Aqueous protective coating for wheel surfaces and method for its production

Abstract

This invention relates to the field of coating technology and discloses a water-based protective coating for wheel and axle surfaces and its preparation method. The water-based protective coating is prepared from a primer and a topcoat. This application uses bis[3-(trimethoxysilyl)propyl]amine and 3,3-dithiodipropionic acid as raw materials to prepare a self-healing additive, which is dispersed in the primer and topcoat. Firstly, it improves the adhesion between the primer and the wheel and axle metal; secondly, it improves the interlayer adhesion between the primer and the topcoat; and thirdly, it forms a Si-O-Si film on the topcoat surface, improving the coating's water resistance and acid and alkali resistance. The water-based protective coating prepared by this application has excellent comprehensive performance. Using this coating can help reduce the surface treatment costs of wheel and axle for railway operators, reduce coating usage costs, improve the service life of wheel and axle, and optimize the operating efficiency of the railway system.

Description

Technical Field

[0001] This invention relates to the field of coating technology, specifically to a water-based protective coating for wheel and axle surfaces and its preparation method. Background Technology

[0002] With the rapid development of China's high-speed rail technology, wheel and axle, as key load-bearing components for train operation, have protective coating technologies that directly affect vehicle safety, durability, and maintenance costs. Wheel and axle are constantly exposed to complex environments such as rainwater, salt spray, and mechanical wear, making them susceptible to corrosion and fatigue damage, which in turn affects operational stability. Currently, the performance of domestic wheel and axle protective coatings lags significantly behind that of Weckeler products from Germany, and a mature pure water-based wheel and axle protective coating solution has not yet emerged internationally. With increasingly stringent environmental protection requirements, the development of water-based wheel and axle protective coatings has become imperative.

[0003] Existing coating materials are prone to adhesion failure or detachment under high-frequency impact and complex loads. The friction, vibration, and impact forces experienced by wheel axles during operation far exceed those of ordinary rail transit equipment. Traditional epoxy or polyurethane coatings, due to insufficient toughness, are prone to stress concentration leading to cracking (such as alligator cracks) or localized peeling, exposing the substrate and accelerating corrosion. Simultaneously, the coating surface is easily scratched or abraded by particles such as sand and hail, resulting in a sharp decline in protective performance. High-speed rail wheel axle coatings need to remain stable under dynamic friction and cyclic loads, but existing materials lack intelligent repair mechanisms. For example, after localized failure due to mechanical wear or microcracks, the coating cannot fill the defects through self-healing, relying solely on frequent downtime for maintenance. Although the academic community has explored self-healing coatings based on microcapsules or shape memory polymers, these technologies have not yet achieved large-scale application, and their high cost and repair efficiency fail to meet the operational needs of high-speed rail.

[0004] Based on this, this application provides a water-based protective coating for wheel and axle surfaces to solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to overcome one or more shortcomings in the prior art and provide a water-based protective coating for wheel and axle surfaces, wherein the coating prepared by this method has excellent comprehensive performance.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] A water-based protective coating for wheel and axle surfaces, the water-based protective coating being prepared from a primer and a topcoat;

[0008] The primer is composed of the following components by weight percentage: 35%-55% single-component waterborne alkyd resin, 2%-8% film-forming aid, 0.5%-2% dispersant, 0.2%-1% wetting agent, 3%-10% pigment, 15%-30% filler, 2-5% self-healing agent, 0.2%-0.8% anti-settling agent, 0.5%-1.5% anti-flash rust agent, 0.2%-0.8% pH adjuster, and 15%-25% deionized water;

[0009] The topcoat is composed of the following components by weight percentage: 50%-70% single-component waterborne acrylic resin, 5%-12% film-forming aid, 0.5%-2% dispersant, 0.2%-1% wetting agent, 1%-5% pigment, 12%-25% filler, 2-5% self-healing agent, 2%-5% anti-settling agent, 0.5%-1% pH adjuster, and 6%-15% deionized water.

[0010] Preferably, the film-forming aid is one or more of ethylene glycol butyl ether, diethylene glycol butyl ether, propylene glycol methyl ether, dipropylene glycol methyl ether, propylene glycol butyl ether, dipropylene glycol butyl ether, tripropylene glycol butyl ether, and ethylene glycol.

[0011] Preferably, the dispersant is a silicate dispersant (such as dispersant LAPONITE® S482), a phosphate dispersant (sodium hexametaphosphate, sodium tripolyphosphate), a polyacrylate dispersant (such as dispersant Dispex® Ultra PA 4550AN), a polyurethane dispersant (such as dispersant Efka® PU 4061), or a polyetheramine dispersant (such as Houtch® D-156).

[0012] Preferably, the wetting agent is a polyether-modified organosilicon wetting agent (such as wetting agent CW-7137) or a fluoropolymer wetting agent (such as wetting agent Thetawet FS-8100).

[0013] Preferably, the pigment is one or more of titanium dioxide, carbon black, iron oxide red, iron oxide yellow, iron oxide orange, iron oxide black, and zinc oxide; the filler is one or more of barium sulfate, talc, kaolin, calcium carbonate, silica powder, mica powder, and wollastonite; and the anti-settling agent is one or more of fumed silica and bentonite.

[0014] Preferably, the flash rust inhibitor is one or more of the following: organic complex flash rust inhibitors (such as flash rust inhibitor CK-69, flash rust inhibitor HY-75), long carbon chain multi-component carboxylic acid flash rust inhibitors (flash rust inhibitor DX319, flash rust inhibitor DX309), isomeric organic amine flash rust inhibitors (flash rust inhibitor DX319), and nitrite flash rust inhibitors (flash rust inhibitor FH-4Y, flash rust inhibitor HY-75).

[0015] Preferably, the pH adjuster is one or more of ammonia, sodium hydroxide aqueous solution, N,N-dimethylethanolamine, and 2-amino-2-methyl-1-propanol.

[0016] Preferably, the preparation method of the self-healing functional additive includes the following steps:

[0017] Under ice-water bath conditions, bis[3-(trimethoxysilyl)propyl]amine, 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl), and 4-dimethylaminopyridine (DMAP) were added to chloroform, stirred and mixed to disperse, and then 3,3-dithiodipropionic acid was added. The mixture was stirred and reacted at room temperature for 24-28 hours. After the reaction was completed, the mixture was rotary evaporated and dried to obtain the self-healing functional additive.

[0018] In this process, under the catalysis of EDC·HCl and DMAP, the secondary amine group in bis[3-(trimethoxysilyl)propyl]amine reacts with the carboxyl group in 3,3-dithiodipropionic acid to obtain a self-healing functional additive. The preparation method is simple. It contains disulfide bonds. When the coating is subjected to mechanical force (such as scratches or cracks) and breaks, the disulfide bonds (-SS-) are homolytically cleaved, forming two highly reactive sulfur free radicals. These radicals exchange with nearby disulfide bonds, reforming new disulfide bonds, thus "stitching" the cracks together at the molecular level. Furthermore, the disulfide bonds are dynamic covalent bonds, capable of repeated breaking and recombination at the same location, achieving self-healing. This solves the problems of poor self-healing performance of existing wheel and axle coatings and the high cost of adding self-healing materials. The synthetic reaction route of the self-healing functional additive in this application is as follows:

[0019] ;

[0020] Preferably, the mass ratio of bis[3-(trimethoxysilyl)propyl]amine, EDC·HCl, DMAP, and 3,3-dithiodipropionic acid is 4-4.5:2.2-2.5:0.3-0.32:1.

[0021] Preferably, the method for preparing the aqueous protective coating for the wheel and axle surface includes the following steps:

[0022] Step 1: Mix the dispersant, pigment, filler, 1 / 2 self-healing functional additive, anti-settling agent, and 1 / 2 deionized water evenly, then grind them to prepare a filler slurry. Then add the single-component waterborne alkyd resin, 1 / 2 self-healing functional additive, wetting agent, anti-flash rust agent, pH adjuster, film-forming aid, and 1 / 2 deionized water to the slurry, stir for 30-40 minutes, mix evenly, and obtain the primer.

[0023] In this process, the self-healing additive is added in two stages: The first stage, with half added, involves the formation of Si-OH (silanol groups) in the presence of deionized water. This Si-OH undergoes a dehydration condensation reaction with the hydroxyl groups on the surfaces of pigments and fillers, firmly adhering to their surfaces. Subsequently, when mixed with waterborne alkyd resin, the additive is uniformly dispersed in the primer matrix without agglomeration, thus fully utilizing the functions of the pigments and fillers. The second stage, with the remaining half added, involves the hydrolysis of the siloxane structure in the additive under the action of deionized water, producing Si-OH. This Si-OH then undergoes a condensation reaction with the hydroxyl groups in the waterborne alkyd resin, introducing the additive into the resin molecules and increasing the resin's cross-linking degree. When the primer is applied to the metal surface of the wheel axle, the unreacted Si-OH can condense with the hydroxyl groups on the metal surface, forming strong covalent bonds, further enhancing the primer's adhesion to the wheel axle. Simultaneously, the silanol groups also condense with each other, forming a dense Si-O-Si three-dimensional network structure, making the coating denser and tougher. Under external force, it can absorb and disperse external stress, significantly improving the coating's mechanical properties.

[0024] Step 2: Mix the dispersant, pigment, filler, 1 / 2 self-healing functional additive, anti-settling agent, and 1 / 2 deionized water evenly, then grind them to prepare a filler slurry. Then add the single-component water-based acrylic resin, 1 / 2 self-healing functional additive, wetting agent, pH adjuster, film-forming aid, and 1 / 2 deionized water to the slurry, stir for 30-40 minutes, mix evenly, and obtain the topcoat.

[0025] Step 3: Wipe the surface of the wheel axle with butyl acetate to remove stains and dust. Apply a one-component water-based alkyd resin primer using a high-pressure airless sprayer, controlling the thickness to 50-60 μm. Allow it to dry naturally until surface dry. Then apply a one-component acrylic topcoat using a high-pressure airless sprayer, controlling the thickness to 130-150 μm. Dry for 12 hours and then bake at 60℃ for 2 hours to obtain a water-based protective coating for the wheel axle surface.

[0026] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0027] This application describes the preparation of a self-healing additive using bis[3-(trimethoxysilyl)propyl]amine and 3,3-dithiodipropionic acid as raw materials. The additive has a novel structure and a simple preparation method. It is added to the primer and topcoat in two stages. The first addition disperses fillers and pigments, ensuring their uniform distribution in both the primer and topcoat. The second addition, under the influence of deionized water, causes the Si-OH groups in the primer to undergo a condensation reaction with the hydroxyl groups on the metal surface, increasing the adhesion between the primer and the metal substrate of the wheel axle. During the topcoat film formation process, siloxane-containing molecular segments migrate and accumulate on the surface, where the Si-OH groups undergo a self-condensation reaction, forming a dense Si-O-Si film. Due to the low surface energy of Si-O-Si, the coating surface exhibits excellent hydrophobicity and acid / alkali resistance, thereby improving the service life of the coating in high-humidity, salt-spray environments.

[0028] Furthermore, during the application of the topcoat over the primer, the topcoat contains deionized water. Under the action of the deionized water, the Si-OH in the primer undergoes dehydration condensation with the Si-OH generated in the topcoat in the surface-dried state, forming a Si-O-Si structure at the interface between the primer and the topcoat. This increases the bonding strength between the primer and the topcoat, significantly improves the interlayer adhesion between the primer and the topcoat, and further enhances the mechanical properties of the coating. Detailed Implementation

[0029] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0030] All raw materials used in this application are commercially available products.

[0031] Example 1

[0032] This embodiment provides a method for preparing a self-healing functional additive:

[0033] Under ice-water bath conditions, 17.1 g of bis[3-(trimethoxysilyl)propyl]amine, 9.4 g of EDC·HCl, and 1.22 g of DMAP were added to chloroform and stirred to mix and disperse. Then, 4.2 g of 3,3-dithiodipropionic acid was added. The mixture was stirred and reacted at room temperature for 24-28 h. After the reaction was completed, the mixture was rotary evaporated and dried to obtain the self-healing functional additive.

[0034] Example 2

[0035] This embodiment provides a method for preparing a self-healing functional additive:

[0036] Under ice-water bath conditions, 18g of bis[3-(trimethoxysilyl)propyl]amine, 9.5g of EDC·HCl, and 1.3g of DMAP were added to chloroform and stirred to mix and disperse. Then, 4.2g of 3,3-dithiodipropionic acid was added. The mixture was stirred and reacted at room temperature for 24-28 hours. After the reaction was completed, the mixture was rotary evaporated and dried to obtain the self-healing functional additive.

[0037] Example 3

[0038] This embodiment provides a method for preparing a water-based protective coating for wheel and axle surfaces:

[0039] Step 1: Mix the dispersant, pigment, filler, 1 / 2 self-healing functional additive, anti-settling agent, and 1 / 2 deionized water evenly, then grind them to prepare a filler slurry. Then add the single-component waterborne alkyd resin, 1 / 2 self-healing functional additive, wetting agent, anti-flash rust agent, pH adjuster, film-forming aid, and 1 / 2 deionized water to the slurry, stir for 30 minutes, mix evenly, and obtain the primer.

[0040] Step 2: Mix the dispersant, pigment, filler, 1 / 2 self-healing functional additive, anti-settling agent, and 1 / 2 deionized water evenly, then grind them to prepare a filler slurry. Then add the single-component water-based acrylic resin, 1 / 2 self-healing functional additive, wetting agent, pH adjuster, film-forming aid, and 1 / 2 deionized water to the slurry, stir for 30 minutes, mix evenly, and obtain the topcoat.

[0041] Step 3: Wipe the surface of the wheel axle with butyl acetate to remove stains and dust. Apply a one-component water-based alkyd resin primer using a high-pressure airless spraying device, controlling the thickness to 50 μm. Allow it to dry naturally. Then apply a one-component acrylic topcoat using a high-pressure airless spraying device, controlling the thickness to 130 μm. Dry for 12 hours and then bake at 60°C for 2 hours to obtain a water-based protective coating for the wheel axle surface.

[0042] In this embodiment, the primer is composed of the following components by weight percentage: 37.3% single-component waterborne alkyd resin, 2% ethylene glycol butyl ether film-forming aid, 2% sodium hexametaphosphate dispersant, 0.2% wetting agent CW-7137, 10% iron oxide yellow pigment, 20% kaolin filler, 1% self-healing functional aid (the self-healing functional aid in Example 1), 0.5% bentonite anti-settling agent, 1.5% flash rust inhibitor CK-69, 0.5% N,N-dimethylethanolamine pH adjuster, and 25% deionized water;

[0043] The topcoat is composed of the following components by weight percentage: 52.5% single-component waterborne acrylic resin, 8% ethylene glycol film-forming aid, 2% dispersant LAPONITE® S482, 0.5% wetting agent CW-7137, 5% iron oxide yellow pigment, 15% kaolin filler, 1% self-healing agent (the self-healing agent in Example 1), 5% bentonite anti-settling agent, 1% ammonia pH adjuster, and 10% deionized water.

[0044] Example 4

[0045] This embodiment provides a method for preparing a water-based protective coating for wheel and axle surfaces:

[0046] Step 1: Mix the dispersant, pigment, filler, 1 / 2 self-healing functional additive, anti-settling agent, and 1 / 2 deionized water evenly, then grind them to prepare a filler slurry. Then add the single-component waterborne alkyd resin, 1 / 2 self-healing functional additive, wetting agent, anti-flash rust agent, pH adjuster, film-forming aid, and 1 / 2 deionized water to the slurry, stir for 40 minutes, mix evenly, and obtain the primer.

[0047] Step 2: Mix the dispersant, pigment, filler, 1 / 2 self-healing functional additive, anti-settling agent, and 1 / 2 deionized water evenly, then grind them to prepare a filler slurry. Then add the single-component water-based acrylic resin, 1 / 2 self-healing functional additive, wetting agent, pH adjuster, film-forming aid, and 1 / 2 deionized water to the slurry, stir for 35 minutes, mix evenly, and obtain the topcoat.

[0048] Step 3: Wipe the surface of the wheel axle with butyl acetate to remove stains and dust. Apply a one-component water-based alkyd resin primer using a high-pressure airless spraying device, controlling the thickness to 55 μm. Allow it to dry naturally. Then apply a one-component acrylic topcoat using a high-pressure airless spraying device, controlling the thickness to 150 μm. Dry for 12 hours and then bake at 60°C for 2 hours to obtain a water-based protective coating for the wheel axle surface.

[0049] In this embodiment, the primer is composed of the following components by weight percentage: 46% single-component waterborne alkyd resin, 5% dipropylene glycol butyl ether film-forming aid, 1% dispersant Efka® PU 4061, 1% wetting agent CW-7137, 7% iron oxide red pigment, 15% kaolin filler, 2% self-healing functional aid (the self-healing functional aid in Example 1), 0.8% bentonite anti-settling agent, 1.5% flash rust inhibitor FH-4Y, 0.7% ammonia pH adjuster, and 20% deionized water;

[0050] The topcoat is composed of the following components by weight percentage: 50% single-component waterborne acrylic resin, 8% diethylene glycol butyl ether film-forming aid, 1.5% sodium hexametaphosphate dispersant, 0.2% wetting agent CW-7137, 5% iron oxide red pigment, 22% kaolin filler, 2% self-healing functional aid (the self-healing functional aid in Example 1), 2% bentonite anti-settling agent, 1% 2-amino-2-methyl-1-propanol pH adjuster, and 8.3% deionized water.

[0051] Example 5

[0052] This embodiment provides a method for preparing a water-based protective coating for wheel and axle surfaces:

[0053] Step 1: Mix the dispersant, pigment, filler, 1 / 2 self-healing functional additive, anti-settling agent, and 1 / 2 deionized water evenly, then grind them to prepare a filler slurry. Then add the single-component waterborne alkyd resin, 1 / 2 self-healing functional additive, wetting agent, anti-flash rust agent, pH adjuster, film-forming aid, and 1 / 2 deionized water to the slurry, stir for 35 minutes, mix evenly, and obtain the primer.

[0054] Step 2: Mix the dispersant, pigment, filler, 1 / 2 self-healing functional additive, anti-settling agent, and 1 / 2 deionized water evenly, then grind them to prepare a filler slurry. Then add the single-component water-based acrylic resin, 1 / 2 self-healing functional additive, wetting agent, pH adjuster, film-forming aid, and 1 / 2 deionized water to the slurry, stir for 40 minutes, mix evenly, and obtain the topcoat.

[0055] Step 3: Wipe the surface of the wheel axle with butyl acetate to remove stains and dust. Apply a one-component water-based alkyd resin primer using a high-pressure airless sprayer, controlling the thickness to 60µm. Allow it to dry naturally. Then apply a one-component acrylic topcoat using a high-pressure airless sprayer, controlling the thickness to 140µm. Dry for 12 hours and then bake at 60℃ for 2 hours to obtain a water-based protective coating for the wheel axle surface.

[0056] In this embodiment, the primer is composed of the following components by weight percentage: 42.2% single-component waterborne alkyd resin, 2% tripropylene glycol butyl ether film-forming aid, 0.5% sodium hexametaphosphate dispersant, 0.5% wetting agent CW-7137, 10% iron oxide orange pigment, 25% mica powder filler, 2.5% self-healing functional aid (the self-healing functional aid in Example 2), 0.5% fumed silica anti-settling agent, 1% flash rust inhibitor DX319, 0.5% ammonia pH adjuster, and 15% deionized water;

[0057] The topcoat is composed of the following components by weight percentage: 58% single-component waterborne acrylic resin, 10% film-forming aid propylene glycol methyl ether, 0.5% sodium tripolyphosphate dispersant, 1% wetting agent Thetawet FS-8100, 1% iron oxide orange pigment, 15% mica powder filler, 2.5% self-healing agent (the self-healing agent in Example 2), 5% fumed silica anti-settling agent, 1% ammonia pH adjuster, and 6% deionized water.

[0058] Comparative Example 1

[0059] The difference between this comparative example and Example 3 is that bis[3-(trimethoxysilyl)propyl]amine is used instead of the self-healing functional additive of the present invention.

[0060] Comparative Example 2

[0061] The difference between this comparative example and Example 3 is that 3,3-dithiodipropionic acid is used instead of the self-healing functional additive of the present invention.

[0062] Comparative Example 3 is a commercially available sample from overseas.

[0063] Adhesion was tested using the cross-cut adhesion test according to EN ISO 2409:2020 standard.

[0064] Salt spray resistance test was conducted in accordance with EN ISO 9227:2022 standard.

[0065] Table 1:

[0066]

[0067] The adhesion rating of 0 is the best. As can be seen from the table, the water-based protective coating prepared by this invention has excellent adhesion and good salt spray resistance.

[0068] Resistance to flying sand was tested according to BS EN 13261-2024 standard;

[0069] Cupping tests were performed according to the requirements in BS EN 13261-2024 and the method in EN ISO 1520:2006.

[0070] Table 2:

[0071]

[0072] The sand-resistant rating is 4, which is the best. As shown in the table, the coating prepared by this invention has good sand-resistant properties. The higher the cupping test result, the better the flexibility. The coating prepared by this application has good flexibility.

[0073] In accordance with the requirements of BS EN 13261-2024, the coating bending test was performed using the method of EN ISO 1519:2011.

[0074] The pull strength of the coating was tested using a pull tester. The coating was cut in the middle with a knife. The cut strip was pressed to make full contact with the fracture surface. It was then placed in an oven at 60°C and heated for 1 hour. After cooling to room temperature, the above steps were repeated 3 times. The pull strength after repair was tested using a pull tester. The self-repair efficiency (%) = pull strength after repair / pull strength before cutting × 100%.

[0075] Table 3:

[0076]

[0077] As shown in the table, the coating prepared by the present invention has good mechanical properties and self-healing efficiency. When subjected to external damage, it can self-repair and thus improve the service life of the wheel axle.

[0078] Water resistance was tested according to ISO 2812-2 / ASTM D870.

[0079] According to ISO 2812-2:2018, the acid, alkali and salt resistance properties were tested.

[0080] Table 4:

[0081]

[0082] As shown in the table, the coating prepared by this invention has excellent water resistance, acid and alkali resistance, and salt resistance, and is better than a certain commercially available product abroad, and has broad application prospects.

[0083] In summary, the above test data prove that the water-based protective coating for wheel and axle surfaces prepared by the method of this invention meets the protection requirements for Class III wheel and axle in BS EN 13261:2024, and its performance is superior to that of foreign samples in terms of mechanical properties, self-healing properties, salt spray resistance, resistance to 40°C water, resistance to 3% sulfuric acid, 10% sodium hydroxide, and 10% sodium chloride.

[0084] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A water-based protective coating for wheel and axle surfaces, characterized in that, The water-based protective coating is prepared from a primer and a topcoat; The primer is composed of the following components by weight percentage: 35%-55% single-component waterborne alkyd resin, 2%-8% film-forming aid, 0.5%-2% dispersant, 0.2%-1% wetting agent, 3%-10% pigment, 15%-30% filler, 2-5% self-healing agent, 0.2%-0.8% anti-settling agent, 0.5%-1.5% anti-flash rust agent, 0.2%-0.8% pH adjuster, and 15%-25% deionized water; The topcoat is composed of the following components by weight percentage: 50%-70% single-component waterborne acrylic resin, 5%-12% film-forming aid, 0.5%-2% dispersant, 0.2%-1% wetting agent, 1%-5% pigment, 12%-25% filler, 2-5% self-healing agent, 2%-5% anti-settling agent, 0.5%-1% pH adjuster, and 6%-15% deionized water.

2. The water-based protective coating for wheel and axle surfaces according to claim 1, characterized in that, The film-forming aid is one or more of ethylene glycol butyl ether, diethylene glycol butyl ether, propylene glycol methyl ether, dipropylene glycol methyl ether, propylene glycol butyl ether, dipropylene glycol butyl ether, tripropylene glycol butyl ether, and ethylene glycol.

3. The water-based protective coating for wheel and axle surfaces according to claim 1, characterized in that, The dispersant is a silicate dispersant, a phosphate dispersant, a polyacrylate dispersant, a polyurethane dispersant, or a polyetheramine dispersant.

4. The water-based protective coating for wheel and axle surfaces according to claim 1, characterized in that, The wetting agent is a polyether-modified organosilicon wetting agent or a fluoropolymer wetting agent.

5. The water-based protective coating for wheel and axle surfaces according to claim 1, characterized in that, The pigment is one or more of titanium dioxide, carbon black, iron oxide red, iron oxide yellow, iron oxide orange, iron oxide black, and zinc oxide; the filler is one or more of barium sulfate, talc, kaolin, calcium carbonate, silica powder, mica powder, and wollastonite; and the anti-settling agent is one or more of fumed silica and bentonite.

6. The water-based protective coating for wheel and axle surfaces according to claim 1, characterized in that, The flash rust inhibitor is one or more of the following: organic complex flash rust inhibitors, long carbon chain multi-component carboxylic acid flash rust inhibitors, isomeric organic amine flash rust inhibitors, and nitrite flash rust inhibitors.

7. The water-based protective coating for wheel and axle surfaces according to claim 1, characterized in that, The pH adjuster is one or more of ammonia, sodium hydroxide aqueous solution, N,N-dimethylethanolamine, and 2-amino-2-methyl-1-propanol.

8. The water-based protective coating for wheel and axle surfaces according to claim 1, characterized in that, The preparation method of the self-healing functional additive includes the following steps: Under ice-water bath conditions, bis[3-(trimethoxysilyl)propyl]amine, EDC·HCl, and DMAP were added to chloroform, stirred and mixed to disperse, and then 3,3-dithiodipropionic acid was added. The mixture was stirred and reacted at room temperature for 24-28 hours. After the reaction was completed, the mixture was rotary evaporated and dried to obtain the self-healing functional additive.

9. The water-based protective coating for wheel and axle surfaces according to claim 8, characterized in that, The mass ratio of bis[3-(trimethoxysilyl)propyl]amine, EDC·HCl, DMAP, and 3,3-dithiodipropionic acid is 4-4.5:2.2-2.5:0.3-0.32:

1.

10. A method for preparing an aqueous protective coating for axle surfaces as described in any one of claims 1-9 comprises the following steps: Step 1: Mix the dispersant, pigment, filler, 1 / 2 self-healing functional agent, anti-settling agent, and 1 / 2 deionized water evenly, then grind to prepare a filler slurry. Next, add the single-component waterborne alkyd resin, 1 / 2 self-healing functional agent, wetting agent, anti-flash rust agent, pH adjuster, film-forming aid, and 1 / 2 deionized water to the mixture. Stir for 30-40 minutes until well mixed to obtain the primer; Step 2: Mix the dispersant, pigment, filler, 1 / 2 self-healing functional additive, anti-settling agent, and 1 / 2 deionized water evenly, then grind them to prepare a filler slurry. Then add the single-component water-based acrylic resin, 1 / 2 self-healing functional additive, wetting agent, pH adjuster, film-forming aid, and 1 / 2 deionized water to the slurry, stir for 30-40 minutes, mix evenly, and obtain the topcoat. Step 3: Wipe the surface of the wheel axle with butyl acetate to remove stains and dust. Apply a one-component water-based alkyd resin primer using a high-pressure airless sprayer, controlling the thickness to 50-60 μm. Allow it to dry naturally. Then apply a one-component acrylic topcoat using a high-pressure airless sprayer, controlling the thickness to 130-150 μm. Dry for 12 hours and then bake at 60°C for 2 hours to obtain a water-based protective coating for the wheel axle surface.

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