Polyolefin non-woven fabric composite reverse osmosis membrane support material and preparation method thereof

Abstract

The invention discloses a polyolefin non-woven fabric composite reverse osmosis membrane support material and a preparation method thereof, and belongs to the technical field of water treatment membranes. The supporting material is of a double-layer pore structure without a polysulfone middle layer, and is formed by hot-pressing and compounding a plasma-silane grafted composite hydrophilic modified battery diaphragm polypropylene ultrafiltration functional layer and a nano silicon dioxide doped polyolefin non-woven fabric reinforced supporting layer through covalent bonds; the battery diaphragm comprises the following raw materials in parts by weight: 20-25 parts of a battery diaphragm polypropylene base material, 70-75 parts of nano silicon dioxide doped polyolefin non-woven fabric, 6-12 parts of a composite hydrophilic modification system and the like. The preparation method comprises the steps of base material modification, reinforced non-woven fabric preparation, hot-pressing compounding and the like, the supporting material has the characteristics of high flux, high mechanical strength and thin layer, the raw material cost is reduced compared with that of a traditional product, and the supporting material is applied to the fields of seawater desalination and high-salt industrial wastewater treatment and has a wide industrialization prospect.

Description

Technical Field

[0001] This invention belongs to the field of water treatment membrane technology, specifically relating to a polyolefin nonwoven fabric composite reverse osmosis membrane support material and its preparation method. Background Technology

[0003] Existing technologies attempt to replace traditional supports with polyolefin materials. Patent application CN113117539A discloses a reverse osmosis membrane with a modified polyethylene microporous membrane and a polysulfone layer. While this reduces substrate costs, it still requires a polysulfone layer, failing to address thickness and process complexity issues. Patent application CN116036881A discloses a reverse osmosis membrane using hydrophilically modified PP / PE as a support for lithium-ion battery separators. This single-layer structure with a single polyolefin substrate eliminates the polysulfone layer, but suffers from insufficient mechanical strength under high pressure and a difficulty in simultaneously maintaining the integrity of the microporous structure after hydrophilic modification. Furthermore, existing polyolefin modification methods (such as plasma treatment and dopamine coating) either damage the microporous structure leading to decreased flux or have long modification cycles and poor hydrophilic durability, failing to meet the demands of industrial applications.

[0004] In summary, existing technologies cannot simultaneously achieve synergistic optimization of high porosity, high mechanical strength, high hydrophilicity, and low cost. There is an urgent need for an innovative composite structure and modification process to break through the technical bottleneck of polyolefin-based support materials. Summary of the Invention

[0005] One of the objectives of this invention is to provide a polyolefin nonwoven composite reverse osmosis membrane support material, which solves the problems of high cost, large thickness, low strength, and poor flux of traditional supports.

[0006] The second objective of this invention is to provide a method for preparing a polyolefin nonwoven composite reverse osmosis membrane support material, which is used to prepare the aforementioned polyolefin nonwoven composite reverse osmosis membrane support material.

[0007] The objective of this invention can be achieved through the following technical solutions: In a first aspect, a polyolefin nonwoven fabric composite reverse osmosis membrane support material is a double-layer porous structure without a polysulfone intermediate layer, which is formed by covalent hot-pressing an upper ultrafiltration functional layer and a lower reinforcing support layer. By weight, the raw material composition includes: 20-25 parts of polypropylene substrate for battery separator, 70-75 parts of nano-silica-doped polyolefin nonwoven fabric, 6-12 parts of composite hydrophilic modification system, and 0.8-1.5 parts of crosslinking accelerator. The ultrafiltration functional layer is a plasma-grafted silane-modified hydrophilic composite battery separator polypropylene, and the reinforcing support layer is a polyolefin nonwoven fabric doped with nano-silica.

[0008] The upper layer uses plasma-silane grafted modified polypropylene battery separator as the ultrafiltration functional layer to trap impurities; the lower layer uses nano-silica-doped polyolefin nonwoven fabric as the reinforcing support layer to provide mechanical rigidity and conductivity. The two are covalently bonded and hot-pressed together, and the proportions of raw materials are adjusted (20-25 parts polypropylene substrate, 70-75 parts nonwoven fabric, etc.) to ultimately achieve the comprehensive characteristics of high throughput, high strength, and thinness of the support.

[0009] Furthermore, the overall thickness of the composite support material is 100-130 μm, which is more than 20% thinner than the traditional polysulfone / polyester support. By relying on the composite of a thin battery separator polypropylene substrate and a polyolefin nonwoven fabric, the traditional polysulfone interlayer is eliminated, reducing thickness redundancy and thus increasing the membrane element winding area.

[0010] Furthermore, the polypropylene substrate of the battery separator has a thickness of 20-25 μm, is wet-formed, has an original porosity of 45%-55%, and a pore size distribution of 0.01-0.1 μm. This thickness range enables the support to be thin while ensuring structural strength; the 45%-55% porosity and 0.01-0.1 μm pore size can balance ultrafiltration retention effect and water permeability, while the wet-formation process ensures the connectivity and uniformity of the pores.

[0011] Furthermore, the raw material for the nano-silica-doped polyolefin nonwoven fabric is a blend of polypropylene and polyethylene in a mass ratio of (7-8):(2-3); the nano-silica particles have a diameter of 10-50 nm and are modified with silane coupling agent KH550, with a doping amount of 1%-3% of the nonwoven fabric mass. The blending of PP and PE in this ratio balances the rigidity and toughness of the nonwoven fabric; the 10-50 nm silica particles can be uniformly dispersed, avoiding agglomeration; KH550 modification improves the interfacial compatibility between silica and the polymer matrix; the 1%-3% doping amount enhances the strength of the nonwoven fabric without damaging its pore structure.

[0012] Furthermore, the composite hydrophilic modification system includes a plasma treatment aid and a silane grafting modifier; the plasma treatment aid is an argon-oxygen mixture with a volume ratio of (8-9):1; the silane grafting modifier is a 10%-15% (w / w) aqueous solution of acrylic acid-γ-methacryloyloxypropyltrimethoxysilane copolymer. Argon serves as the plasma working gas to maintain stable discharge, while oxygen provides active particles; the controlled ratio of these two gases prevents excessive oxidation of the substrate. The 10%-15% silane grafting agent concentration ensures sufficient grafting rate to improve hydrophilicity without clogging the substrate micropores due to excessive concentration.

[0013] Furthermore, the crosslinking promoter is a 2%-3% (w / w) aqueous solution of tetrabutyl titanate. This 2%-3% tetrabutyl titanate aqueous solution can act as a crosslinking bridging agent, promoting the covalent bonding of surface active groups between the upper ultrafiltration functional layer and the lower reinforcing support layer during hot-pressing composite processes, thereby improving peel strength and structural stability.

[0014] Secondly, a method for preparing a polyolefin nonwoven composite reverse osmosis membrane support material includes the following steps: S1. After pretreating the polypropylene substrate for the battery separator, plasma activation and silane grafting modification are performed sequentially to obtain a hydrophilic modified polypropylene ultrafiltration functional layer. S2. Modified nano-silica is dispersed in water with polypropylene chopped fibers and polyethylene chopped fibers, and then wet-formed to prepare reinforced polyolefin nonwoven fabric. Surface activation is then performed to obtain the reinforced nonwoven fabric. S3. The hydrophilic modified polypropylene ultrafiltration functional layer is sprayed with a crosslinking accelerator and then hot-pressed to obtain a polyolefin nonwoven fabric composite reverse osmosis membrane support material. First, the polypropylene substrate is subjected to plasma activation and silane grafting modification to introduce active sites and improve hydrophilicity, laying the foundation for subsequent composite. Then, a nano-silica-doped reinforced nonwoven fabric is prepared, and its interfacial compatibility with the upper substrate is ensured through modification and activation. Subsequently, a hot-pressing composite process is used in conjunction with a crosslinking promoter to achieve covalent bonding of the two-layer structure, ensuring interlayer stability. Finally, a polyolefin nonwoven fabric composite reverse osmosis membrane support material with high flux, high mechanical strength and thin-layer characteristics is obtained.

[0015] Furthermore, the parameters for plasma activation in step S1 are: gas flow rate 50-100 mL / min, processing power 120-180 W, and time 2-5 min; the parameters for silane grafting modification are: ammonium persulfate initiator addition 0.1%-0.3%, reaction temperature 50-60℃, and reaction time 1.5-3 h. The plasma parameters control the activation intensity to avoid damaging the substrate, while simultaneously introducing sufficient hydroxyl and carboxyl active sites onto the surface; the silane grafting parameters regulate the reaction rate. A 0.1%-0.3% initiator dosage and a temperature of 50-60℃ ensure uniform grafting reaction without affecting the substrate porosity.

[0016] Furthermore, the modification parameters for nano-silica in step S2 are as follows: the amount of silane coupling agent KH550 is 5%-8% of the silica mass, ultrasonic dispersion for 30-60 min, and reaction at 80℃ for 2 h. A 5%-8% KH550 dosage can completely coat the silica surface, and ultrasonic dispersion for 30-60 min breaks up agglomerated particles; 80℃ is the suitable temperature for silane hydrolysis and condensation, and the 2 h reaction ensures that the modifier and silica are fully combined, improving interfacial compatibility.

[0017] Furthermore, in step S1, the moisture content of the pretreated polypropylene substrate is ≤0.05%; in step S2, the power of the plasma treatment of the nonwoven fabric is 100-150W, and the time is 1-2 minutes. Low moisture content avoids side reactions during plasma activation and prevents moisture from interfering with the hydrolysis of the modifier during silane grafting, ensuring a uniform modification effect.

[0018] Furthermore, the parameters for hot-pressing lamination in step S3 are: temperature 130-150℃, pressure 0.8-1.5MPa, and time 40-80s. 130-150℃ is close to the softening point of polypropylene, which facilitates the bonding of the two layers; 0.8-1.5MPa pressure ensures tight lamination while preventing pore collapse; and 40-80s time balances lamination efficiency and structural stability.

[0019] Furthermore, the composite support material prepared by the method can be applied to seawater desalination and high-salt industrial wastewater treatment, directly replacing traditional polysulfone / polyester nonwoven fabric supports.

[0020] The beneficial effects of this invention are: (1) The polyolefin nonwoven composite reverse osmosis membrane support material prepared by the present invention is a double-layer porous structure without polysulfone intermediate layer. It is composed of a battery separator polypropylene ultrafiltration functional layer (20-25 parts) modified by plasma-silane grafting and hydrophilic composite, and a polyolefin nonwoven reinforced support layer (70-75 parts) doped with nano-silica, which is covalently bonded and hot-pressed together. The composite process is supplemented with 6-12 parts of composite hydrophilic modification system and 0.8-1.5 parts of crosslinking promoter to regulate the interface properties, and finally a composite support material with high flux, high mechanical strength and thin-layer characteristics is obtained.

[0021] (2) The double-layer pore structure adopted in this invention achieves synergistic optimization of flux and mechanical strength. The upper ultrafiltration functional layer ensures the retention performance of impurities; the lower reinforcing support layer can effectively reduce the water flow mass resistance, and the combination of the two improves the water flux of the composite support material. At the same time, the 1%-3% nano silica doped in the lower polyolefin nonwoven fabric, after being modified by the silane coupling agent KH550, forms a strong interfacial bond with the polypropylene / polyethylene blend matrix, which greatly improves the tensile strength and solves the problem of easy deformation under high pressure of single polyolefin substrate.

[0022] (3) The plasma-silane grafting composite modification process balances hydrophilicity and microporous structure integrity. First, argon-oxygen mixed gas plasma activation introduces active sites such as hydroxyl and carboxyl groups onto the polypropylene surface of the battery separator. Then, a 10%-15% (w / w) aqueous solution of acrylic acid-γ-methacryloyloxypropyltrimethoxysilane copolymer is used for grafting modification, avoiding the micropore clogging problem caused by traditional single modification processes. The high hydrophilicity of the modified support surface facilitates the uniform coating of subsequent functional layers, providing an ideal substrate for preparing high-performance reverse osmosis membranes, while also enhancing the interfacial bonding between the support layer and subsequent functional layers.

[0023] (4) The polysulfone-free design of this invention significantly reduces cost and thickness. The overall thickness of the composite of the 20-25μm thin polypropylene substrate and the 80-100μm nonwoven fabric is only 100-130μm, which effectively increases the membrane element roll area and filtration efficiency. At the same time, plasma activation combined with tetrabutyl titanate crosslinking promoter enables the two-layer structure to form a stable covalent bond, which improves the interlayer peel strength. In addition, the preparation process of this invention has strong industrial compatibility and can be produced based on existing lithium battery separator and nonwoven fabric production lines. Moreover, the process has no toxic solvent emissions, meets environmental protection requirements, is easy to scale up and mass-produce, and can directly replace the traditional support for seawater desalination and high-salt wastewater treatment, with significant economic and social benefits. Detailed Implementation

[0024] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0025] Example 1

[0026] 1. Raw material composition

[0027] The battery separator consists of 22 parts of polypropylene substrate (22 μm thickness, wet molding, 50% original porosity, pore size distribution 0.03-0.08 μm), 72 parts of nano-silica-doped polyolefin nonwoven fabric (polypropylene: polyethylene = 7.5:2.5, nano-silica particle size 20 nm, doping amount 2%), 8 parts of composite hydrophilic modification system (argon-oxygen mixed gas volume ratio 8.5:1, 12% concentration of acrylic acid-γ-methacryloyloxypropyltrimethoxysilane copolymer aqueous solution), and 1 part of tetrabutyl titanate crosslinking accelerator (2.5% mass concentration aqueous solution).

[0028] 2. Preparation steps

[0029] 2.1 Composite hydrophilic modification of polypropylene substrate for battery separator

[0030] (1) Pretreatment: The polypropylene substrate was cut into 20cm×20cm pieces and dried in a vacuum drying oven at 75℃ for 2.5h. The moisture content was measured to be 0.03%. (2) Plasma activation: The dried substrate is placed in a plasma treatment instrument, and an argon-oxygen mixed gas is introduced at a flow rate of 80 mL / min. The treatment power is set to 150 W and the treatment time is 3 min. Hydroxyl and carboxyl active sites are introduced on the surface of the substrate. (3) Silane grafting modification: Prepare a 12% acrylic acid-γ-methacryloxypropyltrimethoxysilane copolymer aqueous solution, add 0.2% initiator ammonium persulfate, and stir evenly; immerse the plasma-activated substrate in the solution, react in a water bath at 55℃ for 2h, and stir once every 30min during the reaction; after the reaction, rinse 3 times with deionized water until neutral, and dry at 85℃ for 1.5h to obtain a hydrophilic modified polypropylene ultrafiltration functional layer.

[0031] 2.2 Preparation of reinforced nonwoven fabrics

[0032] (1) Modification of nano-silica: Take 10g of 20nm nano-silica, disperse it in 100mL of anhydrous ethanol, add 0.6g of silane coupling agent KH550, sonicate for 45min, stir at 80℃ for 2h, centrifuge, and vacuum dry at 60℃ for 5h to obtain modified nano-silica; redisperse the dried modified nano-silica in deionized water, sonicate for 30min, and prepare an aqueous dispersion with a mass fraction of 2% for later use.

[0033] (2) Wet papermaking: Weigh 75g of polypropylene chopped fibers (fiber length 5-10mm) and 25g of polyethylene chopped fibers (fiber length 5-10mm) at a mass ratio of 7.5:2.5. Put the fibers into a debonding machine and add an appropriate amount of deionized water for mechanical debonding and dispersion to make the fibers completely dispersed to form a uniform slurry. Slowly add the nano-silica dispersion prepared in step (1) into the slurry and continue stirring for 30min to make the modified nano-silica uniformly adsorbed on the fiber surface. Control the silica doping amount to 2% of the total fiber mass. (3) Forming and drying: The uniformly mixed slurry is fed into a wet forming machine (inclined wire paper machine), and after dewatering and pressing, a wet paper web is formed. The wet paper web is then fed into a multi-stage drying system and dried in stages at 80-105℃ to constant weight. Then, it is hot rolled at 130-140℃ to obtain nano-silica-doped reinforced polyolefin nonwoven fabric. (4) Surface activation: The reinforced polyolefin nonwoven fabric is placed in a plasma treatment instrument, the power is set to 120W under an argon atmosphere, and the treatment time is 1.5min to obtain the reinforced nonwoven fabric.

[0034] 2.3 Preparation of Polyolefin Nonwoven Composite Reverse Osmosis Membrane Support Material

[0035] The hydrophilic modified polypropylene ultrafiltration functional layer was aligned with the reinforcing nonwoven fabric, and a tetrabutyl titanate crosslinking accelerator aqueous solution was uniformly sprayed in the middle. The mixture was placed in a hot press and the temperature was set to 140℃, the pressure to 1.2MPa, and the time to 60s. After lamination, the mixture was placed in a vacuum drying oven at 60℃ and dried for 1h to obtain a polyolefin nonwoven fabric composite reverse osmosis membrane support material.

[0036] Example 2

[0037] The difference between this embodiment and Example 1 is that the plasma treatment power is 120W and the time is 2min; the concentration of the silane grafted aqueous solution is 10%. The remaining raw materials and preparation process are the same as in Example 1.

[0038] Example 3

[0039] The difference between this embodiment and Example 1 is that the nano-silica doping amount is 3%; the hot-pressing pressure is 1.5 MPa. All other raw materials and preparation processes remain the same as in Example 1.

[0040] Example 4

[0041] The difference between this embodiment and Example 1 is that the ratio of polypropylene to polyethylene in the nonwoven polyolefin fabric is 7:3. All other raw materials and preparation processes remain the same as in Example 1.

[0042] Example 5

[0043] The difference between this embodiment and Example 1 is that the amount of crosslinking accelerator added is 1.5 parts. All other raw materials and preparation processes remain the same as in Example 1.

[0044] Example 6

[0045] The difference between this embodiment and Example 1 is that the plasma treatment uses an argon-oxygen volume ratio of 9:1; and the silane grafting reaction time is 3 hours. All other raw materials and preparation processes remain the same as in Example 1.

[0046] Example 7

[0047] The difference between this embodiment and Example 1 is that the nano-silica particle size is 50nm; the hot-pressing temperature is 150℃ and the time is 80s. The remaining raw materials and preparation process are the same as in Example 1.

[0048] Comparative Example 1

[0049] The difference between this embodiment and Example 1 is that a polysulfone casting solution (concentration 18%) with a thickness of 15 μm is coated on the surface of the polypropylene ultrafiltration functional layer. All other raw materials and preparation processes remain the same as in Example 1.

[0050] Comparative Example 2

[0051] The difference between this embodiment and Example 1 is that the polyolefin nonwoven fabric composite is omitted, and a modified polypropylene substrate is used directly as the support. All other raw materials and preparation processes remain the same as in Example 1.

[0052] Comparative Example 3

[0053] The difference between this embodiment and Example 1 is that silane grafting modification is omitted, and only plasma activation is performed. All other raw materials and preparation processes remain the same as in Example 1.

[0054] Comparative Example 4

[0055] The difference between this embodiment and Example 1 is that plasma activation is omitted, and silane grafting modification is performed directly. All other raw materials and preparation processes remain the same as in Example 1.

[0056] Comparative Example 5

[0057] The difference between this embodiment and Example 1 is that plasma activation and crosslinking accelerator are omitted, and a simple physical hot-pressing composite is used. All other raw materials and preparation processes remain the same as in Example 1.

[0058] Comparative Example 6

[0059] The difference between this embodiment and Example 1 is that a commercially available polyester nonwoven fabric (80 μm thick) and a polysulfone layer (30 μm thick) are used as the support. All other raw materials and preparation processes remain the same as in Example 1.

[0060] Performance testing

[0061] 1. Porosity test: Immersion method, referring to GB / T21650.2-2008 "Determination of Pore Size Distribution and Porosity of Solid Materials by Mercury Intrusion Porosimetry and Gas Adsorption Method"; 2. Water flux test: Refer to GB / T32360-2015 "Test Method for Ultrafiltration Membranes", the test conditions are pressure 1.5MPa and temperature 25℃; 3. Mechanical property testing: Tensile strength is tested according to GB / T1040.3-2006 "Determination of tensile properties of plastics"; 4. Aging resistance test: Double 85 aging test, referring to GB / T3511-2018 "Weather resistance of vulcanized rubber or thermoplastic rubber", the conditions are 85℃, 85%RH, and aging time is 30 days; 6. Long-term operational stability test: Calculate the pure water flux retention rate of the support material after 90 days of continuous operation at 1.5 MPa pressure.

[0062] The results are shown in Table 1: Table 1

[0063] As shown in Table 1, the tensile strength of Examples 1-7 remained stable at 62-70 MPa, significantly higher than the 35 MPa of Comparative Example 2 and the 50 MPa of Comparative Example 6. This demonstrates the rigid support of the lower nonwoven fabric and the reinforcing effect of nano-silica, addressing the issue of easy deformation under high pressure in single polyolefin substrates. Regarding porosity, the porosity of the examples was 48.5%-49.2%, slightly higher than the 42% of Comparative Example 2 and the 44.5% of Comparative Example 5. This is attributed to the good preservation of the pore structure by the wet molding process and the low-damage characteristics of the plasma-silane grafting composite modification to the micropores, providing a structural basis for high flux. Regarding water flux, the water flux of the examples was 2.0-2.3 L / m³. 2 The flux density (·h·bar) is significantly higher than that of the comparative example because the removal of the polysulfone layer reduces filtration resistance, while the higher porosity ensures the smooth flow of water. In contrast, the presence of the polysulfone layer in the comparative example increases mass transfer resistance, which in turn leads to a decrease in flux.

[0064] The performance improvement of the support material due to composite hydrophilic modification is mainly reflected in two aspects, as shown in the table: one is the effect on water flux. The water flux of Examples 1-7 remained stable at 2.0-2.3 L / m. 2 The concentration of ·h·bar was 0.9 L / m², while that of Comparative Example 3 (plasma treatment only) was 0.9 L / m². 2 • h·bar, Comparative Example 4 (silane grafting only) is 1.2 L / m 2 The flux density (·h·bar) was significantly lower than in the examples. This indicates that a single modification process cannot simultaneously achieve both hydrophilicity and the integrity of the pore structure, while the plasma-silane grafting composite modification can introduce hydrophilic groups while avoiding clogging of micropores, thus ensuring high throughput. Secondly, regarding the impact on long-term stability, the flux retention rate of the examples after 90 days was ≥91%, while that of Comparative Example 3 was 65% and Comparative Example 4 was 72%, indicating that the hydrophilic groups introduced by the composite modification are more firmly bonded and less prone to detachment or degradation during long-term operation. Simultaneously, the performance degradation of the examples after 30 days of double 85 aging was ≤2.5%, far lower than the 8.5% of Comparative Example 3 and 7.8% of Comparative Example 4, further confirming that the composite modification process imparts superior aging resistance and hydrophilic durability to the support material.

[0065] Covalent crosslinking is a crucial factor in ensuring interlayer stability. Comparative Example 5 employed a physical composite process without plasma activation or crosslinking accelerators, yet its flux retention rate after long-term operation was only 45%, far lower than the over 91% of the examples. This indicates that plasma activation can introduce active groups onto the surfaces of polypropylene and nonwoven fabrics, achieving covalent bonding in conjunction with crosslinking accelerators, significantly improving the interlayer bonding strength and long-term operational stability of the materials.

[0066] The polysulfone-free design optimizes both thickness and cost. The membrane thickness in this embodiment is 85-90 μm, thinner than Comparative Examples 1 and 6. The polysulfone-free design directly reduces the thickness of the intermediate layer, increasing the film element winding area. In terms of cost, the elimination of the polysulfone layer and the use of existing lithium-ion battery separator production lines result in lower raw material costs compared to Comparative Examples 1 and 6, demonstrating a significant cost advantage for industrialization.

[0067] The performance degradation rate of the examples after 30 days of dual 85 aging was ≤2.5%, far lower than that of Comparative Example 2 (18.5%), Comparative Example 5 (12.0%), and Comparative Example 6 (15.0%), indicating that the composite structure combined with covalent bonds can improve the structural stability of the material and reduce performance degradation during aging. In terms of long-term operation, the flux retention rate of the examples after 90 days was ≥91%, while that of Comparative Example 5 was only 45% and that of Comparative Example 6 was only 60%, demonstrating that the composite support material of the present invention possesses excellent long-term service stability under harsh operating conditions.

[0068] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A polyolefin nonwoven composite reverse osmosis membrane support material, characterized in that, The polyolefin nonwoven fabric composite reverse osmosis membrane support material is a double-layer porous structure without a polysulfone intermediate layer, which is formed by covalent hot pressing of the upper polyolefin battery separator ultrafiltration functional layer and the lower reinforcing support layer. By weight, the raw material composition includes: 20-25 parts of polypropylene substrate for battery separator, 70-75 parts of nano-silica-doped polyolefin nonwoven fabric, 6-12 parts of composite hydrophilic modification system, and 0.8-1.5 parts of crosslinking accelerator. The ultrafiltration functional layer is a plasma-grafted silane-modified hydrophilic composite battery separator polypropylene, and the reinforcing support layer is a polyolefin nonwoven fabric doped with nano-silica.

2. The polyolefin nonwoven composite reverse osmosis membrane support material according to claim 1, characterized in that, The battery separator polypropylene substrate has a thickness of 20-25 μm, is wet-formed, has an original porosity of 45%-55%, and a pore size distribution of 0.01-0.1 μm; the overall thickness of the polyolefin nonwoven composite reverse osmosis membrane support material is 100-130 μm.

3. The polyolefin nonwoven composite reverse osmosis membrane support material according to claim 1, characterized in that, The raw material for the nano-silica-doped polyolefin nonwoven fabric is a blend of polypropylene and polyethylene in a mass ratio of (7-8):(2-3); the nano-silica particles have a diameter of 10-50 nm and are modified with silane coupling agent KH550, with a doping amount of 1%-3% of the nonwoven fabric mass.

4. The polyolefin nonwoven composite reverse osmosis membrane support material according to claim 1, characterized in that, The composite hydrophilic modification system includes a plasma treatment aid and a silane grafting modifier; the plasma treatment aid is an argon-oxygen mixture with a volume ratio of (8-9):1; the silane grafting modifier is a 10%-15% aqueous solution of acrylic acid-γ-methacryloyloxypropyltrimethoxysilane copolymer.

5. The polyolefin nonwoven composite reverse osmosis membrane support material according to claim 1, characterized in that, The crosslinking accelerator is a 2%-3% (w / w) aqueous solution of tetrabutyl titanate.

6. A method for preparing a polyolefin nonwoven composite reverse osmosis membrane support material, characterized in that, A method for preparing the polyolefin nonwoven composite reverse osmosis membrane support material according to any one of claims 1-5 includes the following steps: S1. After pretreating the polypropylene substrate for the battery separator, plasma activation and silane grafting modification are performed sequentially to obtain a hydrophilic modified polypropylene ultrafiltration functional layer. S2. Modified nano-silica is dispersed in water with polypropylene chopped fibers and polyethylene chopped fibers, and then wet-formed to prepare reinforced polyolefin nonwoven fabric. Surface activation is then performed to obtain the reinforced nonwoven fabric. S3. The hydrophilic modified polypropylene ultrafiltration functional layer is coated with a crosslinking accelerator and then hot-pressed to obtain a polyolefin nonwoven composite reverse osmosis membrane support material.

7. The method for preparing a polyolefin nonwoven composite reverse osmosis membrane support material according to claim 6, characterized in that, The parameters for plasma activation in step S1 are: gas flow rate 50-100 mL / min, processing power 120-180 W, and time 2-5 min; the parameters for silane grafting modification are: ammonium persulfate initiator addition 0.1%-0.3%, reaction temperature 50-60℃, and reaction time 1.5-3 h.

8. The method for preparing a polyolefin nonwoven composite reverse osmosis membrane support material according to claim 6, characterized in that, The modification parameters for nano-silica in step S2 are as follows: the amount of silane coupling agent KH550 is 5%-8% of the mass of silica, ultrasonic dispersion for 30-60 min, and reaction at 80℃ for 2 h.

9. The method for preparing a polyolefin nonwoven composite reverse osmosis membrane support material according to claim 6, characterized in that, In step S1, the moisture content of the pretreated polypropylene substrate is ≤0.05%; in step S2, the power of the plasma treatment of the nonwoven fabric is 100-150W, and the time is 1-2min. The parameters for hot-pressing composite in step S3 are: temperature 130-150℃, pressure 0.8-1.5MPa, and time 40-80s.

10. The method for preparing a polyolefin nonwoven composite reverse osmosis membrane support material according to claim 6, characterized in that, The polyolefin nonwoven composite reverse osmosis membrane support material prepared by the above method can be applied to seawater desalination and high-salt industrial wastewater treatment.

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