follows by mass: 60-95 parts polymer matrix, 0-10 parts compatibilizer, and 5-40 parts hydrophilic second phase; the melt index of the polymer matrix is ​​0.3-100 g / 10 min.

[0069] S2: The composite masterbatch is melt-spun at a temperature of 170–230°C, with air as the cavity fluid at a flow rate of 0.01–0.2 L / min; wherein the melt draw ratio is 2500–9500%.

[0070] After melt spinning, the fibers are cooled by a combination of air cooling and spray cooling processes. The fibers are first cooled by air (room temperature air, temperature range 5-30℃, air cooling length 0.1-4m), then cooled by spray cooling (spray cooling water temperature range 0-90℃, spray cooling length 0.1-2m), and finally cooled by air (room temperature air, temperature range 5-30℃, air cooling length 0.5-2m). After cooling, the fibers are wound to obtain the initial spun hollow fibers.

[0071] S3: The virgin hollow fiber is subjected to isothermal heat treatment at a temperature of 90-150℃ for 20-120 minutes, and then naturally cooled to obtain heat-treated hollow fiber.

[0072] S4: The heat-treated hollow fibers are subjected to cold stretching to expand the pores and hot stretching to form pores in sequence. The cold stretching ratio is 10-50% and the cold stretching temperature is room temperature (5-30℃). The hot stretching ratio is 60-290% and the hot stretching temperature is 80-150℃. The thermal stress is then isothermally eliminated for 30-120 minutes to obtain a hydrophilic hollow fiber membrane with an asymmetric multi-scale structure.

[0073] The hollow fiber membrane preparation method provided by this invention addresses the problem that existing membrane-making processes cannot produce asymmetric hollow fiber membranes with a dense skin layer and a porous support layer in a green manner. Based on the goal of green, solvent-free preparation of asymmetric hollow fiber membranes, this invention combines air cooling and spray cooling processes, utilizing rapid freezing to solidify the molecular chains on the fiber surface, thereby constructing a dense outer skin layer structure. A hydrophilic second phase is co-incorporated into the polymer matrix, and a compatibilizer is used to regulate the interfacial compatibility between the two phases. A bimodal porous support layer is constructed by combining the lamellar separation mechanism and the phase separation mechanism. This invention organically combines melt blending, melt spinning, multiple cooling, and stretching technologies. With the synergistic cooperation of various process technologies and their parameters, a hydrophilic hollow fiber membrane with an asymmetric multi-scale structure of a dense skin layer and a bimodal porous support layer is prepared. This effectively improves the membrane's porosity, hydrophilicity, and selectivity, increases the membrane's pure water flux, and also endows the membrane with good retention and antifouling properties.

[0074] The method for preparing hollow fiber membranes with asymmetric structures provided by this invention has a short process flow and simple operation. It is also green and sustainable, with no waste liquid generated during the production process. It can be mass-produced using existing equipment, which is very conducive to its promotion and practical application.

[0075] According to a second aspect of the present invention, a hollow fiber membrane with an asymmetric structure prepared by any of the above-described preparation methods is provided.

[0076] The hollow fiber membrane with an asymmetric structure provided by this invention is a hollow fiber membrane with an asymmetric multi-scale structure having a dense skin layer and a bimodal pore support layer. It has high flux, high rejection rate and high antifouling performance, and also has high mechanical properties. It can be used alone in wastewater treatment, oxygenation membrane and special separation fields.

[0077] In a preferred embodiment, the pore size of the dense skin layer of the hollow fiber membrane of the present invention is ≤100nm.

[0078] In a preferred embodiment, the bimodal pore support layer of the hollow fiber membrane of the present invention (having two different levels and independent pore size distributions) is composed of small micropores (100nm < pore size ≤ 1μm) formed by lamellar separation and large micropores (1μm < pore size ≤ 10μm) formed by separation at the two-phase interface.

[0079] In a preferred embodiment, the inner surface, outer surface, and wall of the hollow fiber membrane of the present invention are all hydrophilic. This is achieved by a hydrophilic second phase forming a uniformly dispersed island structure within the hollow fiber membrane wall, combined with a polar compatibilizer that is directionally migrated and uniformly dispersed. The surface water contact angle is ≤85°.

[0080] According to a third aspect of the present invention, an application of the hollow fiber membrane described in any of the preceding claims in the field of membrane separation is provided.

[0081] The hollow fiber membrane with an asymmetric structure provided by this invention can be specifically applied in the fields of wastewater treatment, seawater desalination, biomedicine, and gas separation, achieving outstanding application results.

[0082] The present invention will be further illustrated by the following examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or purchased directly from the market.

[0083] Example 1

[0084] A method for preparing a hollow fiber membrane with an asymmetric structure includes the following steps:

[0085] 1) According to the mass fraction, 90 parts of polypropylene T30s (melt index 3.0 g / 10 min, melting point 166℃), 5 parts of polypropylene grafted maleic anhydride (grafting rate 1 wt.%) and 10 parts of ethylene-vinyl alcohol copolymer (ethylene content 27 mol%) in the film-forming components were dried separately. The dried materials were thoroughly mixed and then fed into a twin-screw extruder with a length-to-diameter ratio of 50 for extrusion granulation. Then, the mixture was dried at 80℃ for 12 hours to obtain the composite masterbatch for later use.

[0086] 2) The composite masterbatch was melt-spun using a single-screw spinning machine. The temperature of each section of the spinning machine from the feed port to the die head was controlled as follows: 170℃, 180℃, 190℃, 200℃, 200℃, 200℃, 190℃, 185℃. Air was used as the cavity fluid with a flow rate of 0.06L / min. The melt draw ratio during the spinning process was 6000%.

[0087] Then, it is first air-cooled (cooling length 1m, room temperature 5-30℃), then spray-cooled (cooling length 1m, water temperature 20℃), then air-cooled again (cooling length 3.5m, room temperature 5-30℃), and then wound into initial spun hollow fibers;

[0088] 3) The virgin hollow fibers are subjected to isothermal heat treatment at a temperature of 150℃ for 60 minutes, and then naturally cooled to obtain heat-treated hollow fibers.

[0089] 4) The heat-treated hollow fibers were sequentially subjected to cold stretching for pore expansion and hot stretching for pore formation. The cold stretching ratio was 20% and the cold stretching temperature was room temperature (20℃). The hot stretching ratio was 180% and the hot stretching temperature was 140℃. Thermal stress was then isothermally relieved for 60 minutes to obtain a hydrophilic polypropylene / ethylene-vinyl alcohol hollow fiber membrane with an asymmetric multi-scale structure. This asymmetric hollow fiber membrane has an inner surface and longitudinal cross-sectional structure as shown below. Figure 1 As shown, the relevant structure and performance parameters of the membrane are listed in Table 1 of the experimental examples.

[0090] Example 2

[0091] A method for preparing a hollow fiber membrane with an asymmetric structure includes the following steps:

[0092] 1) According to the mass fraction, 90 parts of polypropylene T30s (melt index 3.0 g / 10 min, melting point 166℃), 5 parts of polypropylene grafted maleic anhydride (grafting rate 1 wt.%) and 10 parts of ethylene-vinyl alcohol copolymer (ethylene content 27 mol%) in the film-forming components were dried separately. The dried materials were thoroughly mixed and then fed into a twin-screw extruder with a length-to-diameter ratio of 50 for extrusion granulation. Then, the mixture was dried at 80℃ for 12 hours to obtain the composite masterbatch for later use.

[0093] 2) The composite masterbatch was melt-spun using a single-screw spinning machine. The temperature of each section of the spinning machine from the feed port to the die head was controlled as follows: 170℃, 180℃, 190℃, 200℃, 200℃, 200℃, 190℃, 185℃. Air was used as the cavity fluid with a flow rate of 0.06L / min. The melt draw ratio during the spinning process was 6000%.

[0094] Then, it is first air-cooled (cooling length 1.5m, room temperature 5-30℃), then spray-cooled (cooling length 1m, water temperature 20℃), then air-cooled again (cooling length 3m, room temperature 5-30℃), and then wound into initial spun hollow fibers.

[0095] 3) The virgin hollow fibers are subjected to isothermal heat treatment at a temperature of 150℃ for 60 minutes, and then naturally cooled to obtain heat-treated hollow fibers.

[0096] 4) The heat-treated hollow fibers were subjected to cold stretching for pore expansion and hot stretching for pore formation in sequence. The cold stretching ratio was 20% and the cold stretching temperature was room temperature (20℃). The hot stretching ratio was 180% and the hot stretching temperature was 140℃. The thermal stress was then relieved isothermally for 60 minutes to obtain a hydrophilic polypropylene / ethylene-vinyl alcohol hollow fiber membrane with an asymmetric multi-scale structure, i.e., a hollow fiber membrane with an asymmetric structure. The relevant structure and performance parameters of the membrane are shown in Table 1 of the experimental example.

[0097] Example 3

[0098] A method for preparing a hollow fiber membrane with an asymmetric structure includes the following steps:

[0099] 1) According to the mass fraction, 90 parts of polypropylene T30s (melt index 3.0 g / 10 min, melting point 166℃), 5 parts of polypropylene grafted maleic anhydride (grafting rate 1 wt.%) and 10 parts of ethylene-vinyl alcohol copolymer (ethylene content 27 mol%) in the film-forming components were dried separately. The dried materials were thoroughly mixed and then fed into a twin-screw extruder with a length-to-diameter ratio of 50 for extrusion granulation. Then, the mixture was dried at 80℃ for 12 hours to obtain the composite masterbatch for later use.

[0100] 2) The composite masterbatch was melt-spun using a single-screw spinning machine. The temperature of each section of the spinning machine from the feed port to the die head was controlled as follows: 170℃, 180℃, 190℃, 200℃, 200℃, 200℃, 190℃, 185℃. Air was used as the cavity fluid with a flow rate of 0.06L / min. The melt draw ratio during the spinning process was 6000%.

[0101] Then, it is first air-cooled (cooling length 2m, room temperature 5-30℃), then spray-cooled (cooling length 1m, water temperature 20℃), then air-cooled again (cooling length 2.5m, room temperature 5-30℃), and then wound into initial spun hollow fibers;

[0102] 3) The virgin hollow fibers are subjected to isothermal heat treatment at a temperature of 150℃ for 60 minutes, and then naturally cooled to obtain heat-treated hollow fibers.

[0103] 4) The heat-treated hollow fibers were subjected to cold stretching for pore expansion and hot stretching for pore formation in sequence. The cold stretching ratio was 20% and the cold stretching temperature was room temperature (20℃). The hot stretching ratio was 180% and the hot stretching temperature was 140℃. The thermal stress was then relieved isothermally for 60 minutes to obtain a hydrophilic polypropylene / ethylene-vinyl alcohol hollow fiber membrane with an asymmetric multi-scale structure, i.e., a hollow fiber membrane with an asymmetric structure. The relevant structure and performance parameters of the membrane are shown in Table 1 of the experimental example.

[0104] Example 4

[0105] A method for preparing a hollow fiber membrane with an asymmetric structure includes the following steps:

[0106] 1) By mass fraction, 90 parts of polypropylene T30s (melt index 3.0 g / 10 min, melting point 166℃), 5 parts of polypropylene grafted maleic anhydride (grafting rate 1 wt.%), and 10 parts of ethylene-vinyl alcohol copolymer (ethylene content 27 mol%) in the film-forming components were dried separately. The dried materials were thoroughly mixed and then fed into a twin-screw extruder with a length-to-diameter ratio of 50 for extrusion granulation. The mixture was then dried at 80℃ for 12 hours to obtain the composite masterbatch for later use.

[0107] 2) The composite masterbatch was melt-spun using a single-screw spinning machine. The temperature of each section of the spinning machine from the feed port to the die head was controlled as follows: 170℃, 180℃, 190℃, 200℃, 200℃, 200℃, 190℃, 185℃. Air was used as the cavity fluid with a flow rate of 0.06L / min. The melt draw ratio during the spinning process was 6000%.

[0108] Then, it is first air-cooled (cooling length 1.5m, room temperature 5-30℃), then spray-cooled (cooling length 0.5m, water temperature 20℃), then air-cooled again (cooling length 3.5m, room temperature 5-30℃), and then wound into initial spun hollow fibers;

[0109] 3) The virgin hollow fibers are subjected to isothermal heat treatment at a temperature of 150℃ for 60 minutes, and then naturally cooled to obtain heat-treated hollow fibers.

[0110] 4) The heat-treated hollow fibers were subjected to cold stretching for pore expansion and hot stretching for pore formation in sequence. The cold stretching ratio was 20% and the cold stretching temperature was room temperature (20℃). The hot stretching ratio was 180% and the hot stretching temperature was 140℃. The thermal stress was then relieved isothermally for 60 minutes to obtain a hydrophilic polypropylene / ethylene-vinyl alcohol hollow fiber membrane with an asymmetric multi-scale structure, i.e., a hollow fiber membrane with an asymmetric structure. The relevant structure and performance parameters of the membrane are shown in Table 1 of the experimental example.

[0111] Comparative Example 1

[0112] A method for preparing a polypropylene hollow fiber membrane with a symmetrical structure includes the following steps:

[0113] 1) Polypropylene T30s (melt index 3.0 g / 10 min, melting point 166℃) was spun using a single-screw spinning machine. The temperature of each section of the spinning machine from the feed port to the die head was controlled as follows: 170℃, 180℃, 190℃, 200℃, 200℃, 200℃, 190℃, 180℃. Air was used as the cavity fluid with a flow rate of 0.06 L / min. The melt draw ratio during the spinning process was 6000%.

[0114] Afterwards, it is air-cooled (cooling length 5m, room temperature 5~30℃), and then wound into initial spun hollow fibers;

[0115] 2) The virgin hollow fibers are subjected to isothermal heat treatment at a temperature of 150℃ for 60 minutes, and then naturally cooled to obtain heat-treated hollow fibers.

[0116] 3) The heat-treated hollow fibers were sequentially subjected to cold stretching for pore expansion and hot stretching for pore formation. The cold stretching ratio was 20% and the cold stretching temperature was room temperature (20℃). The hot stretching ratio was 180% and the hot stretching temperature was 140℃. Thermal stress was then isothermally relieved for 60 minutes to obtain a polypropylene hollow fiber membrane with a symmetrical structure. The inner surface and longitudinal cross-sectional structure of the membrane are as follows: Figure 2 As shown, the relevant structure and performance parameters of the membrane are listed in Table 1 of the experimental examples.

[0117] Comparative Example 2

[0118] A method for preparing a polypropylene hollow fiber membrane with a dense skin structure includes the following steps:

[0119] 1) Polypropylene T30s (melt index 3.0 g / 10 min, melting point 166℃) was spun using a single-screw spinning machine. The temperature control of each section of the spinning machine from the feed port to the die head was: 170℃, 180℃, 190℃, 200℃, 200℃, 200℃, 190℃, 180℃. Air was used as the cavity fluid with a flow rate of 0.06 L / min. The melt draw ratio during the spinning process was 6000%.

[0120] Then, it is first air-cooled (cooling length 1.5m, room temperature 5-30℃), then spray-cooled (cooling length 1m, water temperature 20℃), then air-cooled again (cooling length 3m, room temperature 5-30℃), and then wound into initial spun hollow fibers.

[0121] 2) The virgin hollow fibers are subjected to isothermal heat treatment at a temperature of 150℃ for 60 minutes, and then naturally cooled to obtain heat-treated hollow fibers.

[0122] 3) The heat-treated hollow fibers were sequentially subjected to cold stretching for pore expansion and hot stretching for pore formation. The cold stretching ratio was 20% and the cold stretching temperature was room temperature (20℃). The hot stretching ratio was 180% and the hot stretching temperature was 140℃. Thermal stress was then isothermally relieved for 60 minutes to obtain a polypropylene hollow fiber membrane with a dense skin structure. The inner surface and longitudinal cross-sectional structure of the membrane are as follows: Figure 3 As shown, the relevant structure and performance parameters of the membrane are listed in Table 1 of the experimental examples.

[0123] Comparative Example 3

[0124] A method for preparing a hydrophilic polypropylene / ethylene-vinyl alcohol hollow fiber membrane with a bimodal pore structure includes the following steps:

[0125] 1) According to the mass fraction, 90 parts of polypropylene T30s (melt index 3.0 g / 10 min, melting point 166℃), 5 parts of polypropylene grafted maleic anhydride (grafting rate 1 wt.%) and 10 parts of ethylene-vinyl alcohol copolymer (ethylene content 27 mol%) in the film-forming components were dried separately. The dried materials were thoroughly mixed and then fed into a twin-screw extruder with a length-to-diameter ratio of 50 for extrusion granulation. The mixture was then dried at 80℃ for 12 hours to obtain composite masterbatch for later use.

[0126] 2) The composite masterbatch was melt-spun using a single-screw spinning machine. The temperature of each section of the spinning machine from the feed port to the die head was controlled as follows: 170℃, 180℃, 190℃, 200℃, 200℃, 200℃, 190℃, 185℃. Air was used as the cavity fluid with a flow rate of 0.06L / min. The melt draw ratio during the spinning process was 6000%.

[0127] Afterwards, it is air-cooled (cooling length 5m, room temperature 5~30℃), and then wound into initial spun hollow fibers;

[0128] 3) The virgin hollow fibers are subjected to isothermal heat treatment at a temperature of 150℃ for 60 minutes, and then naturally cooled to obtain heat-treated hollow fibers.

[0129] 4) The heat-treated hollow fibers were subjected to cold stretching to expand the pores and hot stretching to form pores. The cold stretching ratio was 20% and the cold stretching temperature was room temperature (20℃). The hot stretching ratio was 180% and the hot stretching temperature was 140℃. The thermal stress was then relieved isothermally for 60 minutes to obtain a hydrophilic polypropylene / ethylene-vinyl alcohol hollow fiber membrane with a bimodal pore structure. The relevant structure and performance parameters of the membrane are shown in Table 1 of the experimental example.

[0130] Test case

[0131] The hollow fiber membranes prepared in Examples 1-4 and Comparative Examples 1-3 were subjected to performance tests, and the test results are shown in Table 1.

[0132] Table 1 Membrane structure and performance parameters

[0133]

[0134]

[0135] The hydrophilic polypropylene / ethylene-vinyl alcohol hollow fiber membrane with an asymmetric multi-scale structure prepared by the method of this invention is shown in [reference needed]. Figure 1 It is evident that it has a dense outer skin structure and a bimodal pore support layer structure. The bimodal pore support layer contains two types of pore structures (micropores and macropores). The separation of oriented lamellar crystals forms the micropore structure, while the separation of the two-phase interface forms the macropore structure. This will significantly increase the membrane porosity and thus increase the pure water flux of the membrane.

[0136] The polypropylene hollow fiber membrane prepared in Comparative Example 1 exhibits a uniform symmetrical pore structure, see... Figure 2 The hollow fiber membrane prepared in Comparative Example 2 exhibits a dense outer skin structure, lacking a dense outer skin. While the outer skin is beneficial for improving the membrane's antifouling and selective permeability, it negatively impacts the membrane's flux.

[0137] The difference between Comparative Example 2 and Comparative Example 1 is that Comparative Example 2 combines air cooling and spray cooling processes to construct a dense outer skin structure. Therefore, the membrane prepared in Comparative Example 2 can exhibit better rejection rate and antifouling performance, but its pure water flux is significantly reduced.

[0138] The difference between Comparative Example 3 and Comparative Example 1 is that a hydrophilic second phase was introduced in Comparative Example 3, and a double microporous structure was constructed by using lamellar separation and phase interface separation. Therefore, the porosity and hydrophilicity of the membrane prepared in Comparative Example 3 were greatly improved, and the pure water flux of the membrane was significantly improved. At the same time, the retention performance and antifouling performance of the membrane obtained in Comparative Example 3 were also slightly improved compared with Comparative Example 1, but were still significantly weaker than Comparative Example 2.

[0139] The difference between Examples 1-4 and Comparative Examples 1-3 is that Examples 1-4 simultaneously constructed a dense skin layer and a bimodal pore support layer, resulting in a significant improvement in the membrane's rejection rate and antifouling performance, with pure water flux exceeding that of Comparative Examples 1-3, demonstrating superior overall performance. The difference between Examples 1-4 lies in the gradual increase in the first-stage air-cooling length and the adjustment of the spray cooling process on the fiber surface molecular chains, achieving control of the asymmetric multi-scale structure. As the first-stage air-cooling length increases, the thickness of the outer skin layer gradually decreases, while the porosity of the bimodal pore support layer gradually increases, leading to a gradual increase in the membrane's pure water flux.

[0140] As shown in Table 1, the hydrophilic polypropylene / ethylene-vinyl alcohol hollow fiber membrane with an asymmetric multi-scale structure exhibits higher porosity, better hydrophilicity, higher pure water flux, and higher antifouling properties, while also possessing good retention performance. Therefore, the asymmetric multi-scale structure constructed using the method of this invention can effectively improve the overall performance of polymer hollow fiber membranes.

[0141] Therefore, the hollow fiber membrane preparation method provided by this invention, through the synergistic cooperation of each step and its process parameters, achieves the goal of constructing an asymmetric membrane structure with a dense skin layer and a bimodal pore support layer in a green and solvent-free manner, and prepares a hydrophilic hollow fiber membrane with high porosity, good permeability, good retention performance and antifouling performance. At the same time, the preparation method provided by this invention has the advantages of being simple, easy to implement and low in cost, and is easy to promote for large-scale production.

[0142] In summary, the preparation method provided by this invention is green and solvent-free, simple, easy to implement, and low in cost. At the same time, the hollow fiber membrane prepared by this method is an asymmetric multi-scale hollow fiber membrane with a dense skin layer and a bimodal pore support layer. It has high flux, high rejection rate, and high antifouling performance, as well as high mechanical properties. It can be used alone in wastewater treatment, oxygenation membranes, and special separation fields.

[0143] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for producing a hollow fiber membrane having an asymmetric structure, characterized by, Includes the following steps: (a) After the masterbatch is melt-spun, it is subjected to first air cooling, spray cooling and second air cooling in sequence to obtain initial spun hollow fiber; The film-forming components of the masterbatch include a polymer matrix and a hydrophilic second phase; The masterbatch comprises the following film-forming components in parts by weight: 60-95 parts of polymer matrix and 5-40 parts of hydrophilic second phase; The polymer matrix includes at least one of homopolymer polypropylene, copolymer polypropylene, high-density polyethylene, linear low-density polyethylene, and polymethylpentene; The melt index of the polymer matrix is ​​0.3~100g / 10min; The hydrophilic second phase includes at least one of ethylene-vinyl alcohol copolymer, nylon 6, nylon 66, surface-hydroxylated silica, surface-aminated silica, surface-hydroxylated titanium dioxide, and surface-aminated titanium dioxide. The conditions for the first air cooling include: air cooling temperature of 5~30℃ and air cooling length of 0.1~4m; The conditions for spray cooling include: water temperature of spray cooling 0~90℃, and spray cooling length 0.1~2m; (b) The initial spun hollow fibers obtained in step (a) are subjected to isothermal heat treatment to obtain heat-treated hollow fibers; The isothermal heat treatment temperature is 90~150℃, and the isothermal heat treatment time is 20~120min; (c) The heat-treated hollow fibers obtained in step (b) are subjected to cold stretching to expand the pores and hot stretching to create pores in sequence, and then the thermal stress is eliminated isothermally to obtain the hollow fiber membrane with the asymmetric structure. The conditions for cold drawing and expanding include: cold drawing ratio of 10~50% and cold drawing temperature of 5~30℃. The conditions for hot stretching to create pores include: a hot stretching ratio of 60-290% and a hot stretching temperature of 80-150℃.

2. The production method according to claim 1, characterized by, The conditions for the second air cooling include: an air temperature range of 5~30℃ and an air cooling length of 0.5~2m.

3. The preparation method according to claim 1, characterized in that, The film-forming components of the masterbatch also include a compatibilizer; The compatibilizer is present in a mass fraction of less than 10 parts; The compatibilizer includes at least one of polypropylene grafted with maleic anhydride, polypropylene grafted with acrylic acid, polypropylene grafted with glycidyl methacrylate, polyethylene grafted with maleic anhydride, polyethylene grafted with acrylic acid, and polyethylene grafted with glycidyl methacrylate.

4. A hollow fiber membrane with an asymmetric structure prepared by the preparation method according to any one of claims 1-3, characterized in that, The hollow fiber membrane has a dense skin layer and a bimodal pore support layer.

5. The hollow fiber membrane according to claim 4, characterized in that, The bimodal porous support layer is composed of small micropores formed by lamellar separation and large micropores formed by the separation of two-phase interfaces; The hollow fiber membrane has hydrophilic properties on its inner surface, outer surface, and wall.

6. The hollow fiber membrane according to claim 4, characterized in that, The pore size of the nanopores in the dense skin layer is less than 100 nm.

7. The hollow fiber membrane according to claim 5, characterized in that, The pore size of the micropores formed by the separation of the lamellar crystals is less than 1 μm and greater than 100 nm; The pore size of the large micropores formed by the separation of the two-phase interface is less than 10 μm and greater than 1 μm.

8. The application of the hollow fiber membrane according to any one of claims 4-7 in the field of membrane separation.

9. The application according to claim 8, characterized in that, The membrane separation field includes at least one of the following: wastewater treatment, seawater desalination, biomedicine, and gas separation.

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