Asymmetric hollow fiber membrane

Abstract

An asymmetric hollow fiber membrane is described comprising a porous substrate layer and a skin layer. And the surface layer is a copolymer of polymethylpentene and polypropylene. Also described are gas separation articles made using such hollow fiber membranes, as well as methods of making and using such hollow fiber membranes and gas separation articles.

Description

Technical Field

[0001] The present disclosure relates to a hollow fiber membrane having a porous substrate layer and a surface layer, wherein the surface layer comprises a copolymer of polymethylpentene and polypropylene. Also described are a gas separation article prepared using such a hollow fiber membrane, and methods of preparing and using such a hollow fiber membrane and a gas separation article. Summary of the invention

[0002] Briefly, in one aspect, the present disclosure provides an asymmetric hollow fiber membrane, comprising a porous substrate and a skin layer covering the porous substrate, the porous substrate comprising a polyolefin polymer and having a plurality of pores surrounding an internal cavity, wherein the skin layer comprises a copolymer comprising at least 80 weight percent of 4-methyl-1-pentene units and at least 5 weight percent of propylene units, wherein the total weight percent of 4-methyl-1-pentene units and propylene units is at least 98% based on the total weight of the monomer units in the copolymer. In some cases, an additional porous substrate layer may cover the skin layer.

[0003] In another aspect, the present disclosure provides a separation article comprising a plurality of such asymmetric hollow fiber membranes arranged substantially in parallel in an array and secured together.

[0004] In another aspect, the present disclosure provides a method of preparing such an asymmetric hollow fiber membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Figure 1 A cross section of an exemplary asymmetric hollow fiber membrane is illustrated.

[0006] Figure 2 A cross-section of another exemplary asymmetric hollow fiber membrane is illustrated.

[0007] Figure 3 A cross-section of another exemplary asymmetric hollow fiber membrane is illustrated. DETAILED DESCRIPTION

[0008] Microporous hollow fibers can be used to separate components from a fluid stream on the basis of, for example, size, phase, and charge. Microporous hollow fibers typically employ materials with controlled porosity and pore sizes of about a few microns, and have many uses, including, for example, separation, filtration, diffusion, and barrier applications. These broad applications have been used in practice in medical devices, electrochemical devices, chemical processing devices, pharmaceutical devices, and water purification, for example.

[0009] The performance of microporous hollow fiber membranes is generally a complex function of the specific end-use application and the composition and structure of the hollow fiber (e.g., hollow fiber diameter, wall thickness, porosity, and pore size). Typically, these and other variables are regulated for specific end-use applications. For example, membranes having a gas permeable separation layer can be used to provide selective gas / gas and gas / liquid passage.

[0010] Membrane contactors that can be used for gas / liquid separation applications can be made using asymmetric microporous hollow fiber membranes. Asymmetric microporous hollow fiber membranes (i.e., hollow fiber membranes comprising two or more layers) allow certain dissolved gases to selectively pass through while blocking liquid water or other aqueous liquids. Such hollow fiber membranes can be used in membrane contactors to achieve gas / liquid separation in applications, such as degassing aqueous printing inks during printing, and separating dissolved gases such as carbon dioxide or methane from aqueous brine for enhanced oil recovery. Other exemplary uses include: removing dissolved gases from fluids used in electroplating or microelectronic components, controlling dissolved gases in beer, wine and alcoholic beverages, extracorporeal membrane oxygenation, enriching oxygen from air, and carbon capture.

[0011] U.S. Patent US10,010,835B2 describes an asymmetric hollow fiber membrane, which includes a thermoplastic polymer substrate defining a plurality of micropores and a polymethylpentene (PMP) polymer surface layer positioned on the thermoplastic polymer substrate. International Patent Publication WO2020 / 136568A1 describes an asymmetric hollow fiber membrane, which includes a porous substrate having a plurality of pores and a surface layer covering the porous substrate. The porous substrate comprises a first semi-crystalline thermoplastic polyolefin (co)polymer and a nucleating agent that can effectively achieve nucleation. The surface layer comprises a second semi-crystalline thermoplastic polyolefin copolymer derived by polymerizing up to 98 weight percent of 4-methyl-1-pentene monomer and at least 2 weight percent of linear or branched alpha olefin monomers.

[0012] Despite the success of these and other asymmetric hollow fiber membranes, further improvements are still needed. For example, it is desirable to provide the required porosity level and selectively to improve interlaminar adhesion between the substrate and the skin layer without the use of a nucleating agent, and to increase tensile strength (breaking force). Unexpectedly, the inventors have found that one or more of these advantages can be achieved when using a polyolefin substrate layer combined with a skin layer comprising a polymethylpentene copolymer (comprising at least 5 weight percent polypropylene).

[0013] Generally speaking, asymmetric hollow fiber membranes include at least two layers that form a boundary around a hollow central cavity. Figure 11 shows a cross section of an exemplary asymmetric hollow fiber membrane. The boundary 180 includes a porous substrate 150 and a skin 160 surrounding the cavity 110 of the asymmetric hollow fiber membrane 100. In this example, the porous substrate 150 includes a surface 120 of the cavity 110, i.e., the surface of the boundary 180 immediately adjacent to the cavity. The skin 160 includes the outer surface 130 of the asymmetric hollow fiber membrane 100, i.e., the surface of the boundary 180 exposed to the surrounding environment.

[0014] In some embodiments, the relative positions of the layers may be reversed. Figure 2 As shown, boundary 280 includes porous substrate 250 and skin 260 surrounding lumen 210 of asymmetric hollow fiber membrane 200. In this example, porous substrate 250 includes outer surface 230 of asymmetric hollow fiber membrane 200 while skin 260 includes surface 220 of lumen 210.

[0015] As used herein, the terms "covering layer" and "covering" refer to a layer surrounding a common cavity and do not require any specific spatial orientation of the layer. Figure 1 , the porous substrate 150 covers the surface layer 160, and the surface layer 160 covers the porous substrate 150. Similarly, referring to Figure 2 , the porous substrate 250 covers the surface layer 260 , and the surface layer 260 covers the porous substrate 250 .

[0016] In general, porous substrates include polyolefin polymers, for example, polyolefin homopolymers or copolymers. In some embodiments, polymers include polypropylene, for example, polypropylene homopolymers. In general, any suitable polypropylene can be used. Suitable polypropylene homopolymers include, for example, Total Petrochemicals purchased from Houston, Texas under the trade name FINA, Houston, Texas, Lyondel-Basell Industries purchased from Pasadena, California under the trade name PRO-FAX, USA (INEOS Olefoins & Polymers, USA (Carson, California) purchased from Spring, Texas, ExxonMobil Chemical Company (Exxon-Mobil Chemical Company, Spring, Texas) purchased from Carson, California under the trade name INEOS.

[0017] The thickness of the porous substrate layer can depend on the specific application in which the microporous asymmetric hollow fibers are employed. In some embodiments, the porous substrate is at least 5 microns thick, e.g., at least 10 microns or at least 20 microns thick. In some embodiments, the porous substrate is no greater than 20 microns thick, e.g., no greater than 100 microns or even no greater than 50 microns thick, e.g., 5 μm to 200 μm, 10 μm to 100 μm, 15 μm to 75 μm, 20 μm to 50 μm, or even 25 microns to 35 microns thick.

[0018] The surface layer comprises a copolymer of 4-methyl-1-pentene and propylene repeating units. Generally speaking, the copolymer of the surface layer (also referred to as the surface layer resin) comprises at least 80 weight percent of 4-methyl-1-pentene units and at least 5 weight percent of propylene units, wherein based on the total number of monomer units in the surface layer resin, the total weight percentage of 4-methyl-1-pentene units and propylene units is at least 98 weight percent. As used herein, the term "monomer unit" refers to the structure of a monomer in its polymerized form. For example, for propylene (CH2=CH-CH3), the corresponding propylene unit is -CH2-CH(CH3)-.

[0019] Generally speaking, the surface layer resin mainly comprises 4-methyl-1-pentene units, that is, at least 80 wt %. In some embodiments, based on the total weight of the monomer units in the surface layer resin, the surface layer resin comprises at least 84 wt %, for example, at least 90 wt % of 4-methyl-1-pentene units. In some embodiments, the surface layer resin comprises not more than 95 wt %, for example, not more than 94 wt % of 4-methyl-1-pentene units (e.g., 84 wt % to 94 wt %, or even 90 wt % to 94 wt % of 4-methyl-1-pentene units).

[0020] Unlike the PMP-based surface resins of the prior art, the surface resins of the present disclosure contain a relatively high amount (i.e., at least 5 weight percent, e.g., at least 6 weight percent, based on the total weight of the monomer units in the surface resin) of polypropylene. In some embodiments, the surface resin contains no more than 20 weight percent, e.g., no more than 16 weight percent, no more than 10 weight percent, or even no more than 8 weight percent of propylene units (e.g., 6 weight percent to 16 weight percent, e.g., 6 weight percent to 8 weight percent of propylene units).

[0021] In some embodiments, the total weight percentage of 4-methyl-1-pentene units and propylene repeating units is at least 99 weight percent, at least 99.5 weight percent, or even 100 weight percent based on the total weight of the monomer units in the surface layer resin. In some embodiments, the surface layer resin may contain additional comonomers, for example, at least one additional α-olefin comonomer, such as ethylene, 1-hexene, and 1-octene. If present, the surface layer resin may contain a combined amount of 0.05 weight percent to 2 weight percent, for example, 0.1 weight percent to 1 weight percent, or even 0.1 weight percent to 0.5 weight percent of such additional comonomers.

[0022] In some embodiments, the surface layer is impermeable to liquids, for example, a solid surface layer without pores or a microporous surface layer that is impermeable to liquids but permeable to gases. In some embodiments, the surface layer can efficiently promote gas absorption and / or degassing of liquids even though the surface layer is impermeable to liquids.

[0023] The thickness of the skin can depend on the specific application of wherein adopting microporous asymmetric hollow fiber.Generally speaking, reducing the thickness of the skin causes higher gas flux and more efficient asymmetric microporous asymmetric hollow fiber membrane.In some embodiments, the thickness of the skin can be no more than 20 microns, for example, no more than 5 microns or even no more than 3 microns.In some embodiments, the skin is at least 0.1 micron thick, for example, at least 0.5 micron thick, for example, 0.1 micron to 20 microns thick, for example, 0.1 micron to 10 microns, 0.5 micron to 10 microns, 0.5 micron to 5 microns or 1 micron to 3 microns thick.

[0024] In some cases, the boundary may include one or more additional layers, any of which may include the outer surface of the asymmetric hollow fiber membrane, the surface of the cavity, or an intermediate layer. Similarly, when one or more additional layers are present, one or both of the porous substrate and the surface layer may be an intermediate layer. In any case, each layer may be described as covering the other layers.

[0025] For example, an asymmetric hollow fiber membrane may include at least three layers forming a boundary around a hollow central lumen. Figure 3 380 includes a first porous substrate 350, a skin layer 360, and a second porous substrate 370 that surround the cavity 310 of the asymmetric hollow fiber membrane 300. In this example, the first porous substrate 350 includes a surface 320 of the cavity 310, that is, the surface of the boundary 380 that is adjacent to the cavity. The second porous substrate 370 includes an outer surface 330 of the asymmetric hollow fiber membrane 300, that is, the surface of the boundary 380 that is exposed to the surrounding environment. The skin layer 360 is sandwiched between the first porous substrate 350 and the second porous substrate 370.

[0026] In some cases, the first porous substrate and the second porous substrate comprise the same material. In some cases, different materials can be used to form the first porous substrate and the second porous substrate. For example, in some cases, the materials used to form the first porous substrate and the second porous substrate can be independently selected from the above materials.

[0027] The second porous substrate can act as a protective layer, thereby protecting the thin surface layer from damage during subsequent processing (such as stretching). Generally speaking, the thickness of the second porous substrate is less than the thickness of the first porous substrate. In some cases, the thickness of the second porous substrate is not greater than 10 microns, for example, not greater than 5 microns or even not greater than 3 microns. In some cases, the thickness of the second porous substrate is at least 0.5 microns, for example, at least 1 micron or even not greater than 2 microns. In some cases, the second porous substrate is 0.5 microns to 10 microns thick, for example, 0.5 microns to 5 microns, 0.5 microns to 3 microns, or even 1 micron to 3 microns thick.

[0028] Depending on the desired asymmetric hollow fiber structure and the desired asymmetric hollow fiber composition, various known preparation methods can be used to make the hollow fiber membranes described herein. Microporous membranes can be made according to various preparation techniques, such as wet, particle stretching, and dry stretching (also known as CELGARD).

[0029] Generally speaking, in the wet process (also referred to as phase inversion, extraction or TIPS), a polymer raw material is mixed with an oil, a solvent and / or another material. The mixture is extruded and pores are formed when such oil, solvent and / or other material is removed. These membranes can be stretched before or after removing the oil, solvent and / or other material.

[0030] Generally, in the particle stretching method, a polymer raw material is mixed with microparticles, the mixture is extruded, and pores are formed during stretching when the interface between the polymer and the microparticles is broken due to the stretching force.

[0031] The dry process differs from the wet process and the particle stretching process in that the porous asymmetric hollow fibers are typically prepared without the addition of processing oils, oils, solvents, plasticizers, etc. or particulate materials. Thus, although other methods may be used, the substrates and skin materials disclosed herein may be used in a dry stretching process to prepare asymmetric hollow fiber membranes having a desired porosity without the need for liquid or particulate additives. Thus, preferably, the porous asymmetric hollow fibers are prepared via The microporous membrane is formed by a process (also known as an "extrusion, annealing, stretching" or "dry stretching" process) whereby a semi-crystalline polymer is extruded to provide an asymmetric hollow fiber precursor, and porosity is induced in the microporous substrate by stretching the extruded precursor.

[0032] In a typical dry process, the resins of the skin layer and the substrate layer (along with any optional additional layers) are coextruded through an annular coextrusion die to form an asymmetric fiber precursor. The asymmetric fiber precursor is then stretched to form an asymmetric hollow fiber comprising a porous substrate layer and a skin layer surrounding a hollow cavity.

[0033] In general, the asymmetric fiber precursor is stretched to form an open porous structure by a uniaxial stretch of at least 10% and at most 500% (e.g., 50% to 300% or 100% to 200%). Single-stage or multi-stage cold stretching can be used, optionally followed by single-stage or multi-stage hot stretching to perform stretching. In general, the cold stretching temperature can be, for example, 20°C to 90°C, for example, 30°C to 70°C. The hot stretching temperature can be, for example, 100°C to 200°C, for example, 120°C to 170°C.

[0034] In some embodiments, the asymmetric fiber precursor can be annealed prior to stretching. For example, the asymmetric fiber precursor can be exposed to a temperature of, for example, 100° C. to 150° C. for, for example, 5 minutes to 30 minutes.

[0035] In some embodiments, the asymmetric hollow fiber membrane can be heat treated or heat set after stretching to reduce the stress in the fiber. The heat setting temperature is usually selected to be at least 5 ° C, at least 10 ° C or even at least 15 ° C higher than the hot stretching temperature. The heat setting duration is usually selected to be at least 30 seconds, at least one minute or at least 90 seconds. Alternatively or in addition, the asymmetric hollow fiber membrane can be subjected to a stress relaxation step to allow the fiber length to shrink to a certain extent, for example, at least 2% or even at least 5%.

[0036] In some embodiments, when a skin layer is adjacent to a porous substrate, the polymer of the skin layer can diffuse into the pores of the porous substrate. Such diffusion can improve interlayer adhesion, but can also reduce gas transport through the porous layer.

[0037] Examples The materials used in the examples are summarized in Table 1.

[0038] Table 1: Summary of materials used in the preparation of the examples .

[0039]

[0040] Skin resin analysis. After dissolving the two skin resins (PMP-B and PMP-C) in trichloroethylene (TCE), the skin resins were analyzed at elevated temperatures using a BRIKER 600 MHz spectrometer (Bruker Company) with an inverse cryoprobe at 400 K. 13C NMR analysis. The spectra were compared with polyethylene and polypropylene. Each component was analyzed quantitatively, as summarized in Table 2. As shown, these resins contain much higher amounts of propylene than PMP-A (which has the highest level of comonomer in the PMP grades and is typically used for the skin layers in coextruded hollow fiber membranes).

[0041] Table 2: Composition of PMP surface layer resin (weight %) .

[0042] name 4-Methyl-1-pentene Propylene Ethylene 1-Hexene Total monomers PMP-A 94.0 -- 3.9 2.3 6.1 PMP-B 85.3 14.4 0.3 -- 14.7 PMP-C 92.8 6.9 0.3 -- 7.2

[0043] Differential scanning calorimetry was performed on each of the PMP resins using a heating ramp of 10.0°C to 200°C per minute and a cooling ramp of 10.0°C to -20°C per minute. PMP-A exhibited a melting peak at 224°C and a crystallization peak at 211°C. In contrast, PMP-B exhibited neither a melting peak nor a crystallization peak. PMP-C exhibited a melting peak at 130°C, but no crystallization peak. Thus, PMP-B and PMPs clearly exhibited different behaviors from semi-crystalline PMPs such as PMP-A.

[0044] Asymmetric fiber precursor (AFP) is prepared as follows. Extrude skin layer resin and substrate resin by two independent single screw extruders. The two melts are fed into the fiber spinning die by a melt gear pump. The die has an annular hole and a central hole for core air supply. The asymmetric hollow fiber precursor through co-extrusion is quenched by an air ring, which provides fiber with a uniform environment. The fiber in its molten state is pulled out by three groups of godet rollers with a fiber drawing speed of 100 meters per minute. The material for substrate and skin layer, extruder temperature and melt pump rate (cubic centimeters per minute) and die temperature are shown in Table 3.

[0045] Table 3: Asymmetric fiber precursor formation parameters .

[0046]

[0047] Asymmetric hollow fiber membranes are prepared by annealing and then dry stretching of asymmetric fiber precursors. In order to anneal, asymmetric fiber precursor bundles (about 25 cm long) are prepared by tying them together at one end. Each bundle is then annealed in a convection oven set at a temperature as shown in Tables 4 to 6. At the set temperature, the annealing time for each asymmetric fiber precursor bundle is 15 minutes. In order to perform dry (hot / cold) stretching, the bundle (annealed or unannealed) is clamped in a temperature-controlled environmental chamber of an Instron Mechanical Tester (Model 5969, Norwood, Massachusetts). 127 mm (5 inch) long fibers are cold stretched at a stretching rate of 600 mm / min at 25°C, and then hot stretched at a stretching rate of 30 mm / min at 120°C. The total extension ratio after 10% relaxation is shown in Tables 4 to 6.

[0048] The resulting CO2 flow rate (GPU) and CO2 / N2 selectivity were measured as follows.

[0049] The gas permeability test is used as the integrity test for non-porous skin and for the performance test of hollow fiber membrane.By using epoxy resin adhesive, hollow fiber is sealed together in 0.65cm (1 / 4 inch) OD nylon tube to prepare the ring module.By cutting the sealing tube with a razor blade, the cavity of each fiber is exposed.Each ring module comprises 10 fibers with about 10.2cm (4 inches) effective length.

[0050] Use a custom designed test bench to carry out gas permeability test. The bench is equipped with gas cylinders, pressure gauges and online gas flow meters of pure gas (CO2 and N2). The principle of this test is to supply pure gas into the fiber cavity and measure the rate at which gas leaks into the surrounding environment through the fiber wall. Use CO2 and N2 to test each fiber ring module respectively. The gas pressure in the fiber cavity is usually set at about 207kPa (30psi). Monitor both gas pressure and gas flow rate by data acquisition software, and obtain data when both pressure and gas flow rate are stable.

[0051] The gas permeation rate (GPU) of each fiber membrane was calculated as follows

[0052]

[0053] Where: Q is the gas flow rate (scc / sec);

[0054] ΔP is the gas pressure differential reading (cm Hg); and

[0055] A is the outer surface area of ​​the fiber (cm 2 )

[0056] Fiber gas selectivity has been used as an indicator of surface integrity. According to literature reports (Polymer, 1989, 30, p. 1357), the selectivity of PMP is usually in the range of 11 to 13. Any fiber with a gas selectivity below 8 is considered to have a defective surface. The CO2 / N2 selectivity of the fiber is calculated from the gas permeation rate of each gas as shown below.

[0057]

[0058] PMP-C (AFP-2 and AFP-3) and PMP-B (AFP-4) were used as skin layer resins to prepare samples annealed at 25° C. The process conditions and results are summarized in Table 4.

[0059] Table 4: Dry-stretched asymmetric hollow fiber membranes .

[0060]

[0061] *Not measured, based on a fiber diameter of 300 microns.

[0062] Additional samples were prepared using PMP-C modified by addition of polypropylene (AFP-5 and AFP-6) as the skin resin. PP-1 was used as the base resin in all samples. The samples were annealed at 25°C and the results are summarized in Table 5 and compared with Sample 1 having an unmodified PMP-C skin resin.

[0063] Table 5: Dry-stretched asymmetric hollow fiber membranes with modified skin layers .

[0064] sample Precursor Stretch ratio Fiber OD (micrometer) <![CDATA[CO2(GPU)]]> <![CDATA[Selectivity CO2 / N2]]> 1 AFP-2 100％ 259 33±1 15±2 6 AFP-5 100％ 262 17.5 10.5 7 AFP-6 200％ 257 15.9 11.3

[0065] Samples were also prepared using PMP-C and modified PMP-C as the skin resin. The base resin was PP-1 in all samples. These samples were annealed at a temperature greater than 100°C. The results are in Table 6.

[0066] Table 6: Asymmetric hollow fiber membranes annealed at high temperature .

[0067]

[0068] *Not measured, based on a fiber diameter of 300 microns.

[0069] Three-layer asymmetric fiber precursor (AFP) is prepared as follows. Surface resin and substrate resin are extruded by two independent single screw extruders. The two melts are fed into another fiber spinning die by a melt gear pump. The die has an annular hole and a central hole for core air supply. In the die, the substrate resin melt is separated into two streams to sandwich the surface melt before leaving the die face. The co-extruded asymmetric hollow fiber precursor is quenched by an air ring, which provides fiber with a uniform environment. The fiber in its molten state is pulled out by three groups of godet rollers at a fiber drawing speed of 100 meters per minute. The material, extruder temperature and melt pump rate (cubic centimeters per minute) and die temperature for substrate material and surface material are shown in Table 7.

[0070] A cross section of AFP-10 was analyzed by electron scanning microscopy (SEM, Model Hitachi TM4000 plus II, obtained from Hitachi High-Tech Corporations, Japan). A three-layer structure can be clearly seen: a thick inner layer (44.3 microns), a thin middle layer (1.21 microns), and a thin outer layer (2.59 microns). The middle layer is formed of the surface layer material; and both the inner layer and the outer layer are formed of the substrate material.

[0071] Table 7: Parameters of three-layer asymmetric fiber precursor formation

[0072]

[0073] Three-layer asymmetric hollow fiber membranes were prepared from these asymmetric fiber precursors by annealing followed by dry stretching. Asymmetric fiber precursor bundles (about 25 cm long) were prepared by tying them together at one end for annealing. Since the skin layer is protected by the second substrate layer, it becomes more practical to process these bundles at elevated temperatures without generating defects in the skin layer.

[0074] Each bundle was annealed in a convection oven set at a temperature of 140°C. The annealing time for each asymmetric fiber precursor bundle was 15 minutes at the set temperature. For dry (hot / cold) stretching, the bundle was clamped in a temperature-controlled environmental chamber of an Instron Mechanical Tester (Model 5969, Norwood, Massachusetts). 127 mm (5 inch) long fibers were cold stretched at a stretching rate of 600 mm / min at 25°C and subsequently hot stretched at a stretching rate of 30 mm / min at 120°C. The total extension ratio after 10% relaxation is shown in Table 8.

[0075] The resulting CO2 flow rate (GPU) and CO2 / N2 selectivity were measured according to the above method. These results are also summarized in Table 8.

[0076] Table 8: Dry-stretched three-layer asymmetric hollow fiber membrane .

[0077]

[0078] Samples 14 to 17 were obtained from the same three-layer asymmetric fiber precursor, but with different stretch ratios. It can be seen that the gas flux increases with increasing the stretch ratio up to 150%; then decreases with further increasing the stretch ratio. However, the gas selectivity remains constant. It appears that thinner skin layers are formed with increasing stretch ratio without generating any micropores or defects in the protected skin layer. However, when stretched up to 180%, the resistance of the porous substrate layer to gas flow can become significant, which can cause the flux to drop.

[0079] Sample 14 was further characterized by examining the external and internal surfaces using high-resolution electron scanning microscopy (Field Emission-SEM, Model Hitachi S-4700, obtained from Hitachi High-Technologies Corporation, Japan). It was found that both the external and internal surfaces had a porous microstructure.

[0080] The separation article can be prepared from the asymmetric hollow fiber membrane of the present disclosure using known methods (such as those described in International Publication No. WO 2021 / 105838 A1). For example, a plurality of asymmetric hollow fiber membranes can be arranged substantially in parallel in an array pattern and fastened together by, for example, weaving or tying the individual hollow fiber membranes together using ropes, lines, yarns, etc. In some embodiments, the array can be pleated, folded, or wound into a tube or box.

Claims

1. An asymmetric hollow fiber membrane comprising a first porous substrate and a skin layer covering the porous substrate, wherein the first porous substrate comprises a polyolefin polymer and has a plurality of pores surrounding an internal cavity, wherein the skin layer comprises a copolymer, the copolymer comprising at least 80 weight percent of 4-methyl-1-pentene units and at least 5 weight percent of propylene units, wherein the total weight percent of 4-methyl-1-pentene units and propylene units based on the total weight of the monomer units in the copolymer is at least 98%.

2. An asymmetric hollow fiber membrane according to claim 2, wherein the copolymer of the surface layer contains 84 weight percent to 94 weight percent of 4-methyl-1-pentene units and 6 weight percent to 16 weight percent of propylene units, optionally wherein the copolymer of the surface layer contains 90 weight percent to 94 weight percent of 4-methyl-1-pentene units and 6 weight percent to 8 weight percent of propylene units.

3. The asymmetric hollow fiber membrane according to claim 1 or 2, wherein the copolymer of the skin layer further comprises 0.1 weight percent to 0.5 weight percent of at least one additional α-olefin comonomer.

4. The asymmetric hollow fiber membrane according to any one of claims 1 to 3, wherein the polyolefin polymer comprises polypropylene.

5. The asymmetric hollow fiber membrane according to any one of claims 1 to 4, wherein the pores have a diameter of 0.01 to 1 micron.

6. The asymmetric hollow fiber membrane according to any one of claims 1 to 5, wherein the asymmetric hollow fiber membrane has a porosity of 5% to 80%, optionally wherein the porosity is at least 20%.

7. The asymmetric hollow fiber membrane according to any one of claims 1 to 6, wherein the skin layer is non-porous.

8. The asymmetric hollow fiber membrane according to any one of claims 1 to 7, wherein the skin layer has a thickness of 0.1 to 20 microns, optionally 0.5 to 10 microns.

9. The asymmetric hollow fiber membrane according to any one of claims 1 to 10, wherein the first porous substrate has a thickness of 5 micrometers to 200 micrometers.

10. The asymmetric hollow fiber membrane according to any one of claims 1 to 9, wherein the first porous substrate comprises a surface of the cavity.

11. The asymmetric hollow fiber membrane according to any one of claims 1 to 10, wherein the skin layer comprises an outer surface of the asymmetric hollow fiber membrane.

12. The asymmetric hollow fiber membrane according to any one of claims 1 to 9, wherein the skin layer comprises a surface of the cavity.

13. The asymmetric hollow fiber membrane according to any one of claims 1 to 9, further comprising a second porous substrate, the second porous substrate comprising a polyolefin polymer and having a plurality of pores, wherein the first porous substrate comprises a surface of the cavity, the second porous substrate comprises an outer surface of the asymmetric hollow fiber membrane, and the surface layer is sandwiched between the first porous substrate and the second porous substrate.

14. The asymmetric hollow fiber membrane of claim 13, wherein the second porous substrate has a thickness of 0.5 micrometers to 10 micrometers.

15. A separation article comprising a plurality of asymmetric hollow fiber membranes according to any one of claims 1 to 14 arranged substantially in parallel in an array and fastened together.

16. The separation article of claim 15, wherein the plurality of asymmetric hollow fiber membranes are arranged in an array, wherein the array is pleated, folded, or rolled into a cartridge or box.

17. The separation product according to claim 15 or 16, wherein the separation product has a CO2 / N2 selectivity of at least 10, optionally wherein the separation product has a CO2 / N2 selectivity of at most 20.

18. The separation product of any one of claims 15 to 17, wherein the separation product has a CO2 gas flux of at least 15 GPU, optionally wherein the separation product has a CO2 gas flux of at most 65 GPU.

19. A method for preparing an asymmetric hollow fiber membrane according to any one of claims 1 to 14, the method comprising: coextruding a substrate resin comprising polypropylene and a skin resin comprising the copolymer to form an asymmetric fiber precursor, the copolymer comprising at least 80 weight percent of 4-methyl-1-pentene units and at least 5 weight percent of propylene units; and The asymmetric fiber precursor is stretched to form the asymmetric hollow fiber membrane, wherein the asymmetric hollow fiber membrane has a skin layer covering a porous substrate, the skin layer being composed of the skin layer resin, the porous substrate being composed of the substrate resin surrounding the inner cavity, wherein the porous substrate includes the plurality of pores.

20. The method according to claim 19, further comprising: The asymmetric fiber precursor is annealed.

Images