Super-hydrophobic anti-drag composite coating based on modified ZIF-8, preparation method of super-hydrophobic anti-drag composite coating and coating aluminum mesh

Abstract

The invention discloses a super-hydrophobic anti-drag composite coating based on modified ZIF-8, a preparation method of the super-hydrophobic anti-drag composite coating and a coating aluminum mesh. According to the scheme, the ZIF-8 particles are subjected to unique hydroxylation pretreatment and composite silane grafting modification, so that the ZIF-8 particles integrate the physical roughness construction function and the chemical hydrophobic / lubricating function, the ZIF-8 particles serve as core filler to be compounded with fluorocarbon resin and PTFE / graphite anti-drag auxiliaries, the dispersion process and spraying curing parameters are optimized, and the anti-drag performance of the coating is improved. And a stable and uniform micro-nano composite structure coating is constructed on the surface of the aluminum mesh. According to the coating, super-hydrophobicity (the water contact angle is larger than or equal to 155 degrees, and the rolling angle is smaller than or equal to 8 degrees) and the fluid drag reduction rate is larger than or equal to 25%) are synchronously achieved, the coating is high in mechanical stability, corrosion resistance and adhesive force, the technical bottlenecks that a traditional coating is single in function and poor in durability are solved, and the process is simple and convenient.

Description

Technical Field

[0001] This invention belongs to the field of functional materials and surface engineering technology, specifically relating to a superhydrophobic drag-reducing composite coating based on modified ZIF-8, its preparation method, and the coated aluminum mesh. Background Technology

[0002] Superhydrophobic coatings, due to their unique "lotus effect," exhibit enormous application potential in areas such as self-cleaning, anti-icing, and corrosion prevention. Simultaneously, in scenarios such as fluid transport pipelines, ship hulls, and underwater vehicles, reducing the frictional resistance between fluids and solid surfaces is significant for improving energy efficiency and reducing operating costs. Theoretically, combining superhydrophobicity with drag reduction properties can produce a synergistic effect: the "cushion layer" (Cassie-Baxter state) formed on a superhydrophobic surface can effectively reduce the solid-liquid contact area, thereby reducing viscous drag.

[0003] Aluminum mesh, as a lightweight, high-strength, porous, and easily moldable metal substrate, is widely used in filtration, separation, protection, and fluid equipment. Constructing a coating on its surface that combines superhydrophobicity and drag reduction can further expand its application value.

[0004] However, most existing research on the development of such functional coatings focuses on single superhydrophobic or drag-reduction properties. Traditional superhydrophobic coatings often lack microstructures optimized for hydrodynamics, resulting in limited drag reduction rates (typically below 20%); while single drag-reduction coatings are easily contaminated and wetted in complex fluid environments, losing their drag-reduction effect.

[0005] Furthermore, zeolite imidazole ester framework materials (ZIFs), especially ZIF-8, possess regular porous structures, high specific surface areas, and tunable morphologies, making them ideal candidates for constructing micro- and nano-rough structures. However, pristine ZIF-8 particles exhibit high surface chemical inertness, poor dispersibility in polymer matrices, and a tendency to agglomerate. Moreover, their surface energy is difficult to control, making it challenging to simultaneously impart excellent hydrophobicity and specific drag-reduction functions to the coating.

[0006] Furthermore, the smooth surface of aluminum mesh makes it prone to oxide layer formation, resulting in weak coating adhesion. When subjected to fluid shear, mechanical wear (such as sand erosion), and salt spray corrosion in marine environments, the micro-nano structure and low surface energy chemical layer of the coating are easily damaged, causing rapid performance degradation and making it difficult to meet long-term use requirements.

[0007] In addition, the preparation of some high-performance coatings relies on complex processes such as chemical vapor deposition and electrospinning, which are costly and make it difficult to achieve uniform and large-scale coating on large-size or complex-shaped aluminum mesh substrates.

[0008] Therefore, in view of the above-mentioned technical problems and defects such as poor functional synergy, limited application of key fillers, insufficient coating durability, and complex preparation process or low compatibility, it is urgent to design and develop a superhydrophobic drag-reducing composite coating based on modified ZIF-8, its preparation method and coated aluminum mesh. Summary of the Invention

[0009] To overcome the shortcomings and difficulties of the existing technology, the first objective of this invention is to provide a superhydrophobic drag-reducing composite coating based on modified ZIF-8; the second objective of this invention is to provide a method for preparing a superhydrophobic drag-reducing composite coating based on modified ZIF-8; the third objective of this invention is to provide a superhydrophobic drag-reducing coated aluminum mesh; and the fourth objective of this invention is to provide a method for preparing a superhydrophobic drag-reducing coated aluminum mesh.

[0010] The first objective of this invention is achieved as follows: the raw material for the superhydrophobic drag-reducing composite coating includes a matrix resin, drag-reducing agent, hydrophobic agent, composite solvent, and modified ZIF-8 particles as the core functional filler; the modified ZIF-8 particles are ZIF-8 particles grafted with hydrophobic agent after surface hydroxylation pretreatment.

[0011] The second objective of this invention is achieved as follows: the method comprises the following steps: (1) Preparation of modified ZIF-8 particles: ZIF-8 powder is subjected to hydroxylation pretreatment to obtain hydroxylated ZIF-8; then the hydroxylated ZIF-8 is grafted with a hydrophobic auxiliary agent under catalytic conditions to obtain the modified ZIF-8 particles. (2) Preparation of coating slurry: Dissolve the matrix resin in a portion of the composite solvent to obtain a resin solution; add the modified ZIF-8 particles and drag-reducing agent obtained in step (1) to the resin solution and perform dispersion treatment to obtain a uniformly dispersed slurry; adjust the viscosity and solid content of the slurry to obtain the coating slurry.

[0012] The third objective of this invention is achieved as follows: the superhydrophobic drag-reducing coated aluminum mesh has a coating formed by the superhydrophobic drag-reducing composite coating on the surface of the aluminum mesh substrate; the thickness of the coating is 80-250 μm, and its surface water contact angle is ≥155° and roll-off angle is ≤8°.

[0013] The fourth objective of the present invention is achieved as follows: the method steps include: (a) aluminum mesh pretreatment: cleaning and drying the aluminum mesh after degreasing; (b) coating application: spraying the coating slurry prepared by the method of any one of claims 5-7 onto the surface of the pretreated aluminum mesh by air spraying, and obtaining the superhydrophobic drag-reducing coated aluminum mesh after pre-baking and curing.

[0014] This invention utilizes a unique hydroxylation pretreatment and composite silane grafting modification of ZIF-8 particles to integrate "physical roughness construction" and "chemical hydrophobic / lubricating functions." These particles serve as the core filler, compounded with fluorocarbon resin and PTFE / graphite drag-reducing agents. Through optimized dispersion processes and spray curing parameters, a stable and uniform micro-nano composite coating is constructed on the surface of an aluminum mesh. This coating simultaneously achieves superhydrophobicity (water contact angle ≥155°, roll-off angle ≤8°) and fluid drag reduction ≥25%, while also exhibiting strong mechanical stability, corrosion resistance, and strong adhesion. It overcomes the technical bottlenecks of traditional coatings, such as single function and poor durability, and the process is simple. Attached Figure Description

[0015] Figure 1 Here are schematic diagrams of the ZIF-8 superhydrophobic coating of the present invention; wherein, (a) is a schematic diagram of the superhydrophobic coating of ZIF-8 on an aluminum mesh substrate and its contact angle; (b) is a schematic diagram of the superhydrophobic coating of ZIF-8 and graphite in a 3:7 ratio and its contact angle; and (c) is a schematic diagram of the superhydrophobic coating of ZIF-8 and graphite in a 7:3 ratio and its contact angle. Figure 2 This is a schematic diagram of the ZIF-8 SEM image of the present invention; wherein, (a) is the particle morphology of ZIF-8 at a scale bar of 500 nm; and (b) is the particle morphology of ZIF-8 at a scale bar of 100 nm. Figure 3 This is a schematic diagram of the ultrasonic cleaning experiment of the present invention; Figure 4 This is a schematic diagram of the self-cleaning experiment of the present invention; where (a) to (d) are uncoated aluminum meshes immersed in pink solution (the state after phenolphthalein is added to NaOH), the solution easily wets and adheres to the surface of the aluminum mesh, and there is still obvious liquid residue after removal; (e) to (h) are superhydrophobic coated samples, the solution is difficult to wet the surface after immersion, the liquid falls off quickly when removed, and the surface remains basically clean, showing significant self-cleaning performance; Figure 5 The diagram shows the durability and wear resistance test results of the present invention; wherein, (a) is a schematic diagram of the ultrasonic cleaning test; and (b) is a schematic diagram of the sandpaper abrasion test. Detailed Implementation

[0016] The present invention will be further described below with reference to the accompanying drawings, but this is not intended to limit the present invention in any way. Any modifications or substitutions made based on the teachings of the present invention shall fall within the protection scope of the present invention.

[0017] like Figures 1-5As shown, the present invention provides a superhydrophobic drag-reducing composite coating based on modified ZIF-8. The raw materials of the superhydrophobic drag-reducing composite coating include a matrix resin, drag-reducing agent, hydrophobic agent, composite solvent, and modified ZIF-8 particles as the core functional filler. The modified ZIF-8 particles are ZIF-8 particles grafted with hydrophobic agents after surface hydroxylation pretreatment.

[0018] The superhydrophobic drag-reducing composite coating comprises, by weight percentage: 8-20% modified ZIF-8 particles, 25-45% matrix resin, 3-10% drag-reducing agent, 4-12% hydrophobic agent, and 25-50% composite solvent. The modified ZIF-8 particles have a particle size of 500 nm - 3 μm; before grafting the hydrophobic additive, the hydroxyl content on the surface of the ZIF-8 particles is 3.2-5.6 mmol / g.

[0019] The matrix resin is selected from one or more of fluorocarbon resin, polysiloxane resin, and polyurethane-modified silicone resin; the drag-reducing agent is polytetrafluoroethylene micro powder and / or graphite micro powder; the hydrophobic agent is selected from one or more of perfluorodecyltriethoxysilane, heptadecafluorodecyltrimethoxysilane, and octadecyltrimethoxysilane.

[0020] To achieve the objective of this invention, a method for preparing a modified ZIF-8 superhydrophobic drag-reducing composite coating is also provided, the method comprising the following steps: (1) Preparation of modified ZIF-8 particles: ZIF-8 powder is subjected to hydroxylation pretreatment to obtain hydroxylated ZIF-8; then the hydroxylated ZIF-8 is grafted with a hydrophobic auxiliary agent under catalytic conditions to obtain the modified ZIF-8 particles. (2) Preparation of coating slurry: Dissolve the matrix resin in a portion of the composite solvent to obtain a resin solution; add the modified ZIF-8 particles and drag-reducing agent obtained in step (1) to the resin solution and perform dispersion treatment to obtain a uniformly dispersed slurry; adjust the viscosity and solid content of the slurry to obtain the coating slurry.

[0021] In step (1), the hydroxylation pretreatment is as follows: ZIF-8 powder is added to a hydrogen peroxide solution with a mass fraction of 10-15%, and stirred at 60-75°C for 4-8 hours; the grafting reaction is as follows: hydroxylated ZIF-8 is dispersed in anhydrous ethanol, a hydrophobic auxiliary agent accounting for 15-40% of the mass of hydroxylated ZIF-8 is added, and hydrochloric acid is used as a catalyst, and stirred at 55-70°C for 8-16 hours.

[0022] In step (2), the dispersion process includes: first, high-speed dispersion at a speed of 3500-6000 r / min for 1.5-3 hours, and then ultrasonic dispersion at a power of 300-500 W for 45-90 minutes; the ultrasonic dispersion is carried out intermittently.

[0023] To achieve the purpose of this invention, a superhydrophobic drag-reducing coated aluminum mesh is also provided. The superhydrophobic drag-reducing coated aluminum mesh has a coating formed by the superhydrophobic drag-reducing composite coating of any one of claims 1-4 on the surface of the aluminum mesh substrate. The thickness of the coating is 80-250 μm, and its surface water contact angle is ≥155° and its roll-off angle is ≤8°.

[0024] At 25℃ and a water flow velocity of 1 m / s, the drag reduction rate of the coated aluminum mesh compared to the uncoated aluminum mesh is ≥25%; And / or, after 50 sandpaper rubs under a 200g load, its surface water contact angle is ≥145°, and the drag reduction rate remains ≥25%; And / or, after 72 hours of 2.5% NaCl salt spray testing, the coating showed no blistering or peeling, and the water contact angle was ≥150°; And / or, according to GB / T 9286-1998 standard, the cross-cut adhesion test shall be performed and the adhesion grade shall be ≤ Grade 1.

[0025] To achieve the objective of this invention, a method for preparing a superhydrophobic drag-reducing coated aluminum mesh is also provided, the method comprising the following steps: (a) Aluminum mesh pretreatment: The aluminum mesh is degreased, cleaned, and dried; (b) Coating application: The coating slurry prepared by any one of claims 5-7 is sprayed onto the surface of the pretreated aluminum mesh by air spraying, and after pre-baking and curing, the superhydrophobic drag-reducing coated aluminum mesh is obtained.

[0026] Specifically, in this embodiment of the invention, a ZIF-8 modified composite superhydrophobic drag-reducing coating is provided. By mass percentage, the raw material composition includes: zinc sulfate hexahydrate 7-9, dimethylimidazole 14-18, methanol 500-800, ZIF-8 8-20, resin 25-45, drag-reducing agent 3-10, hydrophobic agent 4-12, and composite solvent 25-50. The modified ZIF-8 is a ZIF-8 modified by grafting fluorosilane and long-chain alkylsilane after hydroxylation pretreatment, and its particle size is approximately 500nm-3μm.

[0027] The ZIF-8 particles were prepared by weighing 7-9 parts by mass of zinc sulfate hexahydrate and adding it to 500-800 ml of methanol, stirring until homogeneous to form mixed solution A. Then, 14-18 parts by mass of dimethylimidazole were weighed and added to 500-800 ml of methanol solution, stirring until homogeneous to form mixed solution B. Solutions A and B were then mixed together and stirred at 1000-1500 rpm for 12-24 hours to ensure thorough mixing and reaction of the particles, forming solution C. Solution C was then centrifuged repeatedly at 1000-1500 rpm for 12-15 minutes, washed three times with methanol solution, dried in a drying oven at 80-100℃ for 24-48 hours, ground into powder using a mortar and pestle, sieved through an 800-1200 mesh sieve, and set aside for later use. The resin is selected from one or a combination of two of fluorocarbon resin (FEVE), polysiloxane resin, and polyurethane modified silicone resin; wherein the average particle size of the PTFE particles is 2 μm.

[0028] The drag-reducing additives are polytetrafluoroethylene (PTFE) micro powder (particle size 1-5 μm) and graphite micro powder (particle size 10 μm). The hydrophobic additive is one or more of perfluorodecyltriethoxysilane, heptadecafluorodecyltrimethoxysilane, and octadecyltrimethoxysilane in combination; The composite solvent is a mixture of ethyl acetate, N-methylpyrrolidone (NMP), and anhydrous ethanol in a volume ratio of 3:2:2.

[0029] The method for preparing the superhydrophobic drag-reducing coating includes the following steps: (1) ZIF-8 modification: 1) Hydroxylation pretreatment: Add ZIF-8 powder to a 10-15% hydrogen peroxide solution, stir at 60-75℃ for 4-8h, filter, wash with deionized water, and dry at 80-100℃ for 2-4h to obtain hydroxylated ZIF-8; 2) Grafting modification: Disperse hydroxylated ZIF-8 in anhydrous ethanol, add hydrophobic auxiliary agent (15-40% of the mass of hydroxylated ZIF-8), add 0.5-1.5% hydrochloric acid as a catalyst, stir at 55-70℃ for 8-16h, centrifuge, wash with ethanol 3-5 times, and vacuum dry at 100-120℃ for 4-6h to obtain modified ZIF-8; (2) Preparation of coating slurry: 1) Resin dissolution: Add the base resin to the mixed solvent and stir at 45-65℃ for 1-2 hours until completely dissolved to obtain a resin solution; 2) Dispersion of mixed solution: Add modified ZIF-8 and drag-reducing agent to the resin solution in sequence, disperse at 3500-6000r / min for 1.5-3 hours, and then ultrasonically disperse for 45-90 minutes (power 300-500W) to obtain a uniformly dispersed slurry; 3) Viscosity optimization: Add composite solvent to the slurry to adjust the solid content to 25-40% and control the viscosity at 150-300mPa·s (25℃). Stir for 30 minutes and let stand for 1 hour to remove bubbles to obtain the coating slurry; (3) Pretreatment of aluminum mesh: 1) Degreasing: The aluminum mesh is ultrasonically cleaned with acetone for 20-40 minutes to remove surface oil stains, and then rinsed with deionized water; 2) Drying: Dry at 100-120℃ for 30-60 minutes, and set aside for use; (4) Spraying and curing: 1) Spraying: Air spraying process is adopted, with a nozzle diameter of 0.8-1.2mm, a spraying pressure of 0.23-0.25MPa, a spraying distance of 15-30cm, and spraying in 2-4 times with an interval of 15-30min between each spraying; 2) Pre-baking: After each spraying, pre-baking at 60-80℃ for 15-25min; 3) Curing: After the last pre-baking, curing at 130-170℃ for 40-80min, and naturally cooling to room temperature to obtain a superhydrophobic drag-reducing coated aluminum mesh.

[0030] In step (1), the hydroxyl content of hydroxylated ZIF-8 is 3.2-5.6 mmol / g; in step (2), the ultrasonic dispersion adopts an intermittent process, with a 10-minute pause after every 30 minutes of ultrasonication to avoid agglomeration of the slurry due to temperature rise.

[0031] The present invention also provides a superhydrophobic drag-reducing coated aluminum mesh, prepared by the method described in claim 3 or 4, wherein the coating thickness on the aluminum mesh surface is 80-250 μm, and satisfies the following: water contact angle ≥155°, roll-off angle ≤8°; drag reduction rate ≥25% under 25℃ and water flow velocity of 1m / s conditions; water contact angle ≥145° and drag reduction rate ≥25% after 50 sandpaper rubbings under a 200g load; no blistering or peeling of the coating after 72h of 2.5% NaCl salt spray test, water contact angle ≥150°; and adhesion grade ≤1 in cross-cut adhesion test (GB / T 9286-1998).

[0032] The superhydrophobic drag-reducing coated aluminum mesh is used in the inner wall of fluid transport pipelines, ship hull filters, and the surface of underwater vehicles.

[0033] In other words, in the present invention, the preparation method of the superhydrophobic drag-reducing coating material is as follows: 1) Preparation of ZIF-8 granules: Weigh 7-9 parts by mass of zinc sulfate hexahydrate and add it to 500-800 ml of methanol, stirring until homogeneous to form mixed solution A. Then weigh 14-18 parts by mass of dimethylimidazole and add it to 500-800 ml of methanol solution, stirring until homogeneous to form mixed solution B. Mix solutions A and B together and stir at 1000-1500 r / min for 12-24 h to ensure thorough mixing and reaction of the granules, forming solution C. Centrifuge solution C repeatedly at 1000-1500 r / min for 12-15 min, wash three times with methanol solution, and dry in a drying oven at 80-100℃ for 24-48 h. Grind the powder into powder using a mortar and pestle, and sieve it through an 800-1200 mesh sieve for later use; the sieve is a 316L stainless steel sieve.

[0034] 2) Modified ZIF-8 particles: a. Hydroxylation pretreatment: ZIF-8 powder is added to a 10-15% (w / w) hydrogen peroxide solution, stirred at 60-75℃ for 4-8 h, filtered, washed with deionized water, and dried at 80-100℃ for 2-4 h to obtain hydroxylated ZIF-8; b. Grafting modification: Hydroxylated ZIF-8 is dispersed in anhydrous ethanol, a hydrophobic auxiliary agent (15-40% of the mass of hydroxylated ZIF-8) is added, 0.5-1.5% hydrochloric acid is added dropwise as a catalyst, stirred at 55-70℃ for 8-16 h, centrifuged, washed with ethanol 3-5 times, and vacuum dried at 100-120℃ for 4-6 h to obtain modified ZIF-8 particles; wherein, the average particle size of ZIF-8 particles is 500nm-3μm.

[0035] 3) Coating slurry preparation: a. Resin dissolution: Add the base resin to the composite solvent and stir at 45-65℃ for 1-2 hours until the base resin is completely dissolved to obtain a resin solution; b. Mixed solution dispersion: Add modified ZIF-8 powder particles and drag-reducing agent to the resin solution in sequence, and disperse at a high speed of 3500-6000 r / min for 1.5-3 hours, and then ultrasonically disperse at a power of 300-500w for 45-90 minutes to obtain a uniformly dispersed slurry; c. Viscosity optimization: Add composite solvent to the slurry to adjust the solid content to 25-40%, and control the viscosity at 25℃ to 150-300 mPa·s. Continue stirring for 30 minutes and let stand for 1 hour to remove bubbles to obtain the coating slurry; wherein, the base is a 200-mesh high-purity aluminum mesh.

[0036] 4) Pre-treatment of aluminum mesh: a. Degreasing: The aluminum mesh is ultrasonically cleaned with acetone for 20-40 minutes to remove surface oil stains, and then rinsed with deionized water; b. Drying: Dry at 100-120℃ for 30-60 minutes, and set aside for use; 5) Spraying the superhydrophobic coating: a. Spraying: Use air spraying technology, nozzle diameter 0.8-1.2mm, spraying pressure 0.23-0.25MPa, spraying distance 15-30cm, spray in 2-4 coats, with an interval of 15-30min between each coat; b. Pre-baking: Pre-bake at 60-80℃ for 15-25min after each coat; c. Curing: After the final pre-baking, cure at 130-170℃ for 40-80min, then allow to cool naturally to room temperature. The relative humidity of the dryer should be ≤20%.

[0037] Based on the above embodiments, in another embodiment of the present invention, the bifunctional modification of ZIF-8, the hydrophobic-drag reduction synergistic formulation design, and the aluminum mesh pretreatment-spraying process are as follows: 1) Preparation of ZIF-8 granules: Weigh 7-9 parts by mass of zinc sulfate hexahydrate and add it to 500-800 ml of methanol, stirring until homogeneous to form mixed solution A. Then weigh 14-18 parts by mass of dimethylimidazole and add it to 500-800 ml of methanol solution, stirring until homogeneous to form mixed solution B. Mix solutions A and B together and stir at 1000-1500 r / min for 12-24 h to ensure thorough mixing and reaction of the granules, forming solution C. Centrifuge solution C repeatedly at 1000-1500 r / min for 12-15 min, wash three times with methanol solution, and dry in a drying oven at 80-100℃ for 24-48 h. Grind the powder into powder using a mortar and pestle, sieve through an 800-1200 mesh sieve, and set aside for later use.

[0038] 2) Bifunctional modification of ZIF-8: Weigh 8-20g of ZIF-8 powder according to the mass ratio and add it to a hydrogen peroxide solution with a mass fraction of 10-15%. Stir the reaction at 60-75℃ for 4-8h, filter, wash with deionized water, and then dry in a drying oven at 80-100℃ for 2-4h to obtain hydroxylated ZIF-8 powder. Next, the hydroxylated ZIF-8 powder was dispersed in anhydrous ethanol solution, and a hydrophobic auxiliary agent (15-40% of the mass of hydroxylated ZIF-8) was added. 0.5-1.5% hydrochloric acid was added dropwise as a catalyst, and the reaction was stirred at 55-70℃ for 8-16 hours. After centrifugation and washing with ethanol 3-5 times, the mixture was dried under vacuum at 100-120℃ for 4-6 hours to obtain the modified ZIF-8 powder.

[0039] 3) Preparation of coating slurry: Fluorocarbon resin and polysiloxane resin are mixed, and the matrix resin is added to the mixed solvent. The mixture is stirred at 45-65℃ for 1-2 hours until completely dissolved to obtain a resin solution. This solution combines low surface energy and adhesion, providing a stable substrate for the coating. Modified ZIF-8 and drag-reducing agents are added to the resin solution sequentially, and the mixture is dispersed at high speed (3500-6000 r / min) for 1.5-3 hours, followed by ultrasonic dispersion for 45-90 minutes (power 300-500W) to obtain a uniformly dispersed slurry. The ultrasonic dispersion is performed intermittently, with a 10-minute pause after every 30 minutes of ultrasonication to prevent the slurry from agglomerating due to temperature rise. A composite solvent is then added to the slurry to adjust the solid content to 25-40% and the viscosity to 150-300 mPa·s (25℃). After stirring for 30 minutes, the mixture is allowed to stand for 1 hour to remove bubbles, resulting in the coating slurry.

[0040] 4) Aluminum mesh pretreatment: Place the aluminum mesh in an acetone beaker and ultrasonically clean for 20-40 minutes to remove surface oil stains, then rinse with deionized water; dry at 100-120℃ for 30-60 minutes and set aside for later use.

[0041] 5) Spraying and curing: Air spraying process is adopted, with a nozzle diameter of 0.8-1.2mm, a spraying pressure of 0.23-0.25MPa, and a distance of 15-30cm from the aluminum mesh. Spraying is done in 2-4 coats with an interval of 15-30min between each coat. After each coat, pre-bake at 60-80℃ for 15-25min. After the last pre-bake, cure at 130-170℃ for 40-80min and allow to cool naturally to room temperature to obtain a superhydrophobic drag-reducing coated aluminum mesh.

[0042] Example 1: Preparation of Superhydrophobic Drag-Reducing Coating I. Preparation of superhydrophobic drag-reducing coating materials: (1) Preparation of ZIF-8 particles: Weigh 7 parts by weight of analytical grade zinc sulfate hexahydrate and add it to 500 mL of anhydrous methanol. Stir magnetically for 20 min until completely dissolved to obtain mixed solution A. Weigh 14 parts by weight of analytical grade 2-methylimidazole and add it to 500 mL of anhydrous methanol. Stir magnetically for 15 min until completely dissolved to obtain mixed solution B. Slowly add mixed solution A to mixed solution B and stir continuously at 1000 r / min for 12 h. A white suspension precipitate is visible to the naked eye, which is mixed solution C. Transfer mixed solution C to a centrifuge tube and centrifuge at 1000 r / min for 12 min. Discard the supernatant. Resuspend the precipitate in methanol solution and repeat the centrifugation-washing operation 3 times until the supernatant is clear. The precipitate was transferred to a vacuum drying oven and dried at a constant temperature of 80℃ for 24 hours. After removal, it was ground in an agate mortar for 10 minutes and sieved through an 800-1200 mesh standard sieve to obtain 10-20 parts of ZIF-8 powder, which was then sealed and protected from light for later use.

[0043] (2) Modified ZIF-8 particles: 8 parts of the prepared ZIF-8 particles were added to a 10% hydrogen peroxide solution and stirred at 60°C for 4 hours. After filtration, the particles were washed with deionized water and dried at 80°C for 2 hours to obtain hydroxylated ZIF-8. The modified ZIF-8 powder particles were dispersed in anhydrous ethanol solution, 1.5 parts of hydrophobic additive were added, and 0.5 parts of hydrochloric acid were added dropwise as a catalyst. The mixture was stirred at 55°C for 8 hours, washed three times with ethanol, centrifuged, and dried at 100°C under vacuum for 4 hours to obtain modified ZIF-8 powder particles.

[0044] (3) Preparation of coating slurry: 25 parts of the base resin were added to the mixed solvent and stirred at 45°C for 1 hour until completely dissolved to obtain a resin solution; 10 parts of modified ZIF-8 and 3 parts of drag-reducing agent PTFE micro powder (particle size 2μm) were added to the resin solution in sequence, and the mixture was dispersed at 3500r / min for 1.5 hours, and then ultrasonically dispersed for 45 minutes (power 300W) to obtain a uniformly dispersed slurry; composite solvent was added to the slurry to adjust the solid content to 25-40%, and the viscosity was controlled at 150mPa·s at 25°C. After stirring for 30 minutes, the mixture was allowed to stand for 1 hour to remove bubbles to obtain the coating slurry; II. Preparation of superhydrophobic drag-reducing coatings: Aluminum mesh pretreatment: The aluminum mesh is ultrasonically cleaned with acetone for 20 minutes to remove surface oil, and then rinsed with deionized water; it is then dried at 100℃ for 30 minutes and set aside for use. Spraying and curing: Air spraying process is adopted, with a nozzle diameter of 0.8mm, a spraying pressure of 0.23MPa, a spraying distance of 15cm, and spraying in 2 times with an interval of 15min between each spraying; Pre-baking: after each spraying, pre-baking at 60℃ for 15min; after the last pre-baking, curing at 130℃ for 40min, and then naturally cooling to room temperature to obtain a superhydrophobic drag-reducing coated aluminum mesh.

[0045] Example 2: Preparation of superhydrophobic drag-reducing coating I. Preparation of superhydrophobic drag-reducing coating materials: (1) Preparation of ZIF-8 particles: Weigh 8 parts by weight of analytical grade zinc sulfate hexahydrate and add it to 650 mL of anhydrous methanol. Stir magnetically for 20 min until completely dissolved to obtain mixed solution A. Weigh 16 parts by weight of analytical grade 2-methylimidazole and add it to 650 mL of anhydrous methanol. Stir magnetically for 20 min until dissolved to obtain mixed solution B. Slowly add mixed solution A to mixed solution B and stir continuously at 1300 r / min for 18 h. A white suspension precipitate is visible to the naked eye, which is mixed solution C. Transfer mixed solution C to a centrifuge tube and centrifuge at 1300 r / min for 13 min. Discard the supernatant. Resuspend the precipitate in methanol solution and repeat the centrifugation-washing operation 3 times until the supernatant is clear. The precipitate was transferred to a vacuum drying oven and dried at a constant temperature of 90℃ for 36 hours. After removal, it was ground in an agate mortar for 15 minutes and sieved through an 800-1200 mesh standard sieve to obtain 10-20 parts of ZIF-8 powder, which was then sealed and protected from light for later use.

[0046] (2) Modified ZIF-8 particles: 12 parts of the prepared ZIF-8 particles were added to a 13% hydrogen peroxide solution and stirred at 70°C for 6 hours. After filtration, the particles were washed with deionized water and dried at 90°C for 3 hours to obtain hydroxylated ZIF-8. The modified ZIF-8 powder particles were dispersed in anhydrous ethanol solution, 3.6 parts of hydrophobic additive were added, and 1 part of hydrochloric acid was added dropwise as a catalyst. The mixture was stirred at 60°C for 12 hours, washed with ethanol 4 times, centrifuged, and dried at 110°C under vacuum for 5 hours to obtain modified ZIF-8 powder particles.

[0047] (3) Preparation of coating slurry: 25 parts of the base resin were added to the mixed solvent and stirred at 65°C for 1.5 h until completely dissolved to obtain a resin solution; 7 parts of modified ZIF-8 and 3 parts of drag-reducing agent graphite powder (particle size 10 μm) were added to the resin solution in sequence, and dispersed at 5000 r / min for 2 h, and then ultrasonically dispersed for 75 min (power 400 W) to obtain a uniformly dispersed solution; in order to adjust the viscosity, composite solvent was added to the slurry to adjust the solid content to 25-40%, and the viscosity was controlled at 150 mPa·s at 25°C. After stirring for 30 min, the mixture was allowed to stand for 1 h to remove bubbles to obtain the coating slurry; II. Preparation of superhydrophobic drag-reducing coatings: Aluminum mesh pretreatment: The aluminum mesh is ultrasonically cleaned with acetone for 30 minutes to remove surface oil, and then rinsed with deionized water; it is then dried at 110℃ for 45 minutes and set aside for use. Spraying and curing: Air spraying process is adopted, with a nozzle diameter of 0.8mm, a spraying pressure of 0.24MPa, a spraying distance of 20cm, and spraying in 3 times with an interval of 20min between each spraying; Pre-baking: after each spraying, pre-baking at 70℃ for 20min; after the last pre-baking, curing at 150℃ for 1h, and naturally cooling to room temperature to obtain a superhydrophobic drag-reducing coated aluminum mesh.

[0048] Example 3: Preparation of superhydrophobic drag-reducing coating I. Preparation of superhydrophobic drag-reducing coating materials: (1) Preparation of ZIF-8 particles: Weigh 9 parts by weight of analytical grade zinc sulfate hexahydrate and add it to 800 mL of anhydrous methanol. Stir magnetically for 30 min until dissolved to obtain mixed solution A. Weigh 18 parts by weight of analytical grade 2-methylimidazole and add it to 800 mL of anhydrous methanol. Stir magnetically for 40 min until dissolved to obtain mixed solution B. Slowly add mixed solution A to mixed solution B and stir continuously at 1500 r / min for 24 h. A white suspension precipitate is visible to the naked eye, which is mixed solution C. Transfer mixed solution C to a centrifuge tube and centrifuge at 1500 r / min for 15 min. Discard the supernatant. Resuspend the precipitate in methanol solution and repeat the centrifugation-washing operation 3 times until the supernatant is clear. The precipitate was transferred to a vacuum drying oven and dried at a constant temperature of 100℃ for 48 hours. After being removed, it was ground in an agate mortar for 20 minutes and then sieved through an 800-1200 mesh standard sieve to obtain 20 parts of ZIF-8 powder, which were then sealed and protected from light for later use.

[0049] (2) Modified ZIF-8 particles: 20 parts of the prepared ZIF-8 particles were added to a 15% hydrogen peroxide solution and stirred at 75°C for 8 hours. After filtration, the particles were washed with deionized water and dried at 100°C for 4 hours to obtain hydroxylated ZIF-8. The modified ZIF-8 powder particles were dispersed in anhydrous ethanol solution, 4 parts of hydrophobic additive were added, and 1.5 parts of hydrochloric acid were added dropwise as a catalyst. The mixture was stirred at 70°C for 16 hours, washed with ethanol 5 times, centrifuged, and dried at 120°C under vacuum for 6 hours to obtain modified ZIF-8 powder particles.

[0050] (3) Preparation of coating slurry: 30 parts of the base resin were added to the mixed solvent and stirred at 65°C for 2 hours until completely dissolved to obtain a resin solution; 7 parts of modified ZIF-8 and 3 parts of drag-reducing agent PTFE micro powder + graphite powder (mass ratio 1:1) were added to the resin solution in sequence, and dispersed at 6000 r / min for 3 hours, and then ultrasonically dispersed for 90 minutes (power 500W) to obtain a uniformly dispersed solution; in order to adjust the viscosity, composite solvent was added to the slurry to adjust the solid content to 25-40%, and the viscosity was controlled at 150 mPa·s at 25°C. After stirring for 30 minutes, the mixture was allowed to stand for 1 hour to remove bubbles to obtain the coating slurry; II. Preparation of superhydrophobic drag-reducing coatings: Aluminum mesh pretreatment: The aluminum mesh is ultrasonically cleaned with acetone for 40 minutes to remove surface oil, and then rinsed with deionized water; it is then dried at 120℃ for 60 minutes and set aside for use. Spraying and curing: Air spraying process is adopted, with a nozzle diameter of 0.8mm, a spraying pressure of 0.25MPa, a spraying distance of 30cm, and spraying in 4 times with an interval of 30min between each spraying; Pre-baking: after each spraying, pre-baking at 80℃ for 25min; after the last pre-baking, curing at 170℃ for 80min, and then naturally cooling to room temperature to obtain a superhydrophobic drag-reducing coated aluminum mesh.

[0051] Test case I. Test Sample Examples 1-3 show the preparation of superhydrophobic drag-reducing coated aluminum meshes, with a control group (coated aluminum meshes prepared with unmodified ZIF-8).

[0052] II. Testing Methods Superhydrophobic properties: The contact angle (average of 5 measurements) and roll-off angle of 5µl deionized water were measured at room temperature using a contact angle meter (Dataphysics-OCA20). Drag Reduction Performance: A small-scale pipeline flow test device was constructed, with aluminum mesh used as the inner lining of the pipeline. The pressure difference at 25℃ and a water flow velocity of 1m / s was tested, and the drag reduction rate η was calculated using the formula: (1) in, The pressure difference on the surface of the aluminum mesh. The pressure difference is the pressure difference on the surface of the coated aluminum mesh.

[0053] Mechanical stability: Sandpaper friction test - Under a load of 200g, the coating surface is rubbed back and forth for 70cm with 400-grit sandpaper, and the contact angle after friction is tested; Ultrasonic cleaning experiment: The superhydrophobic coating was placed in an ultrasonic cleaner with tweezers and ultrasonically cleaned for 2 hours at a power of 200W with a power density of 0.1–0.5W / cm². The contact angle was measured every 15 minutes. After ultrasonic cleaning, the contact angle was measured at 3 different test points on the coating surface, and the average value was used to characterize the hydrophobic performance.

[0054] Salt spray test: The corrosion resistance of the superhydrophobic coating was tested using a salt spray corrosion test chamber (YW / Q-250, China). According to ISO 9277 standard, the coating underwent a copper-accelerated acetic acid salt spray test (CASS) at a temperature of 35±2℃, with a salt spray deposition rate of 1~2 ml / (80cm²∙h). The experimental solution was prepared with the following parameters: sodium chloride concentration 50 g / L, copper chloride concentration 0.26 g / L, and the pH value adjusted to 3.1-3.3 using glacial acetic acid. The test lasted for 72 hours, during which the contact angle of the coating was measured, and changes in appearance were observed.

[0055] This invention successfully prepared a superhydrophobic and drag-reducing composite coating on the surface of aluminum mesh through ZIF-8 bifunctional modification, hydrophobic-drag reduction synergistic formulation design, and optimized preparation process. This coating simultaneously exhibits excellent superhydrophobic properties (water contact angle ≥155°) and high mechanical stability. The process parameters of this invention are controllable, the raw material cost is moderate, and it is easy to achieve large-scale production, showing broad industrial application prospects in fluid transportation, ship navigation, and other fields.

[0056] In other words, the present invention first employs hydrogen peroxide hydroxylation treatment to introduce a large number of hydroxyl active groups onto the surface of ZIF-8, which improves its dispersibility in the resin and provides sites for subsequent grafting reactions. Then, a composite grafting process using fluorosilane and long-chain alkylsilane is used. The fluorine groups reduce surface energy to achieve superhydrophobicity, while the long-chain alkyl groups form a flexible lubricating layer to reduce fluid friction resistance, achieving a synergistic effect of hydrophobicity and drag reduction. The slurry dispersion uses a combined process of "high-speed dispersion + intermittent ultrasonication" to effectively prevent ZIF-8 agglomeration and ensure a uniform coating surface structure. The spraying process employs multiple sprayings and segmented pre-baking to control the uniformity of coating thickness. The curing temperature and time are strictly matched to ensure that the resin is fully cross-linked and firmly bonded to the filler. PTFE and graphite powder are used as drag-reducing agents to synergistically construct a "micro-nano roughness + low friction" composite structure with modified ZIF-8, further improving the coating drag reduction rate. The aluminum mesh undergoes a composite pretreatment of "degreasing-sandblasting-acid washing." Sandblasting forms a physically rough surface, and acid washing removes the surface oxide film, doubly strengthening the adhesion between the coating and the substrate.

[0057] The prepared superhydrophobic drag-reducing coating has a water contact angle ≥155° and a drag reduction rate ≥25%, simultaneously achieving excellent superhydrophobic performance and efficient drag reduction effect. Its performance deteriorates little after mechanical friction and salt spray corrosion, with an adhesion grade ≤1, meeting the long-term use requirements under harsh operating conditions. This process uses conventional air spraying equipment, the raw materials are readily available and the cost is controllable, making it suitable for large-scale production. It can be widely used in fluid transportation, shipbuilding, and other fields, and is particularly suitable for the functional modification of porous aluminum mesh substrates.

Claims

1. A superhydrophobic drag-reducing composite coating based on modified ZIF-8, characterized in that, The raw materials for the superhydrophobic drag-reducing composite coating include a matrix resin, drag-reducing additives, hydrophobic additives, a composite solvent, and modified ZIF-8 particles as the core functional filler; the modified ZIF-8 particles are ZIF-8 particles grafted with hydrophobic additives after surface hydroxylation pretreatment.

2. The superhydrophobic drag-reducing composite coating based on modified ZIF-8 according to claim 1, characterized in that, The superhydrophobic drag-reducing composite coating comprises, by mass percentage: 8-20% modified ZIF-8 particles, 25-45% matrix resin, 3-10% drag-reducing agent, 4-12% hydrophobic agent, and 25-50% composite solvent.

3. A superhydrophobic drag-reducing composite coating based on modified ZIF-8 according to claim 1 or 2, characterized in that, The modified ZIF-8 particles have a particle size of 500 nm - 3 μm; before grafting the hydrophobic additive, the hydroxyl content on the surface of the ZIF-8 particles is 3.2-5.6 mmol / g.

4. A superhydrophobic drag-reducing composite coating based on modified ZIF-8 according to claim 1 or 2, characterized in that, The matrix resin is selected from one or more of fluorocarbon resin, polysiloxane resin, and polyurethane-modified silicone resin; the drag-reducing agent is polytetrafluoroethylene micro powder and / or graphite micro powder; the hydrophobic agent is selected from one or more of perfluorodecyltriethoxysilane, heptadecafluorodecyltrimethoxysilane, and octadecyltrimethoxysilane.

5. A method for preparing a modified ZIF-8 superhydrophobic drag-reducing composite coating as described in any one of claims 1-4, characterized in that, The method includes the following steps: (1) Preparation of modified ZIF-8 particles: ZIF-8 powder is subjected to hydroxylation pretreatment to obtain hydroxylated ZIF-8; then the hydroxylated ZIF-8 is grafted with a hydrophobic auxiliary agent under catalytic conditions to obtain the modified ZIF-8 particles. (2) Preparation of coating slurry: Dissolve the matrix resin in a portion of the composite solvent to obtain a resin solution; add the modified ZIF-8 particles and drag-reducing agent obtained in step (1) to the resin solution and perform dispersion treatment to obtain a uniformly dispersed slurry; adjust the viscosity and solid content of the slurry to obtain the coating slurry.

6. A method for preparing a modified ZIF-8 superhydrophobic drag-reducing composite coating according to claim 5, characterized in that, In step (1), the hydroxylation pretreatment is as follows: ZIF-8 powder is added to a hydrogen peroxide solution with a mass fraction of 10-15%, and stirred at 60-75°C for 4-8 hours; the grafting reaction is as follows: hydroxylated ZIF-8 is dispersed in anhydrous ethanol, a hydrophobic auxiliary agent accounting for 15-40% of the mass of hydroxylated ZIF-8 is added, and hydrochloric acid is used as a catalyst, and stirred at 55-70°C for 8-16 hours.

7. A method for preparing a modified ZIF-8 superhydrophobic drag-reducing composite coating according to claim 5, characterized in that, In step (2), the dispersion process includes: first, high-speed dispersion at a speed of 3500-6000 r / min for 1.5-3 hours, and then ultrasonic dispersion at a power of 300-500 W for 45-90 minutes; the ultrasonic dispersion is carried out intermittently.

8. A superhydrophobic drag-reducing coated aluminum mesh, characterized in that, The superhydrophobic drag-reducing coated aluminum mesh has a coating formed by the superhydrophobic drag-reducing composite coating of any one of claims 1-4 on the surface of the aluminum mesh substrate; the thickness of the coating is 80-250 μm, and its surface water contact angle is ≥155° and roll-off angle is ≤8°.

9. The superhydrophobic drag-reducing coated aluminum mesh according to claim 8, characterized in that, At 25℃ and a water flow velocity of 1m / s, the drag reduction rate of the coated aluminum mesh compared to the uncoated aluminum mesh is ≥25%; And / or, after 50 sandpaper rubs under a 200g load, its surface water contact angle is ≥145°, and the drag reduction rate remains ≥25%; And / or, after 72 hours of 2.5% NaCl salt spray testing, the coating showed no blistering or peeling, and the water contact angle was ≥150°; And / or, according to GB / T 9286-1998 standard, the cross-cut adhesion test shall be performed and the adhesion grade shall be ≤ Grade 1.

10. A method for preparing a superhydrophobic drag-reducing coated aluminum mesh as described in claim 8 or 9, characterized in that, The method steps include: (a) Aluminum mesh pretreatment: The aluminum mesh is degreased, cleaned, and dried; (b) Coating application: The coating slurry prepared by any one of claims 5-7 is sprayed onto the surface of the pretreated aluminum mesh by air spraying, and after pre-baking and curing, the superhydrophobic drag-reducing coated aluminum mesh is obtained.

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