A compression-resistant, collapse-resistant, porous nanomass transfer membrane and a method of making the same

Abstract

The application discloses a kind of porous nanometer mass transfer membrane of anti-pressure-anti-collapse and preparation method thereof, belong to membrane separation technical field;The mass transfer membrane includes macroscopic rigid substrate layer, thermosetting polyimide mesoporous support layer formed by aliphatic diamine crosslinking, and high-crosslinking polyurea ultrathin active layer formed by in-situ reaction of diisocyanate and diamine.Its preparation method mainly includes forming polyimide porous membrane on substrate layer by phase inversion, then in-situ crosslinking is carried out to construct support layer, and finally active layer is formed on the surface of support layer by electrospraying technology.The composite membrane structure significantly improves the overall rigidity and anti-compaction ability, can long-term maintain excellent permeation flux, high desalination rate and stable pore structure under high pressure and high salt, high temperature harsh conditions of ≥250bar, and is suitable for seawater desalination, high-salinity wastewater treatment and other high-pressure separation processes.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation and nanocomposite membrane improvement technology, specifically relating to a pressure-resistant and collapse-resistant porous nanocomposite membrane and its preparation method. Background Technology

[0002] Membrane separation technology, due to its advantages of low energy consumption, compact process, and high separation efficiency, has been widely used in seawater desalination, brackish water treatment, industrial wastewater reuse, organic solvent separation, and mass transfer and separation processes in high-salt and high-temperature systems. Among them, nano-mass transfer membranes, represented by reverse osmosis membranes and nanofiltration membranes, can achieve effective separation of water molecules and solutes at the molecular scale, and are key functional materials for resource utilization in high-salt systems and in challenging separation processes.

[0003] Existing nanoscale mass transfer membranes typically employ a thin-film composite structure, where an ultrathin separation active layer is formed on the surface of a porous support layer. These membranes exhibit high desalination rates and good permeability under normal operating pressures and mild conditions, leading to their widespread application in practical engineering. However, as applications evolve towards high-recovery concentration, deep treatment of concentrated brine, and extreme conditions such as high temperature and high salinity, the shortcomings of nanoscale mass transfer membranes in terms of structural stability and long-term operational reliability are becoming increasingly apparent.

[0004] On the one hand, nanoscale mass transfer membranes are highly dependent on the overall structural rigidity of the membrane under high operating pressure conditions. The support layers widely used in existing membrane materials are mostly thermoplastic polymers such as polysulfone and polyethersulfone, which are prone to creep and irreversible compaction under long-term high pressure. Insufficient structural rigidity leads to shrinkage and deformation accumulation in the support layer pores, which is then transferred to the active layer through the interface, resulting in a reduction in the number of effective mass transfer channels, a significant decrease in permeate flux, and even irreversible degradation of separation performance. Under coupled conditions such as high salt and high temperature, these compaction and structural relaxation phenomena are even more pronounced, severely restricting the application of membranes under extreme conditions.

[0005] On the other hand, the separation performance of nanoscale mass transfer membranes is mainly determined by the structure of the active layer, and existing active layers are mostly prepared by interfacial polymerization. Although this method can rapidly form a dense thin layer, its reaction process has obvious self-inhibition characteristics: as the polymerization reaction proceeds, the already formed dense membrane layer will hinder the further diffusion and reaction of monomers, making the cross-linking reaction often limited to the surface area of ​​the membrane layer, making it difficult to form a highly uniform, three-dimensional interconnected cross-linked network structure. The active layer with limited cross-linking degree is more prone to compaction and free volume collapse under high pressure, thereby causing flux decline and selectivity changes.

[0006] To improve the compaction resistance of nanocompression membranes, existing technologies have attempted modifications such as increasing the degree of crosslinking in the active layer, increasing membrane thickness, or introducing inorganic fillers. However, these methods often have significant limitations: simply increasing the degree of crosslinking or membrane thickness can significantly increase mass transfer resistance, leading to a decrease in permeation flux; while introducing inorganic fillers may cause interfacial defects, uneven dispersion, or insufficient long-term operational stability. Furthermore, existing technologies mostly focus on regulating the performance of the active layer, paying insufficient attention to the crucial role of the support layer in overall compaction resistance, especially lacking a high-rigidity support system capable of maintaining pore structure stability under ultra-high pressure conditions over a long period.

[0007] Therefore, under current technological conditions, nanoscale mass transfer membranes generally face the following technical contradictions: on the one hand, it is necessary to increase the degree of crosslinking and structural rigidity to suppress compaction and improve high-pressure resistance; on the other hand, it is also necessary to balance the membrane's permeability, desalination rate, retention performance, and mass transfer efficiency in various scenarios to avoid overall performance imbalance due to excessive structural density. How to achieve synergistic optimization of various mass transfer membrane properties while significantly improving the overall structural rigidity and anti-compaction ability of nanoscale mass transfer membranes through reasonable membrane structure design and preparation method adjustments remains a key technical problem that urgently needs to be solved in this field.

[0008] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention provides a pressure-resistant and collapse-resistant porous nanomaterial transfer membrane and its preparation method.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: a pressure-resistant and collapse-resistant porous nano-mass transfer membrane, wherein the nano-mass transfer membrane comprises, from top to bottom: an ultrathin highly cross-linked active layer, a mesoporous cross-linked support layer, and a macroscopic rigid substrate layer; the support layer is composed of polyimide (PI) and cross-linked with aliphatic diamine to form a thermosetting porous network; the active layer is a highly cross-linked polyurea formed by the in-situ reaction of diisocyanate and diamine.

[0011] Preferably, the base layer is either polyester (PET) or polypropylene (PP) nonwoven fabric.

[0012] Furthermore, polyester (PET) is preferred as the base layer.

[0013] Preferably, the aliphatic diamine in the support layer includes at least one of 1,4-butanediamine, 1,6-hexanediamine, and 1,8-octanediamine.

[0014] Preferably, the support layer is formed by phase inversion and in-situ crosslinking in a crosslinking solution, wherein the phase inversion solvent system comprises one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), or N-methylpyrrolidone (NMP).

[0015] Preferably, the thickness of the support layer is 40-120 μm, the average pore size is 30-80 nm, and the porosity is 40-50%.

[0016] Preferably, the active layer is formed by the in-situ reaction of a diamine with a diisocyanate, wherein the diisocyanate is selected from one or more of toluene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI), and the diamine includes aromatic diamines and aliphatic diamines.

[0017] Preferably, the molar ratio of -NCO to -NH2 in the active layer is 1.00:(0.95-1.05), the molar ratio of the aromatic diamine to the aliphatic diamine is 70:30-85:15, and the target degree of crosslinking of the active layer is 80-90%.

[0018] A second aspect of this invention provides a method for preparing a pressure-resistant and collapse-resistant porous nanomaterial mass transfer membrane, comprising the following steps: 1) Select a substrate layer for plasma treatment, prepare a phase inversion solution containing polyimide, and then coat it onto the surface of the substrate layer to form a porous membrane; 2) Introduce diamine into the porous membrane obtained in step 1) to perform in-situ crosslinking at 40-70 °C to obtain a thermosetting crosslinked porous support layer; 3) The surface of the cross-linked support layer obtained in step 2) is coated with an ultrathin highly cross-linked active layer by electro-spraying-in-situ cross-linking to obtain a porous nano-mass transfer membrane.

[0019] Preferably, in step 2), the molar ratio of the polyimide repeating unit to the aliphatic diamine is 1:(0.15-0.35), and the crosslinking is carried out at 40-70 °C for 1-4 h.

[0020] Preferably, in step 3), the active layer thickness is 80-200 nm, the effective pore size is 0.35-0.55 nm, the electro-spraying voltage is 10-20 kV, the spraying distance is 10-15 cm, and the ambient relative humidity is 40-70%.

[0021] Furthermore, the solvent system used in the electro-spraying process includes a non-polar organic solvent and an aqueous system, wherein the non-polar organic solvent is selected from hexane and isooctane.

[0022] Preferably, the nano-mass transfer membrane is used for seawater desalination, concentrated brine, treatment of industrial high-concentration salt wastewater, or resource recovery under high temperature and high salinity conditions; under operating pressure ≥250 bar and high salinity and high temperature conditions, the permeation flux retention rate of the nano-mass transfer membrane is not less than 90%.

[0023] This invention achieves an ideal synergistic balance between structural rigidity, compaction resistance, separation performance, and long-term operational stability by employing a three-layer composite structure (macroscopic rigid substrate layer - mesoporous cross-linked polyimide support layer - ultrathin highly cross-linked polyurea active layer) and a matching preparation process, thereby bringing the following technical effects; 1) First, by constructing an in-situ cross-linked polyimide support layer on a macroscopic rigid substrate layer and forming an ultrathin, highly cross-linked polyurea active layer on its surface, the membrane has high compressibility at both the overall and local scales. This effectively suppresses the collapse of the support channels and the irreversible shrinkage of the free volume of the active layer under high pressure conditions, thereby significantly improving the flux retention rate and flux recovery capability after pressure relief.

[0024] 2) Secondly, by synergistically limiting the monomer molar ratio, reaction time, reaction temperature, solvent system, and additives, this invention overcomes the self-inhibition effect commonly present in interfacial polymerization, enabling the active layer to achieve a high degree of cross-linking while maintaining an ultrathin structure. This improves the rigidity and pressure resistance of the film layer while avoiding a significant increase in mass transfer resistance due to excessive densification, achieving an effective balance between high desalination rate and high throughput. The support layer adopts a cross-linked polyimide network structure, which significantly reduces thermoplastic creep and structural fatigue during long-term operation. Combined with the mechanical support of the macroscopically rigid substrate layer, it can maintain stable interlayer bonding and pore structure under complex operating conditions such as high pressure, high salt, and high temperature.

[0025] In summary, through the synergistic optimization of the above-mentioned structural design and preparation methods, the composite membrane obtained by this invention exhibits excellent structural stability, anti-compaction properties, and consistent separation performance under long-term high-pressure operation conditions. This not only reduces the energy consumption increase caused by flux decay over time, but also improves the membrane's performance recovery ability during cleaning and depressurization processes, extends its service life, and reduces operating and maintenance costs. It is suitable for seawater desalination, high-salinity wastewater treatment, and other ultra-high-pressure separation scenarios, and has significant engineering application value. Attached Figure Description

[0026] Figure 1 This is a schematic diagram showing the changes in thickness and pore size before and after compaction. Detailed Implementation

[0027] The present invention will be further described below through specific embodiments. To make the inventive objectives, technical solutions, and beneficial technical effects of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments. It should be understood that the embodiments described in this specification are merely for explaining the present invention and are not intended to limit the present invention.

[0028] Unless otherwise stated, all instruments and reagents used in the examples are commercially available or synthesized using conventional methods and can be used directly without further processing, and all instruments used in the examples are commercially available.

[0029] Example 1:

[0030] Polyester (PET) nonwoven fabric was selected as the macroscopic rigid substrate layer and subjected to plasma treatment to improve surface energy and enhance interfacial adhesion with subsequent film layers. Subsequently, a polyimide phase inversion membrane solution was prepared, with N,N-dimethylformamide as the solvent system. The solution was uniformly coated on the surface of the treated substrate layer, and an initial porous polyimide membrane was formed through the phase inversion process, thereby obtaining a porous support membrane preform attached to the substrate layer. Next, the porous support membrane was placed in a crosslinking solution containing an aliphatic diamine, selected from 1,4-butanediamine, and the molar ratio of polyimide repeating units to aliphatic diamine was 1:0.2. The reaction was carried out at 55 °C for 2 h to allow the aliphatic diamine to crosslink with the reaction sites in the polyimide molecular chain. This transformed the support layer into a thermosetting mesoporous crosslinked network without destroying the original pore structure, resulting in a compression- and collapse-resistant crosslinked porous support layer with a thickness of 80 μm, an average pore size of 50 nm, and a porosity of 40%.

[0031] Subsequently, an ultrathin, highly cross-linked active layer is constructed on the surface of the aforementioned cross-linked support layer. The reaction system containing a diamine and a diisocyanate is sprayed onto the support layer surface via electrospraying. The diisocyanate is selected from toluene diisocyanate, and the diamine includes aromatic and aliphatic diamines, wherein the molar ratio of aromatic to aliphatic diamines is 70:30, and the molar ratio of -NCO to -NH2 is 1:1. During the electrospraying process, the solvent system used includes a non-polar organic solvent and an aqueous phase system, wherein the non-polar organic solvent is selected from n-hexane. During the electro-spraying process, the spraying voltage is controlled at 10-20 kV, the spraying distance is 10-15 cm, and the relative humidity of the environment is 40-70%, so that the diamine and diisocyanate react in situ on the surface of the support layer to form an ultrathin highly cross-linked polyurea with a thickness of 140 nm, an effective pore size of 0.4 nm, and a cross-linking degree of 85%.

[0032] Example 2:

[0033] Polyester (PET) nonwoven fabric was selected as the macroscopic rigid substrate layer and subjected to plasma treatment to improve surface energy and enhance interfacial adhesion with subsequent film layers. Subsequently, a polyimide phase inversion membrane solution was prepared, with N,N-dimethylacetamide as the solvent system. The solution was uniformly coated on the surface of the treated substrate layer, and an initial porous polyimide membrane was formed through the phase inversion process, thereby obtaining a porous support membrane preform attached to the substrate layer. Next, the porous support membrane was placed in a crosslinking solution containing an aliphatic diamine, specifically 1,6-hexanediamine, with a molar ratio of polyimide repeating units to aliphatic diamine of 1:0.15. The reaction was carried out at 70°C for 1 hour, allowing the aliphatic diamine to crosslink with the reaction sites in the polyimide molecular chain. This transformed the support layer into a thermosetting mesoporous crosslinked network without damaging the original pore structure, resulting in a compression- and collapse-resistant crosslinked porous support layer with a thickness of 60 μm, an average pore size of 30 nm, and a porosity of 45%. Subsequently, an ultrathin, highly cross-linked active layer is constructed on the surface of the aforementioned cross-linked support layer. The reaction system containing diamine and diisocyanate is sprayed onto the support layer surface via electrospraying. The diisocyanate is diphenylmethane diisocyanate, and the diamine includes aromatic and aliphatic diamines, wherein the molar ratio of aromatic to aliphatic diamines is 85:15, and the molar ratio of -NCO to -NH2 is 1.00:0.95. During the electrospraying process, the solvent system used includes a non-polar organic solvent and an aqueous phase system, wherein the non-polar organic solvent is isooctane. During the electro-spraying process, the spraying voltage is controlled at 10-20kV, the spraying distance is 10-15cm, and the relative humidity of the environment is 40-70%, so that the diamine and diisocyanate react in situ on the surface of the support layer to form an ultra-thin highly cross-linked polyurea active layer with a thickness of 100nm, an effective pore size of 0.35nm, and a target cross-linking degree of 90%.

[0034] Example 3:

[0035] Polyester (PET) nonwoven fabric was selected as the macroscopic rigid substrate layer and subjected to plasma treatment to improve surface energy and enhance interfacial adhesion with subsequent film layers. Subsequently, a polyimide phase inversion membrane solution was prepared, with N-methylpyrrolidone as the solvent system. The solution was uniformly coated on the surface of the treated substrate layer, and an initial porous polyimide membrane was formed through the phase inversion process, thereby obtaining a porous support membrane preform attached to the substrate layer. Next, the porous support membrane was placed in a crosslinking solution containing an aliphatic diamine, specifically 1,8-octanediamine, with a molar ratio of polyimide repeating units to aliphatic diamine of 1:0.35. The reaction was carried out at 40°C for 4 hours, allowing the aliphatic diamine to crosslink with the reaction sites in the polyimide molecular chain. This transformed the support layer into a thermosetting mesoporous crosslinked network without damaging the original pore structure, resulting in a compression- and collapse-resistant crosslinked porous support layer with a thickness of 120 μm, an average pore size of 75 nm, and a porosity of 50%. Subsequently, an ultrathin, highly cross-linked active layer is constructed on the surface of the aforementioned cross-linked support layer. The reaction system containing a diamine and a diisocyanate, specifically toluene diisocyanate, is sprayed onto the support layer surface via electrospraying. The molar ratio of the aromatic to aliphatic diamine in the diamine is 80:20, and the molar ratio of -NCO to -NH2 is 1.00:1.05. The solvent system used in the electrospraying process includes a non-polar organic solvent and an aqueous phase system; the non-polar organic solvent is isooctane. During the electro-spraying process, the spraying voltage is controlled at 10-20kV, the spraying distance is 10-15cm, and the relative humidity of the environment is 40-70%, so that the diamine and diisocyanate react in situ on the surface of the support layer to form an ultrathin highly cross-linked polyurea with a thickness of 180nm, an effective pore size of 0.55nm, and a target cross-linking degree of 80%.

[0036] Comparative Example 1: The same substrate and support were used, but the support layer was not cross-linked (the diamine cross-linking step was omitted); the active layer was formulated in the same way as in Example 1.

[0037] Comparative Example 2: The support layer was cross-linked as in Example 1, but the active layer had a low degree of cross-linking (the -NCO: -NH2 ratio was adjusted to 1:0.8, with a target DC ≈ 50%).

[0038] Comparative Example 3: The support layer material was replaced with commonly used PSF (thermoplastic) support, and the support thickness / pore structure was matched as closely as possible to Example 1.

[0039] Comparative Example 4: The active layer thickness is increased to 500nm.

[0040] Performance tests were conducted to compare Examples 1-3 with Comparative Examples 1-4, and the experimental conditions were as follows: 1. Mechanical and compressive deformation testing: Wet tensile testing (ASTM similar method) is used to determine tensile strength and wet modulus; Thickness measurement after compaction: The percentage change in thickness before and after compaction is used as the measurement result, as shown in the table below: Note that the above refers to the thickness variation at the overall membrane layer level; And the following table: Note: The above describes the change in pore diameter before and after compaction; 2. Osmosis and Desalination Test (Wet State) Equipment: High-pressure constant-pressure transmembrane device (pressure resistance ≥300 bar).

[0041] Feed: Two sets of NaCl solutions, 35 g / L (near seawater) and 180 g / L (high salt concentration); temperatures of 25℃ and 80℃; stirring rate to avoid concentration difference.

[0042] Procedure: Initially, measure A0 (initial permeability) at 60 bar, then increase the pressure to 250 bar and maintain it for 24 h (or 72 h) while recording the At (t = 24 h) flux; after depressurization for 30 min, measure A90 (flux after depressurization recovery).

[0043] calculate: Compaction degree XC = (A0 - At) / A0.

[0044] Release recovery rate RR = (A90 - At) / A0 or RR = (A90 - A60) / A0.

[0045] Salt rejection rate = (1 - Cp / Cf) × 100%, Cp and Cf were measured; the results are shown in the table below: And the following table: Note that the data in the table above represents the desalination rate at 35 g / L. Example 2 maintained the highest desalination rate at 250 bar because its active layer had the highest cross-linking degree and a smaller initial active pore size. Example 3, due to its lower active layer DC and larger active pore size, showed a significant decrease in desalination rate (-0.4%) under high pressure, demonstrating the influence of active layer DC and pore size on selectivity. Comparative Examples 1 and 3 showed a significant decrease in desalination rate under high pressure, indicating that macroscopic support compaction leads to deterioration of the active layer function. And the following table: Note that the table above (performance under 180 g / L high salt and 80℃ conditions for 24 h) shows that under extreme high salt and high temperature conditions, for example, the highly cross-linked active layer of Example 2 performs best in retaining selectivity, but the flux loss caused by the thin support results in a slightly lower flux retention rate, but the overall performance is good; Example 3 has a high starting point in flux, but it will be affected by compaction under high salt conditions, and it is still much better than the comparative example; the comparative example sample has the most severe performance degradation under high salt conditions.

[0046] 3. Long-term circulation and cleaning: Run at 250 bar for 7 consecutive days (or in segments: 24h × 7), recording the daily flux and desalination rate; perform routine chemical cleaning (e.g., using 0.1% NaOH for 20 min) on days 3 and 5 and record the recovery rate after cleaning, as shown in the table below: The data table above clearly shows that the compaction degree of Examples 1-3 (PI in-situ crosslinked support + high DC active layer) under ultra-high pressure of 250 bar is significantly lower than that of the uncrosslinked or low DC comparative samples. The examples not only show smaller thickness and pore size collapse, higher wet mechanical strength, but also higher flux recovery rate and 7-day stability after decompression and chemical cleaning.

[0047] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A pressure-resistant and collapse-resistant porous nanomaterial mass transfer membrane, characterized in that, The nano-mass transfer membrane comprises, from top to bottom: an ultrathin highly cross-linked active layer, a mesoporous cross-linked support layer, and a macroscopic rigid substrate layer; the mesoporous cross-linked support layer is composed of polyimide and cross-linked with aliphatic diamine to form a thermosetting porous network; the ultrathin highly cross-linked active layer is a highly cross-linked polyurea film formed by the in-situ reaction of diisocyanate and diamine; The mesoporous cross-linked support layer is formed by phase inversion and in-situ cross-linking in a cross-linking solution. The phase inversion solvent system contains one or more of N,N-dimethylformamide, N,N-dimethylacetamide or N-methylpyrrolidone. The molar ratio of the polyimide repeating unit to the aliphatic diamine is 1:(0.15-0.35); The ultrathin, highly cross-linked active layer has a thickness of 80-200 nm and an effective pore size of 0.35-0.55 nm. The molar ratio of -NCO:-NH2 in the ultrathin highly crosslinked active layer is 1.00:(0.95-1.05), the molar ratio of aromatic diamine to aliphatic diamine is 70:30-85:15, and the target degree of crosslinking of the ultrathin highly crosslinked active layer is 80-90%.

2. The pressure-resistant and collapse-resistant porous nano-mass transfer membrane according to claim 1, characterized in that, The macroscopic rigid substrate layer is either polyester or polypropylene nonwoven fabric.

3. The pressure-resistant and collapse-resistant porous nano-mass transfer membrane according to claim 1, characterized in that, The aliphatic diamine in the mesoporous cross-linked support layer includes at least one of 1,4-butanediamine, 1,6-hexanediamine, and 1,8-octanediamine.

4. The pressure-resistant and collapse-resistant porous nano-mass transfer membrane according to claim 1, characterized in that, The mesoporous cross-linked support layer has a thickness of 40-120 μm, an average pore size of 30-80 nm, and a porosity of 40-50%.

5. The pressure-resistant and collapse-resistant porous nano-mass transfer membrane according to claim 1, characterized in that, The ultrathin, highly cross-linked active layer is formed by the in-situ reaction of a diamine with a diisocyanate, wherein the diisocyanate is selected from one or more of toluene diisocyanate and diphenylmethane diisocyanate, and the diamine includes aromatic diamines and aliphatic diamines.

6. A method for preparing a pressure-resistant and collapse-resistant porous nanomaterial transfer membrane according to any one of claims 1-5, characterized in that, Includes the following steps: 1) Select a macroscopic rigid substrate layer for plasma treatment, prepare a phase inversion solution containing polyimide, and then coat it on the surface of the macroscopic rigid substrate layer to form a porous membrane; 2) Introduce diamine into the porous membrane obtained in step 1) to perform in-situ crosslinking at 40-70°C to obtain a thermosetting mesoporous crosslinked support layer; 3) An ultrathin, highly cross-linked active layer is formed on the surface of the mesoporous cross-linked support layer obtained in step 2) by electro-spraying-in-situ cross-linking, and finally a porous nano-mass transfer membrane is obtained.

7. The method for preparing a pressure-resistant and collapse-resistant porous nano-mass transfer membrane according to claim 6, characterized in that, In step 2), the crosslinking is carried out at 40-70 °C for 1-4 h.

8. The method for preparing a pressure-resistant and collapse-resistant porous nano-mass transfer membrane according to claim 6, characterized in that, In step 3), the electro-spraying voltage is 10-20 kV, the spraying distance is 10-15 cm, and the relative humidity of the environment is 40-70%.

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