An in-situ superhydrophilic titanium phosphate coating on stainless steel surface, its preparation method, and its application in oil-water separation.

Abstract

This invention provides an in-situ construction method for a superhydrophilic titanium phosphate coating on a stainless steel surface, its application in oil-water separation, and belongs to the field of oil-water separation technology. This invention employs a two-step hydrothermal method to in-situ construct a uniform crystalline titanium phosphate nanosheet coating on a stainless steel surface. This nanosheet structure with uniform roughness not only exhibits superhydrophilicity but also displays extremely low adhesion to high-viscosity crude oil in the aqueous phase, thus possessing the function of adhering to the original oil. Specifically, the first hydrothermal process uses phosphoric acid and tetrabutyl titanate as phosphating agents to directly phosphate the stainless steel to form a phosphating film. The second hydrothermal process uses titanium oxysulfate, sulfuric acid, phosphoric acid, and hydrogen peroxide as precursors to generate a titanium phosphate coating in-situ on the surface of the aforementioned phosphating film. This invention utilizes a two-step hydrothermal method to achieve in-situ growth of a titanium phosphate coating on an iron-based metal substrate, resulting in strong interfacial adhesion.

Description

Technical Field

[0001] This invention relates to the field of oil-water separation technology, and in particular to an in-situ superhydrophilic titanium phosphate coating on a stainless steel surface, its preparation method, and its application in oil-water separation. Background Technology

[0002] In oil-water separation technology, commonly used separation methods include physical, chemical, and biological methods. Among these, physical methods are widely used in various industries due to their simplicity, lack of secondary pollution, and high separation efficiency. Within physical methods, membrane separation is the most frequently used oil-water separation technique. However, the membrane substrate must be coated with a material to impart specific wettability in order to achieve highly selective oil-water separation.

[0003] To prevent membrane substrates from becoming clogged by oily substances and to achieve continuous oil-water separation, recent research has focused on superhydrophilic-underwater superoleophobic coating materials, such as hydrophilic polymers, inorganic materials, and hydrogels. Furthermore, the market offers a wide variety of membrane substrates, including hollow fiber ultrafiltration membranes, nitrocellulose and polyvinyl fluoride nanofiltration membranes, ceramic membranes, and metal mesh membranes. From an industrial application perspective, stainless steel metal mesh membranes offer advantages such as high mechanical strength, low cost, adjustable pore size, and ease of processing, and are commonly used as substrates for oil-water separation membranes. Methods for preparing coatings on their surface include liquid-phase immersion, spraying, surface grafting, and sol-gel methods. However, due to the chemical inertness of stainless steel, it is difficult to react with other chemical substances to prepare coatings on its surface in situ, which limits the application of stainless steel metal mesh membranes in oil-water separation.

[0004] The prior art CN115873429A discloses a superhydrophilic titanium phosphate oil-water separation coating and its preparation method. This method uses a layer-by-layer assembly method to prepare a superhydrophilic titanium phosphate coating on the surface of a substrate. However, this method uses electrostatic interaction to adsorb the coating precursor material to the substrate surface layer by layer. The interfacial adhesion between the coating and the substrate is a weak interaction, resulting in poor interfacial adhesion of the coating. Summary of the Invention

[0005] In view of this, the present invention aims to provide an in-situ superhydrophilic titanium phosphate coating on a stainless steel surface, its preparation method, and its application in oil-water separation. The method of the present invention enables the in-situ preparation of a superhydrophilic titanium phosphate coating on a ferrous metal substrate. The resulting coating exhibits superhydrophilicity, extremely low adhesion to crude oil in the aqueous phase, and strong interfacial adhesion.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a method for in-situ preparation of a superhydrophilic titanium phosphate coating on a stainless steel surface, comprising the following steps:

[0008] The iron-based metal substrate is mixed with the first phosphating solution and subjected to a first hydrothermal phosphating reaction to obtain an iron-based metal substrate coated with a phosphating film.

[0009] The iron-based metal substrate covered with phosphate film is mixed with a second phosphate solution to carry out a second hydrothermal phosphate reaction, thereby obtaining a superhydrophilic titanium phosphate coating on the surface of the iron-based metal substrate.

[0010] The first phosphating solution comprises phosphoric acid, tetrabutyl titanate, and water;

[0011] The second phosphating solution comprises titanium sulfate, sulfuric acid, phosphoric acid, hydrogen peroxide, and water.

[0012] Preferably, in the first phosphating solution, the volume ratio of phosphoric acid to tetrabutyl titanate is 0.5 to 10:1;

[0013] The volume ratio of phosphoric acid to water is 1 to 4:10.

[0014] Preferably, the temperature of the first hydrothermal phosphating reaction is 160-180°C, and the time is 5-10 hours.

[0015] Preferably, the method for preparing the second phosphating solution includes the following steps:

[0016] Titanium oxysulfate, sulfuric acid, and water are mixed to obtain an aqueous solution of titanium oxysulfate;

[0017] The aqueous solution of titanium oxysulfate was mixed with phosphoric acid and hydrogen peroxide to obtain a second phosphating solution.

[0018] Preferably, in the second phosphating solution, the mass ratio of titanium oxysulfate to sulfuric acid is 0.5-1 g: 0.2-1 mL;

[0019] The mass ratio of titanium oxysulfate to phosphoric acid is 0.5–1 g: 2–6 mL.

[0020] The mass ratio of titanium oxysulfate to hydrogen peroxide is 0.5–1 g: 12–20 mL.

[0021] The mass ratio of titanium oxysulfate to water is 0.5–1 g: 50–200 mL.

[0022] Preferably, the temperature of the second hydrothermal phosphating reaction is 120–140°C, and the time is 8–24 hours.

[0023] Preferably, the iron-based metal substrate is a stainless steel mesh with a pore size of 5–25 μm.

[0024] The present invention provides an in-situ superhydrophilic titanium phosphate coating on a stainless steel surface prepared by the above method, comprising an iron-based metal substrate and a titanium phosphate coating grown in situ on the surface of the iron-based metal substrate, wherein the titanium phosphate coating has a nanosheet structure.

[0025] Preferably, the thickness of the titanium phosphate coating is 5–6 μm.

[0026] This invention provides the application of the above-mentioned in-situ superhydrophilic titanium phosphate coating on stainless steel surface in oil-water separation.

[0027] This invention provides a method for in-situ preparation of a superhydrophilic titanium phosphate coating on a stainless steel surface, comprising the following steps: mixing an iron-based metal substrate with a first phosphating solution and performing a first hydrothermal phosphating reaction to obtain an iron-based metal substrate coated with a phosphating film; mixing the iron-based metal substrate coated with the phosphating film with a second phosphating solution and performing a second hydrothermal phosphating reaction to obtain a superhydrophilic titanium phosphate coating on the surface of the iron-based metal substrate; the first phosphating solution comprises phosphoric acid, tetrabutyl titanate, and water; the second phosphating solution comprises titanium oxysulfate, sulfuric acid, phosphoric acid, hydrogen peroxide, and water. This invention employs a two-step hydrothermal method to in-situ construct a uniform crystalline titanium phosphate nanosheet coating on a stainless steel surface. This nanosheet structure with uniform roughness not only exhibits superhydrophilicity but also shows ultra-low adhesion to high-viscosity crude oil in an aqueous phase, thus possessing the function of adhering to anti-oil. Specifically, the first hydrothermal process uses phosphoric acid and tetrabutyl titanate as phosphating agents to directly phosphate stainless steel, forming a phosphate film. The second hydrothermal process uses titanium oxysulfate, sulfuric acid, phosphoric acid, and hydrogen peroxide as precursors. During this process, due to the acidity of sulfuric acid and phosphoric acid and the oxidizing property of hydrogen peroxide, titanium oxysulfate dissolves and exists as tetravalent titanium ions. During the hydrothermal process, when the titanium ion concentration on the stainless steel surface after the first phosphating reaches a certain level, a titanium phosphate coating is gradually formed in situ on its surface. This invention uses a two-step hydrothermal method to achieve in-situ growth of a titanium phosphate coating on the surface of an iron-based metal substrate, resulting in strong interfacial adhesion. The results of the examples show that the superhydrophilic titanium phosphate coating prepared by the present invention has a contact angle of 0° and excellent hydrophilicity. On the other hand, in the aqueous phase, the contact angles of oil droplets such as dodecane, hexadecane, toluene, petroleum ether, crude oil, and dichloroethane on the sample surface after secondary hydrothermal treatment are all greater than 150°. Furthermore, the 20 kg standard sand drop test shows that the titanium phosphate coating prepared by the present invention has strong interfacial adhesion and good adhesion to iron-based metal substrates.

[0028] Meanwhile, the method provided by this invention is simple to operate, low in cost, and easy to implement for industrial mass production.

[0029] This invention provides the application of the above-mentioned in-situ superhydrophilic titanium phosphate coating on stainless steel surfaces in oil-water separation. The superhydrophilic titanium phosphate coating provided by this invention has superhydrophilicity and the ability to adhere antigens to oils in the aqueous phase, and exhibits excellent oil-water separation performance when used as an oil-water separation membrane. Attached Figure Description

[0030] Figure 1 The images are scanning electron microscope images of the original 316L stainless steel sample, the coating prepared by primary hydrothermal phosphating and secondary hydrothermal treatment in Example 1.

[0031] Figure 2 The X-ray photoelectron spectroscopy spectra of the original 316L stainless steel sample, the coating prepared by the first hydrothermal phosphating and the coating prepared by the second hydrothermal process in Example 1 are shown.

[0032] Figure 3 Photographs showing the contact angle of water on the original 316L stainless steel sample and on the coated sample after secondary hydrothermal treatment in Example 1;

[0033] Figure 4 Photographs of contact angles on the surface of samples containing dodecane, hexadecane, toluene, petroleum ether, crude oil, and dichloroethane after secondary hydrothermal treatment in an aqueous phase.

[0034] Figure 5 Photographs and test results of the adhesion force test on the surface of crude oil samples after secondary hydrothermal treatment in the aqueous phase;

[0035] Figure 6 High and low magnification scanning electron microscope images of the original stainless steel mesh and the sample used as an oil-water separation membrane after secondary hydrothermal treatment.

[0036] Figure 7 Low- and high-magnification scanning electron microscope images of stainless steel mesh with titanium phosphate coating after a 20 kg drop test;

[0037] Figure 8 The graph shows the relationship between water flux and oil content in the effluent as a continuous oil-water separation volume when a stainless steel mesh with a titanium phosphate coating is used as an oil-water separation membrane. Detailed Implementation

[0038] This invention provides a method for in-situ preparation of a superhydrophilic titanium phosphate coating on a stainless steel surface, comprising the following steps:

[0039] The iron-based metal substrate is mixed with the first phosphating solution and subjected to a first hydrothermal phosphating reaction to obtain an iron-based metal substrate coated with a phosphating film.

[0040] The iron-based metal substrate covered with phosphate film is mixed with a second phosphate solution to carry out a second hydrothermal phosphate reaction, thereby obtaining a superhydrophilic titanium phosphate coating on the surface of the iron-based metal substrate.

[0041] The first phosphating solution comprises phosphoric acid, tetrabutyl titanate, and water;

[0042] The second phosphating solution comprises titanium sulfate, sulfuric acid, phosphoric acid, hydrogen peroxide, and water.

[0043] This invention involves mixing an iron-based metal substrate with a first phosphating solution and performing a first hydrothermal phosphating reaction to obtain an iron-based metal substrate coated with a phosphating film. In this invention, the iron-based metal substrate is preferably a stainless steel substrate. This invention does not have specific requirements regarding the type or composition of the stainless steel substrate; any type of stainless steel well-known in the art is suitable for preparing a superhydrophilic titanium phosphate coating using the method of this invention. In this invention, the iron-based metal substrate is preferably a stainless steel mesh, and the pore size of the stainless steel mesh is preferably 5–25 μm, more preferably 10–20 μm.

[0044] Before the first hydrothermal phosphating reaction, the iron-based metal substrate is preferably ultrasonically cleaned. In this invention, the cleaning solution used for ultrasonic cleaning is preferably an organic solvent, more preferably a mixture of acetone and isopropanol, with the volume ratio of acetone to isopropanol preferably being 1:1. In this invention, the ultrasonic cleaning time is preferably 30–40 min, more preferably 35 min; the power is preferably 200–600 W, more preferably 300–500 W. This invention removes contaminants from the surface of the iron-based metal substrate through ultrasonic cleaning.

[0045] After ultrasonic cleaning, the present invention preferably performs a water rinse. The present invention does not have special requirements for the water rinsing method; any water rinsing method well known to those skilled in the art can be used.

[0046] In this invention, the first phosphating solution comprises phosphoric acid, tetrabutyl titanate, and water. In this invention, the volume ratio of phosphoric acid to tetrabutyl titanate is preferably 0.5–10:1, more preferably 4–8:1. In this invention, the mass fraction of phosphoric acid is preferably 85%, and the purity of the tetrabutyl titanate is preferably analytical grade.

[0047] In this invention, the volume ratio of phosphoric acid to water is preferably 8:50.

[0048] In this invention, the first hydrothermal phosphating reaction is preferably carried out in a stainless steel reactor with a polytetrafluoroethylene liner, the temperature of the first hydrothermal phosphating reaction is preferably 160-180°C, more preferably 170°C, and the time is preferably 5-10 hours, more preferably 6-8 hours.

[0049] In this invention, the reagents used in the first hydrothermal phosphating reaction are phosphoric acid and tetrabutyl titanate. When the two are mixed to form a solution, the tetrabutyl titanate first undergoes a hydrolysis reaction with water, as shown in formula (1):

[0050] Ti(O-C4H9)4+4H2O→Ti(OH)4+4C4H9OH formula (1);

[0051] Secondly, the hydrogen ions (H+) in the phosphoric acid solution + The reaction between the titanium ions and the Ti(OH)4 produced after hydrolysis produces tetravalent titanium ions in the solution, as shown in formula (2):

[0052] Ti(OH)₄ + 4H⁺ + =Ti 4+ +4H2O formula (2);

[0053] With H in the solution + The consumption of phosphoric acid shifts the ionization equilibrium to the positive direction, producing hydrogen phosphate and dihydrogen phosphate, as shown in equation (3):

[0054]

[0055] Subsequently, hydrogen phosphate and dihydrogen phosphate ions in the solution react with tetravalent titanium ions to form titanium hydrogen phosphate and titanium phosphate white precipitates, as shown in formulas (4) and (5):

[0056] Ti 4+ +2H2PO4 - =Ti(HPO4)2↓+2H + Equation (4);

[0057] 3Ti 4+ +4HPO4 2- =Ti3(PO4)4↓+4H + Equation (5);

[0058] Subsequently, during the first hydrothermal reaction at 180°C, Fe in the stainless steel dissolves under the action of phosphoric acid to form Fe2+. 2 + This is also a common chemical reaction in the phosphating process of stainless steel, as shown in formula (6):

[0059] Fe + 2H3PO4 = Fe(H2PO4)2 (water-soluble) + H2↑ Equation (6);

[0060] In the solution near the stainless steel surface, as Fe... 2+ As the concentration increases, when Fe 2+ With HPO4 2- PO4 3- When the concentration is greater than the solubility product of titanium hydrogen phosphate and titanium phosphate precipitate produced in the above solution, a displacement reaction occurs, forming a phosphate film on the stainless steel surface, as shown in formulas (7) and (8):

[0061] 2Fe 2++Ti(HPO4)2=2FeHPO4↓+Ti 4+ Equation (7);

[0062] 6Fe 2+ +Ti3(PO4)4=2Fe3(PO4)2↓+3Ti 4+ Equation (8).

[0063] It is important to emphasize that if the precursor solution contains only phosphoric acid and no tetrabutyl titanate, titanium hydrogen phosphate and titanium phosphate precipitates cannot be formed. In this case, the displacement reaction cannot occur during the hydrothermal process, and thus the stainless steel surface cannot be phosphated.

[0064] After the first hydrothermal phosphating reaction, the present invention preferably cools to room temperature and washes the resulting iron-based metal substrate coated with phosphating film with water.

[0065] This invention involves mixing the iron-based metal substrate coated with a phosphate film with a second phosphate solution to perform a second hydrothermal phosphate reaction, thereby obtaining a superhydrophilic titanium phosphate coating on the surface of the iron-based metal substrate. In this invention, the second phosphate solution comprises titanium oxysulfate, sulfuric acid, phosphoric acid, hydrogen peroxide, and water.

[0066] In this invention, in the second phosphating solution, the mass ratio of titanium oxysulfate to sulfuric acid is preferably 0.5-1g:0.2-1mL, more preferably 0.5g:0.6-0.8mL; the mass ratio of titanium oxysulfate to phosphoric acid is preferably 0.5-1g:2-6mL, more preferably 0.5g:4mL; the mass ratio of titanium oxysulfate to hydrogen peroxide is preferably 0.5-1g:12-20mL, more preferably 0.5g:16mL; and the mass ratio of titanium oxysulfate to water is preferably 0.5-1g:50-200mL, more preferably 0.5g:200mL.

[0067] In this invention, the mass concentration of sulfuric acid is preferably 98%, the mass concentration of phosphoric acid is preferably 85%, the mass concentration of hydrogen peroxide is preferably 30%, and the titanium oxysulfate is preferably in powder form with analytical grade purity.

[0068] In this invention, the method for preparing the second phosphating solution preferably includes the following steps:

[0069] Titanium oxysulfate, sulfuric acid, and water are mixed to obtain an aqueous solution of titanium oxysulfate;

[0070] The aqueous solution of titanium oxysulfate was mixed with phosphoric acid and hydrogen peroxide to obtain a second phosphating solution.

[0071] This invention involves mixing titanium oxysulfate, sulfuric acid, and water to obtain an aqueous solution of titanium oxysulfate. In this invention, the mixing method is preferably stirring, and the stirring time is preferably 3–8 hours, more preferably 5–6 hours. In this invention, the mass fraction of titanium oxysulfate in the aqueous solution is preferably 0.2–0.6%, more preferably 0.3–0.5%.

[0072] In this invention, the aqueous solution of titanium oxysulfate is mixed with phosphoric acid and hydrogen peroxide to obtain a second phosphating solution. In this invention, the mixing method is preferably stirring.

[0073] In this invention, the second hydrothermal phosphating reaction is preferably carried out in a reaction vessel with a polytetrafluoroethylene liner, the temperature of the second hydrothermal phosphating reaction is preferably 120-140°C, more preferably 130°C, and the time is preferably 8-24 hours, more preferably 12-18 hours.

[0074] In this invention, the precursors used in the second hydrothermal process are titanium oxysulfate, sulfuric acid, phosphoric acid, and hydrogen peroxide. During this process, due to the acidity of sulfuric acid and phosphoric acid and the oxidizing property of hydrogen peroxide, titanium oxysulfate exists in the form of tetravalent titanium ions after dissolving. During the hydrothermal process, when the titanium ions on the stainless steel surface after the first phosphating reach a certain concentration, a titanium phosphate coating is gradually formed in situ on its surface.

[0075] It is also worth noting that if stainless steel does not undergo the first step of hydrothermal phosphating, the second step of hydrothermal reaction conditions cannot form a titanium phosphate nanocoating on the stainless steel surface.

[0076] The present invention provides an in-situ superhydrophilic titanium phosphate coating on a stainless steel surface prepared by the above method, comprising an iron-based metal substrate and a titanium phosphate coating grown in situ on the surface of the iron-based metal substrate, wherein the titanium phosphate coating has a nanosheet structure.

[0077] In this invention, the thickness of the titanium phosphate coating is preferably 5 to 6 μm.

[0078] This invention provides the application of the above-mentioned in-situ superhydrophilic titanium phosphate coating on stainless steel surfaces in oil-water separation. The superhydrophilic titanium phosphate coating provided by this invention has superhydrophilicity and the ability to adhere antigens to oils in the aqueous phase, and exhibits excellent oil-water separation performance when used as an oil-water separation membrane.

[0079] The following detailed description, in conjunction with embodiments, illustrates the in-situ superhydrophilic titanium phosphate coating on stainless steel surfaces provided by the present invention, its preparation method, and its application in oil-water separation. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0080] Example 1

[0081] A method for in-situ preparation of a superhydrophilic titanium phosphate coating on a stainless steel surface comprises the following steps:

[0082] (1) Substrate pretreatment: The 316L stainless steel substrate is placed in a mixture of acetone and isopropanol in a volume ratio of 1:1. The surface is cleaned using an ultrasonic cleaner for 30 minutes and 300W. This removes organic contaminants from the surface. After rinsing with water, it is ready for use.

[0083] (2) First hydrothermal phosphating: Add 50 mL of water to the polytetrafluoroethylene liner, and add 8 mL of phosphoric acid and 2 mL of tetrabutyl titanate. Stir evenly and immerse the stainless steel substrate treated in step (1) in the solution. Then transfer the polytetrafluoroethylene liner to a stainless steel reactor and hydrothermally react at 180°C for 5 hours in a forced-air drying oven. After cooling to room temperature, remove the phosphated stainless steel substrate and wash it with water for later use.

[0084] (3) Second hydrothermal preparation of the coating: First, a titanium oxysulfate solution was prepared. The specific preparation method was to thoroughly mix 200 mL of water, 0.6 mL of sulfuric acid, and 0.5 g of titanium oxysulfate powder, and stir for 4 hours until the titanium oxysulfate was completely dissolved and the solution was transparent. Then, 16 mL of hydrogen peroxide and 4 mL of phosphoric acid were added to the prepared titanium oxysulfate solution and stirred evenly. Next, the stainless steel substrate that underwent hydrothermal phosphating in step (2) was placed in the above solution, and then the solution was transferred to the polytetrafluoroethylene liner of the reactor and hydrothermally reacted at 120 °C for 8 hours in a forced-air drying oven. After cooling to room temperature, the sample was taken out and washed with clean water. After drying, the final sample was obtained with a coating thickness of 5-6 μm.

[0085] Figure 1 Scanning electron microscope images of 316L stainless steel as a whole (a), after primary hydrothermal phosphating (b), and after secondary hydrothermal coating preparation (c). Figure 1 Scanning electron microscopy reveals that, compared to the original stainless steel, a uniform phosphate film is formed on its surface after the hydrothermal phosphate process. Figure 1 As can be clearly observed in (c), after two hydrothermal treatments, the stainless steel surface is uniformly covered by nanosheets, and the surface roughness of the sample is significantly different from that of the original stainless steel. The change in the surface morphology of the sample also indicates the successful preparation of the coating.

[0086] Figure 2 X-ray photoelectron spectroscopy (XPS) spectra of 316L stainless steel original sample, coating prepared by primary hydrothermal phosphating, and coating prepared by secondary hydrothermal treatment. Figure 2X-ray photoelectron spectroscopy (XPS) analysis also revealed that the original stainless steel contained very little phosphorus (P), while the P content increased significantly after a single hydrothermal phosphating treatment. This also indicates that the hydrothermal reaction successfully phosphating the stainless steel. After a second hydrothermal treatment, many elements on the stainless steel surface disappeared, and distinct peaks for P, O, and Ti elements appeared. Given that hydrothermal methods are commonly used to prepare titanium phosphate materials, it can be concluded that the surface micro / nanosheet structure material is a titanium phosphate coating material.

[0087] Water contact angle tests were performed on 316L stainless steel samples, both raw and coated after secondary hydrothermal treatment. Figure 3 Photographs show the contact angles of water on the surface of a 316L stainless steel sample (a) and on the surface of a coated sample after secondary hydrothermal treatment (b). The contact angle test shows that the original stainless steel sample has moderate hydrophilicity, with a contact angle of approximately 56°. However, after secondary hydrothermal treatment, the contact angle of the sample surface is 0°, indicating its extremely excellent hydrophilicity.

[0088] Contact angle of oil droplets in aqueous phase was tested on 316L stainless steel original sample and coated sample after secondary hydrothermal treatment. Figure 4 The images show the contact angles of oil droplets (dodecane, hexadecane, toluene, petroleum ether, crude oil, and dichloroethane) on the sample surface after secondary hydrothermal treatment in the aqueous phase. It can be seen that the contact angles of these oil droplets on the sample surface after secondary hydrothermal treatment are all greater than 150°, indicating that the sample exhibits excellent superoleophobic properties in the aqueous phase.

[0089] Figure 5 Photograph (a) and test results (b) show the adhesion force of crude oil to the sample surface in the aqueous phase after secondary hydrothermal treatment. Surface force testing shows that the coating material has extremely low adhesion force to crude oil in the aqueous phase. When the applied crude oil pressure is 100, 200, and 300 micronewtons, the adhesion force of crude oil to its surface is 0.07, 2.4, and 3.2 micronewtons, respectively.

[0090] Example 2

[0091] By replacing the 316L stainless steel substrate in Example 1 with a stainless steel mesh, a titanium phosphate nanocoating can be successfully prepared on the surface of the stainless steel mesh according to the method in Example 1.

[0092] Figure 6 High- and low-magnification scanning electron microscope images of the original stainless steel mesh (a) and the sample used as an oil-water separation membrane after secondary hydrothermal treatment (b).

[0093] The titanium phosphate coating on the surface of the obtained stainless steel mesh was subjected to a 20 kg standard drop test (GB / T23988-2009). Figure 7These are low-magnification (a) and high-magnification (b) scanning electron microscope images of a stainless steel mesh with a titanium phosphate coating after a 20 kg drop test. It can be seen that although some of the coating has peeled off, some parts remain clearly visible, indicating that the titanium phosphate coating grown in situ on the stainless steel substrate exhibits strong interfacial adhesion.

[0094] Figure 8 The graph shows the relationship between water flux (left axis) and oil content in the effluent (right axis) as a continuous oil-water separation volume when a stainless steel mesh with a titanium phosphate coating is used as an oil-water separation membrane. It can be seen that in crude oil / water separation, even when the volume of the filtered oil-water mixture increases to 4000 liters, the water flux remains as high as 1800 liters / m² / hour, while the oil content in the effluent remains below 5 mg / L, indicating its significant advantages in oil-water separation.

[0095] Example 3

[0096] (1) Substrate pretreatment is the same as in Example 1.

[0097] (2) First hydrothermal phosphating: Add 50 mL of water to the polytetrafluoroethylene liner, and add 8 mL of phosphoric acid and 8 mL of tetrabutyl titanate. Stir evenly and immerse the stainless steel substrate treated in step (1) in the solution. Then transfer the polytetrafluoroethylene liner to a stainless steel reactor and hydrothermally react at 160°C for 10 h in a forced-air drying oven. After cooling to room temperature, remove the phosphated stainless steel substrate and wash it with water for later use.

[0098] (3) The second hydrothermal preparation of the coating is the same as in Example 1.

[0099] The sample also showed a 0° contact angle with water in air and a 157° contact angle with crude oil in water, indicating that the sample also has excellent superhydrophilic-superoleophobic properties in water.

[0100] Example 4

[0101] (1) Substrate pretreatment is the same as in Example 1.

[0102] (2) The first hydrothermal phosphating was the same as in Example 1.

[0103] (3) Second hydrothermal preparation of the coating: First, a titanium oxysulfate solution was prepared. The specific preparation method was to thoroughly mix 200 mL of water, 0.5 mL of sulfuric acid, and 1 g of titanium oxysulfate powder, and stir for 8 hours until the titanium oxysulfate was completely dissolved and the solution was transparent. Then, 16 mL of hydrogen peroxide and 4 mL of phosphoric acid were added to the prepared titanium oxysulfate solution and stirred evenly. Next, the substrate that underwent hydrothermal phosphating in step (2) was placed in the above solution, and then the solution was transferred to the polytetrafluoroethylene liner of the reactor and hydrothermally reacted at 120°C for 24 hours in a forced-air drying oven. After cooling to room temperature, the sample was taken out and washed with clean water. After drying, the final sample was obtained. The contact angle of the obtained sample with water in air was 0°, and the contact angle with crude oil in water was 154°, indicating that the sample also has excellent superhydrophilic-superoleophobic properties in water.

[0104] Example 5

[0105] (1) Substrate pretreatment is the same as in Example 1.

[0106] (2) The first hydrothermal phosphating was the same as in Example 1.

[0107] (3) Second hydrothermal preparation of the coating: First, a titanium oxysulfate solution was prepared. The specific preparation method was to thoroughly mix 200 mL of water, 0.6 mL of sulfuric acid, and 0.5 g of titanium oxysulfate powder, and stir for 4 hours until the titanium oxysulfate was completely dissolved and the solution was transparent. Then, 20 mL of hydrogen peroxide and 2 mL of phosphoric acid were added to the prepared titanium oxysulfate solution and stirred evenly. Next, the substrate that underwent hydrothermal phosphating in step (2) was placed in the above solution, and then the solution was transferred to the polytetrafluoroethylene liner of the reactor and hydrothermally reacted at 140°C for 8 hours in a forced-air drying oven. After cooling to room temperature, the sample was taken out and washed with clean water. After drying, the final sample was obtained. The contact angle of the obtained sample with water in air was 0°, and the contact angle with crude oil in water was 155°, indicating that the sample also has excellent superhydrophilic-superoleophobic properties in water.

[0108] Comparative Example 1

[0109] Compared with Example 1, the addition of tetrabutyl titanate during the first hydrothermal phosphating process was omitted, and the rest of the operations were the same.

[0110] X-ray photoelectron spectroscopy tests showed that the stainless steel surface did not contain phosphorus, meaning that under these conditions, the stainless steel substrate surface could not be phosphated, making it difficult to carry out a subsequent second hydrothermal reaction.

[0111] Comparative Example 2

[0112] Compared with Example 1, step (2) is omitted, and the rest of the operations are the same.

[0113] The results showed that X-ray photoelectron spectroscopy did not detect elements such as titanium and phosphorus on the surface, and SEM showed that the treated surface did not change significantly compared with the original stainless steel. Therefore, without the first step of hydrothermal phosphating, the second step of hydrothermal reaction conditions cannot construct a titanium phosphate nanocoating on the stainless steel surface.

[0114] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for in-situ preparation of a superhydrophilic titanium phosphate coating on a stainless steel surface, comprising the following steps: The stainless steel substrate is mixed with the first phosphating solution and subjected to the first hydrothermal phosphating reaction to obtain a stainless steel substrate coated with a phosphating film. The stainless steel substrate covered with phosphate film is mixed with a second phosphate solution to carry out a second hydrothermal phosphate reaction, thereby obtaining a superhydrophilic titanium phosphate coating on the surface of the stainless steel substrate. The first phosphating solution comprises phosphoric acid, tetrabutyl titanate, and water; The second phosphating solution comprises titanium sulfate, sulfuric acid, phosphoric acid, hydrogen peroxide, and water.

2. The method according to claim 1, characterized in that, In the first phosphating solution, the volume ratio of phosphoric acid to tetrabutyl titanate is 0.5 to 10:1; The volume ratio of phosphoric acid to water is 1 to 4:10; The phosphoric acid has a mass fraction of 85%.

3. The method according to claim 1 or 2, characterized in that, The temperature of the first hydrothermal phosphating reaction is 160–180°C, and the time is 5–10 h.

4. The method according to claim 1, characterized in that, The method for preparing the second phosphating solution includes the following steps: Titanium oxysulfate, sulfuric acid, and water are mixed to obtain an aqueous solution of titanium oxysulfate; The aqueous solution of titanium oxysulfate was mixed with phosphoric acid and hydrogen peroxide to obtain a second phosphating solution.

5. The method according to claim 1, wherein In the second phosphating solution, the mass ratio of titanium oxysulfate to sulfuric acid is 0.5–1 g: 0.2–1 mL; The mass ratio of titanium oxysulfate to phosphoric acid is 0.5–1 g: 2–6 mL. The mass ratio of titanium oxysulfate to hydrogen peroxide is 0.5–1 g: 12–20 mL. The mass ratio of titanium oxysulfate to water is 0.5–1 g: 50–200 mL; The sulfuric acid has a mass concentration of 98%, the phosphoric acid has a mass concentration of 85%, and the hydrogen peroxide has a mass concentration of 30%.

6. The method according to claim 1 or 5, characterized in that, The temperature of the second hydrothermal phosphating reaction is 120–140°C, and the time is 8–24 hours.

7. The method according to claim 1, characterized in that, The stainless steel substrate is a stainless steel mesh with a pore size of 5–25 μm.

8. The in-situ superhydrophilic titanium phosphate coating on the surface of stainless steel prepared by the method of any one of claims 1 to 7 comprises a stainless steel substrate and a titanium phosphate coating grown in situ on the surface of the stainless steel substrate, wherein the titanium phosphate coating has a nanosheet structure.

9. The in-situ superhydrophilic titanium phosphate coating on stainless steel surface according to claim 8, characterized in that, The thickness of the titanium phosphate coating is 5–6 μm.

10. The application of the in-situ superhydrophilic titanium phosphate coating on stainless steel surface as described in claim 8 or 9 in oil-water separation.

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