A low-temperature curable acrylic cathodic electrophoretic coating and its preparation method

Abstract

The present invention relates to an acrylic cathodic electrophoretic coating, and particularly to a preparation method and application of a low-temperature curing acrylic cathodic electrophoretic coating. The low-temperature curing acrylic cathodic electrophoretic coating prepared by the present invention does not generate smoke and smoke oil during drying, reduces the pollution of the electrophoretic drying furnace, reduces the exhaust gas emission, and is relatively environmentally friendly. The low-temperature curing acrylic cathodic electrophoretic coating prepared by the present invention does not generate bubble points during the coating process, reduces the color change of the electroplated layer caused by baking, and thus improves the coating film quality. The coating of the low-temperature curing acrylic cathodic electrophoretic coating prepared by the present invention has excellent corrosion resistance, good covering ability, high degree of automation of the coating equipment, and high coating utilization rate.

Description

Technical Field

[0001] The present invention relates to an acrylic cathodic electrophoretic coating, and particularly to a preparation method of a low-temperature curing acrylic cathodic electrophoretic coating and its application as a coating. Background Art

[0002] Acrylic cathodic electrophoretic coating is a special type of coating, mainly used in the metal surface treatment process. Through electrophoretic deposition (EPD), charged coating particles are deposited on the workpiece surface under the action of an electric field to form a uniform coating. This technology is particularly suitable for coating workpieces with complex shapes because it can ensure that the coating evenly covers every corner and crevice of the workpiece. Acrylic cathodic electrophoretic coatings are widely used in the automotive industry, household appliances, building materials, and other industries that require high-quality protective and decorative coatings. For example, automotive bodies and parts, the outer shells of household appliances such as refrigerators and washing machines, and door and window frames may all use this coating for surface treatment. Compared with traditional solvent-based coatings, acrylic cathodic electrophoretic coatings contain less volatile organic compounds and are more environmentally friendly. In addition, they can provide excellent rust and corrosion protection, are suitable for equipment and structures used outdoors or in harsh environments, can form a very smooth and shiny surface after application, have excellent decorative effects, and have good adhesion to the substrate, ensuring the durability and long-term use of the coating. The development of this coating and its application technology has greatly improved the quality and efficiency of product surface treatment, and at the same time promoted the application and development of environmentally friendly coatings. Although acrylic cathodic electrophoretic coatings are developing vigorously, the baking temperature range of ordinary acrylic cathodic electrophoretic coatings is generally 160 - 180 °C, which is prone to blistering during high-temperature baking of light industrial components with electroplated layers, affecting their use. Their salt spray resistance is also weaker than that of epoxy cathodic electrophoretic coatings and is not suitable for high-corrosion environments. Therefore, the development of low-temperature curing acrylic cathodic electrophoretic coatings is of great significance for promoting social development. Low-temperature curing cathodic electrophoretic coatings are not only beneficial for coating light industrial components with electroplated layers but also can greatly reduce energy consumption.

[0003] Due to its weather resistance, aesthetic appearance, and environmental protection advantages, acrylic cathodic electrophoretic coatings have become the preferred choice for high-end coatings in the fields of automotive, household appliances, and building materials. The low-temperature curing acrylic cathodic electrophoretic coating prepared by the present invention is increasingly widely used in outdoor and decorative scenarios due to its comprehensive performance. Summary of the Invention

[0004] In order to overcome the deficiencies of the prior art, the present invention provides a low-temperature curing acrylic cathodic electrophoretic coating and a preparation method thereof. The following technical solutions are adopted to achieve this:

[0005] S1. Preparation of modified acrylic resin: Mix methyl methacrylate, butyl acrylate, styrene and dimethylaminoethyl methacrylate evenly in a certain proportion, then continue to add propylene glycol methyl ether and butanol, and keep stirring. Add benzoyl peroxide and sodium dodecyl sulfate, and continue stirring. Then add the above mixture into a four-necked flask, introduce N2 gas or Ar gas, and then install it in a microwave reactor. Set the program, heat the temperature to 75 - 85 °C within 1 - 2 min, react for 90 - 180 min, set the microwave power to 600 - 900 W, cool to 40 - 55 °C after the reaction, and set aside; Take out the four-necked flask from the microwave reactor, then slowly add 1.5 - 2.1 g of glacial acetic acid, and keep stirring for 45 - 60 min; Then transfer the material to a high-speed disperser, and dropwise add deionized water under high-speed shearing at 2000 - 3000 rpm to form a cationic aqueous dispersion with a solid content of 35 - 42%; Finally, adjust the pH to 5.5 - 6.0, filter to remove impurities, and the modified acrylic resin emulsion can be obtained;

[0006] S2. Preparation of modified nanocomposite: Put SiO2, CeO2 and ZrO2 in a ball mill in a certain proportion for ball milling. Take out the ball-milled and mixed powder and add it to a polytetrafluoroethylene liner containing ethanol and N-methylpyrrolidone. Then add carbon nanotubes and methacryloxy silane, stir evenly, set the hydrothermal temperature to 150 - 180 °C, keep the temperature for 10 - 12 h, cool naturally, then centrifuge, wash the sample with ethanol 3 times to obtain a black solid material, transfer it to a ball mill, and the modified nanocomposite can be obtained after ball milling;

[0007] S3. Preparation of electrophoretic bath solution: Add the modified acrylic resin emulsion prepared in step S1, BYK-024 defoamer, BYK-348 leveling agent and 2,2'-dimercaptoethanol into a stirring kettle in a certain proportion and mix evenly, then add titanium dioxide, color pigments and the modified nanocomposite prepared in step S2, and stir at 2000 rpm / min for 3 h; Adjust the solid content of the material to 15 - 20% with deionized water; Then adjust the rotation speed to 500 rpm / min and mix at 55 - 63 °C for 15 - 20 min; Then lower the temperature to 25 - 30 °C, slowly add toluene diisocyanate and methyl ethyl ketoxime, stir evenly, adjust the pH to 5.5 - 6.0 with glacial acetic acid, and pass through a 200-mesh filter to obtain the electrophoretic bath solution;

[0008] S4. Clean and treat the metal surface to be coated to remove impurities such as oil stains and oxide layers; Then use the pretreated workpiece as the cathode, put it into the electrophoretic bath prepared in step S3, deposit the coating under the action of a direct current electric field, and remove the un-deposited coating particles; Finally, heat to completely cure the coating to form the final protective film.

[0009] The specific operation steps of the present invention are as follows:

[0010] S1. Prepare modified acrylic resin: After mixing 3.8 - 5.2 g of methyl methacrylate, 1.8 - 2.3 g of butyl acrylate, 1.1 - 1.9 g of styrene, and 1.6 - 1.8 g of dimethylaminoethyl methacrylate evenly, continue to add 25.8 - 32.5 g of propylene glycol methyl ether and 18.3 - 26.1 g of butanol, and continuously stir for 6 - 11 h. Then add 0.8 - 1.3 g of benzoyl peroxide and 0.3 - 0.5 g of sodium dodecyl sulfate, and continue to stir for 3 - 5 h. Then add the above - mentioned mixed solution into a four - necked flask, introduce N2 gas or Ar gas for 35 - 48 min, and then install it into a microwave reactor. Set the program to raise the temperature to 75 - 85 °C within 1 - 2 min, react for 90 - 180 min, set the microwave power to 600 - 900 W. After the reaction, cool it to 40 - 55 °C for standby; Take out the four - necked flask from the microwave reactor, and then slowly add 1.5 - 2.1 g of glacial acetic acid, and continuously stir for 45 - 60 min; Then transfer the material into a high - speed disperser, and dropwise add deionized water under high - speed shearing at 2000 - 3000 rpm to form a cationic aqueous dispersion with a solid content of 35 - 42%; Finally, adjust the pH to 5.5 - 6.0, filter to remove impurities, and the modified acrylic resin emulsion can be obtained. In this step, the purpose of mixing methyl methacrylate, butyl acrylate, and styrene in proportion is to adjust the hardness and flexibility of the material. The addition of dimethylaminoethyl methacrylate is mainly used to introduce cationic groups. The purpose of benzoyl peroxide is mainly to act as an initiator. The purpose of glacial acetic acid is to act as a neutralizing agent to adjust the pH value of the solution, playing the role of amine neutralization and dispersion. In this process, through the reaction of acid with amine groups (-NH2), cationic groups ( ) are generated, making the resin have a positive charge. The originally hydrophobic resin is transformed into a water - dispersible cationic polymer, ensuring that positively charged particles can stably migrate to the cathode workpiece during electrophoretic coating. In this step, through the strong shearing force generated by high rotation speed, the resin is "torn" into nano - scale fine particles, and at the same time, water molecules wrap around the particle surface to form a stable emulsion. The oil droplets are evenly dispersed in water. After the resin is neutralized by amine, the surface is positively charged ( ). During water dispersion, the positively charged particles cannot agglomerate due to electrostatic repulsion. This process ensures the electrophoretic efficiency of the coating and the uniformity and weather resistance of the final coating.

[0011] S2. Preparation of modified nanocomposites: Put 1.3 - 1.5 g of SiO2, 0.8 - 1.1 g of CeO2 and 0.9 - 1.4 g of ZrO2 into a ball mill, and ball mill at a rotation speed of 300 - 600 r / min for 4 - 6 h. Take out the ball-milled and mixed powder and add it to a 100 mL polytetrafluoroethylene liner containing 32 mL of ethanol and 23 mL of N-methylpyrrolidone. Then add 2.1 - 3.3 g of carbon nanotubes and 0.6 - 0.8 g of methacryloxy silane. After stirring evenly, set the hydrothermal temperature at 150 - 180 °C and the heat preservation time at 10 - 12 h. After natural cooling, centrifuge and wash the sample with ethanol three times to obtain a black solid material. Transfer it to a ball mill and ball mill for 3 - 5 h to obtain the modified nanocomposite. In this step, the functions of nano-SiO2, CeO2 and ZrO2 are to improve scratch resistance and UV resistance. The addition of carbon nanotubes not only enhances the conductivity of the composite material and improves the deposition uniformity, but also can improve the mechanical strength of the coating. In this step, the composite of nano-SiO2, CeO2 and ZrO2 improves the pigment dispersion efficiency and the purity of the coating by virtue of its high hardness and chemical stability. After combining with carbon nanotubes, as a functional filler through nanosizing and surface modification, it significantly enhances the hardness, wear resistance and weather resistance of the coating.

[0012] S3. Preparation of electrophoretic bath solution: Add 53 - 68 g of the modified acrylic resin emulsion prepared in step S1, 0.5 - 0.7 g of BYK-024 defoamer, 0.6 - 0.8 g of BYK-348 leveling agent and 0.2 - 0.4 g of dimercaptoethanol into a stirring kettle and mix evenly. Then add 8 - 11 g of titanium dioxide, 3 - 5 g of color pigment and 2.5 - 3.6 g of the modified nanocomposite prepared in step S2, and stir at 2000 rpm for 32 - 41 min. Adjust the solid content of the material to 15 - 20% with deionized water. Then adjust the rotation speed to 500 rpm and mix at 55 - 63 °C for 15 - 20 min. Then lower the temperature to 25 - 30 °C, slowly add 13 - 18 g of toluene diisocyanate and 3 - 5 g of methyl ethyl ketoxime, and stir for 28 - 36 min. Then adjust the pH to 5.5 - 6.0 with glacial acetic acid and pass through a 200-mesh filter to obtain the electrophoretic bath solution. In this step, the modified acrylic resin emulsion serves as a film-forming matrix to provide positive charges. The addition of titanium dioxide is mainly for covering and weather resistance. The function of methyl ethyl ketoxime is mainly to prevent the coating from drying and crusting on the inner surface of the container, ensure good fluidity of the coating when the can is opened, and also has the function of inhibiting oxidation. The selection of BYK-348 can significantly reduce the surface tension of the coating, help improve the material wettability and prevent cratering. It can promote the leveling of the coating during the drying process, reduce orange peel phenomenon, and improve the smoothness and flatness of the coating.

[0013] S4. Clean and treat the metal surface to be coated, removing impurities such as oil stains and oxide layers; then place the pretreated workpiece as the cathode into the electrophoresis bath prepared in step S3, deposit the coating under the action of a DC electric field, and remove the undeposited coating particles; finally, cure the coating completely by heating to form the final protective film. Specifically: First, spray the metal surface to be coated with an alkaline degreaser for 3 - 5 min, then wash it with two countercurrent running waters at room temperature for 1 - 2 min, immerse it in a zinc-based phosphating solution for 3 - 5 min, control the temperature at 40 - 50 °C to form a gray phosphating film; control the temperature at 28 - 30 °C, the deposition voltage at 150 - 250 V, the electrodeposition time at 2 - 3 min, the electrode spacing at 10 - 20 cm. During the deposition process, the charged resin particles migrate towards the cathode in the electric field and form a wet film through electrodialysis dehydration, with a thickness of 15 - 25 μm. Then, ultrafiltration water circulation is used for rinsing to recover the poorly adhered coating; in the curing stage, first pre-bake at 80 °C for 10 min, then raise the temperature to 115 - 120 °C, and the curing can be completed in 20 min.

[0014] Preferably: In step S1, after mixing 3.8 g of methyl methacrylate, 1.8 g of butyl acrylate, 1.1 g of styrene, and 1.6 g of dimethylaminoethyl methacrylate evenly, continue to add 25.8 g of propylene glycol methyl ether and 18.3 g of butanol, continuously stir for 6 h, add 0.8 g of benzoyl peroxide and 0.3 g of sodium dodecyl sulfate, continue to stir for 3 h, then add the above mixture into a four-necked flask, pass N2 gas for 35 min, then install it into a microwave reactor, set the program, raise the temperature to 75 °C within 1 min, react for 90 min, set the microwave power to 600 W, cool to 40 °C after the reaction is completed and set aside; take out the four-necked flask from the microwave reactor, then slowly add 1.5 g of glacial acetic acid and continuously stir for 45 min; then transfer the material into a high-speed disperser, and dropwise add deionized water under high-speed shearing at 2000 rpm to form a cationic aqueous dispersion with a solid content of 35%; finally, adjust the pH to 5.5, filter to remove impurities, and the modified acrylic resin emulsion can be obtained.

[0015] Preferably: In step S1, after mixing 5.2 g of methyl methacrylate, 2.3 g of butyl acrylate, 1.9 g of styrene, and 1.8 g of dimethylaminoethyl methacrylate uniformly, 32.5 g of propylene glycol methyl ether and 26.1 g of butanol are continuously added, and the mixture is continuously stirred for 11 h. Then, 1.3 g of benzoyl peroxide and 0.5 g of sodium dodecyl sulfate are added, and stirring is continued for 5 h. Then, the above mixture is added to a four-necked flask, and Ar gas is introduced for 48 min. Then, it is loaded into a microwave reactor, the program is set, the temperature is raised to 85 °C within 2 min, and the reaction is carried out for 180 min. The microwave power is set to 900 W. After the reaction, it is cooled to 55 °C and set aside; the four-necked flask is taken out of the microwave reactor, and then 2.1 g of glacial acetic acid is slowly added, and stirring is continued for 60 min; then the material is transferred to a high-speed disperser, and deionized water is added dropwise under high-speed shearing at 3000 rpm to form a cationic aqueous dispersion with a solid content of 42%; finally, the pH is adjusted to 6.0, and impurities are removed by filtration to obtain a modified acrylic resin emulsion.

[0016] Preferably: In step S2, 1.3 g of SiO2, 0.8 g of CeO2, and 0.9 g of ZrO2 are put into a ball mill and ball-milled at a rotation speed of 300 r / min for 4 h. The ball-milled and mixed powder is taken out and added to a 100 mL polytetrafluoroethylene liner containing 32 mL of ethanol and 23 mL of N-methylpyrrolidone. Then, 2.1 g of carbon nanotubes and 0.6 g of methacryloxy silane are added. After stirring evenly, the hydrothermal temperature is set to 150 °C, and the heat preservation time is 10 h. After natural cooling, centrifugal separation is carried out, and the sample is washed 3 times with ethanol to obtain a black solid material, which is transferred to a ball mill and ball-milled for 3 - 5 h to obtain a modified nanocomposite material.

[0017] Preferably: In step S2, 1.5 g of SiO2, 1.1 g of CeO2, and 1.4 g of ZrO2 are put into a ball mill and ball-milled at a rotation speed of 600 r / min for 6 h. The ball-milled and mixed powder is taken out and added to a 100 mL polytetrafluoroethylene liner containing 32 mL of ethanol and 23 mL of N-methylpyrrolidone. Then, 3.3 g of carbon nanotubes and 0.8 g of methacryloxy silane are added. After stirring evenly, the hydrothermal temperature is set to 180 °C, and the heat preservation time is 12 h. After natural cooling, centrifugal separation is carried out, and the sample is washed 3 times with ethanol to obtain a black solid material, which is transferred to a ball mill and ball-milled for 5 h to obtain a modified nanocomposite material.

[0018] Preferably: In step S3, 53 g of the modified acrylic resin emulsion prepared in step S1, 0.5 g of BYK-024 defoamer, 0.6 g of BYK-348 leveling agent, and 0.2 g of dimercaptoethanol are added to a stirring kettle and mixed evenly. Then, 8 g of titanium dioxide, 3 g of color pigment, and 2.5 g of the modified nanocomposite prepared in step S2 are added, and stirred at 2000 rpm / min for 32 min; the solid content of the material is adjusted to 15% with deionized water; then the rotation speed is adjusted to 500 rpm / min, and mixed at 55 °C for 15 min; then the temperature is lowered to 25 °C, 13 g of toluene diisocyanate and 3 g of methyl ethyl ketoxime are slowly added, and stirred for 28 min. Then, the pH is adjusted to 5.5 with glacial acetic acid, and filtered through a 200-mesh sieve to obtain the electrophoretic bath solution.

[0019] Preferably: In step S3, 68 g of the modified acrylic resin emulsion prepared in step S1, 0.7 g of BYK-024 defoamer, 0.8 g of BYK-348 leveling agent, and 0.4 g of dimercaptoethanol are added to a stirring kettle and mixed evenly. Then, 11 g of titanium dioxide, 5 g of color pigment, and 3.6 g of the modified nanocomposite prepared in step S2 are added, and stirred at 2000 rpm / min for 41 min; the solid content of the material is adjusted to 20% with deionized water; then the rotation speed is adjusted to 500 rpm / min, and mixed at 63 °C for 20 min; then the temperature is lowered to 30 °C, 18 g of toluene diisocyanate and 5 g of methyl ethyl ketoxime are slowly added, and stirred for 36 min. Then, the pH is adjusted to 6.0 with glacial acetic acid, and filtered through a 200-mesh sieve to obtain the electrophoretic bath solution.

[0020] Preferably: In step S4, the surface of the metal to be coated is sprayed with an alkaline degreaser for 3 min, then washed with two reverse running waters at room temperature for 1 min, impregnated in a zinc-based phosphating solution for 3 min, and the temperature is controlled at 40 °C to form a gray phosphating film; the temperature is controlled at 28 °C, the deposition voltage is 150 V, the electrodeposition time is 2 min, the distance between the two electrodes is 10 cm. During the deposition process, the charged resin particles migrate to the cathode in the electric field and form a wet film with a thickness of 15 μm through electrodialysis dehydration. Then, it is rinsed with an ultrafiltration water cycle to recover the poorly adhered coating; in the curing stage, it is pre-baked at 80 °C for 10 min first, and then heated to 115 °C for 20 min to complete the curing.

[0021] Preferably: In step S4, the metal surface to be coated is sprayed with an alkaline degreaser for 5 minutes, then washed twice with reverse running water at room temperature for 2 minutes, impregnated in a zinc-based phosphating solution for 5 minutes at a temperature controlled at 50 °C to form a gray phosphating film; the temperature is controlled at 30 °C, the deposition voltage is 250 V, the electrodeposition time is 3 minutes, the distance between the two electrodes is 20 cm, and during the deposition process, the charged resin particles migrate towards the cathode in the electric field and form a wet film with a thickness of 25 μm through electrodialysis dehydration. Then, ultrafiltration water circulation is used for rinsing to recover the poorly adhered coating; in the curing stage, first pre-bake at 80 °C for 10 minutes, and then raise the temperature to 120 °C for 20 minutes to complete the curing.

[0022] Preferably: The zinc-based phosphating solution in step S4 is composed of zinc dihydrogen phosphate, zinc nitrate, sodium nitrite, sodium chlorate, tartaric acid, citric acid, a surfactant, sodium molybdate, and phosphoric acid in a certain proportion.

[0023] Preferably: The zinc-based phosphating solution in step S4 is composed of 3.2 g of zinc dihydrogen phosphate, 1.5 g of zinc nitrate, 0.85 g of sodium nitrite, 0.21 g of sodium chlorate, 0.75 g of tartaric acid, 0.5 g of citric acid, 0.62 g of sodium dodecyl sulfate, 0.3 g of sodium molybdate, 1.5 g of phosphoric acid, and 50 mL of deionized water.

[0024] Advantages of the present invention:

[0025] 1. The drying temperature of the low-temperature curing acrylic cathodic electrophoretic coating prepared by the present invention is low, saving fuel costs.

[0026] 2. When the low-temperature curing acrylic cathodic electrophoretic coating prepared by the present invention is dried, it does not produce smoke and soot, reduces the pollution of the electrophoretic drying furnace, reduces the exhaust gas emissions, and is beneficial to environmental protection.

[0027] 3. When the low-temperature curing acrylic cathodic electrophoretic coating prepared by the present invention is used to coat workpieces such as imitation gold and silver plating, no bubble points will be generated, and the color change of the electroplated layer due to baking is reduced, thereby improving the film quality.

[0028] 4. The coating of the low-temperature curing acrylic cathodic electrophoretic coating prepared by the present invention has excellent corrosion resistance, good covering ability, high degree of automation of the coating equipment, and high coating utilization rate.

[0029] 5. The operation of the present invention is simple, the production cost is low, and it is conducive to mass production.

[0030] 6. The coating prepared by the present invention is smooth and has high gloss, and can be formulated into transparent or high-covering colors.

[0031] 7. The salt spray resistance of the acrylic coating prepared by the present invention is greatly improved, the baking curing temperature is reduced, and the energy consumption is reduced. Brief Description of the Drawings

[0032] Figure 1 、Scanning electron microscope image of the modified nanocomposite prepared in Example 1 of the present invention.

[0033] Figure 2 、Scanning electron microscope image of the modified nanocomposite prepared in Comparative Example 1 of the present invention.

[0034] Figure 3 、Scanning electron microscope image of the modified nanocomposite prepared in Comparative Example 2 of the present invention. Detailed Description of the Invention

[0035] The following specific examples illustrate the embodiments of the present invention. Those skilled in the art can easily understand the other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, without conflict, the following examples and the features in the examples can be combined with each other.

[0036] Example 1

[0037] S1. Preparation of modified acrylic resin: After mixing 3.8 g of methyl methacrylate, 1.8 g of butyl acrylate, 1.1 g of styrene and 1.6 g of dimethylaminoethyl methacrylate evenly, continue to add 25.8 g of propylene glycol monomethyl ether and 18.3 g of butanol, and stir continuously for 6 h. Then add 0.8 g of benzoyl peroxide and 0.3 g of sodium dodecyl sulfate, and continue to stir for 3 h. Then add the above mixture into a four-necked flask, pass N2 gas for 35 min, and then install it into a microwave reactor. Set the program to raise the temperature to 75 °C within 1 min, react for 90 min, set the microwave power to 600 W, cool to 40 °C after the reaction, and set aside; Take out the four-necked flask from the microwave reactor, and then slowly add 1.5 g of glacial acetic acid, and stir continuously for 45 min; Then transfer the material into a high-speed disperser, and drop deionized water under high-speed shearing at 2000 rpm to form a cationic aqueous dispersion with a solid content of 35%; Finally, adjust the pH to 5.5, filter to remove impurities, and the modified acrylic resin emulsion can be obtained. The purpose of mixing methyl methacrylate, butyl acrylate and styrene in proportion in this step is to adjust the hardness and flexibility of the material. The addition of dimethylaminoethyl methacrylate is mainly used to introduce cationic groups. The purpose of benzoyl peroxide is mainly used as an initiator. The purpose of glacial acetic acid is to act as a neutralizing agent to adjust the pH value of the solution, playing the role of amine neutralization and dispersion. This process reacts an acid with an amine group (-NH2) to generate a cationic group ( ), making the resin have a positive charge. The original hydrophobic resin is transformed into a water-dispersible cationic polymer to ensure that positively charged particles can stably migrate to the cathode workpiece during electrophoretic coating. In this step, the strong shear force generated by high-speed rotation "tears" the resin into nano-scale fine particles, and at the same time, water molecules wrap around the particle surface to form a stable emulsion, with oil droplets evenly dispersed in water. After the resin is neutralized by amine, it has a positive charge on the surface ( ), and the positively charged particles cannot agglomerate due to electrostatic repulsion during water dispersion. This process ensures the electrophoretic efficiency of the coating and the uniformity and weather resistance of the final coating.

[0038] S2. Preparation of modified nano-composite materials: Put 1.3 g of SiO2, 0.8 g of CeO2, and 0.9 g of ZrO2 into a ball mill, ball mill at a rotation speed of 300 r / min for 4 h, take out the ball-milled and mixed powder and add it to a 100 mL polytetrafluoroethylene liner containing 32 mL of ethanol and 23 mL of N-methylpyrrolidone. Then add 2.1 g of carbon nanotubes and 0.6 g of methacryloxy silane, stir evenly, set the hydrothermal temperature at 150 °C, keep the temperature for 10 h, cool naturally, then centrifuge and separate, wash the sample with ethanol 3 times to obtain a black solid material, transfer it to the ball mill, and obtain the modified nano-composite material after ball milling for 3 h; in this step, the functions of nano-SiO2, CeO2, and ZrO2 are to improve scratch resistance and ultraviolet resistance. The addition of carbon nanotubes not only enhances the conductivity of the composite material and improves the deposition uniformity, but also can improve the mechanical strength of the coating. In this step, the composite material of nano-SiO2, CeO2, and ZrO2 improves the pigment dispersion efficiency and the purity of the coating by virtue of its high hardness and chemical stability. After combining with carbon nanotubes, through nano-sizing and surface modification as functional fillers, it significantly enhances the coating hardness, wear resistance, and weather resistance.

[0039] S3. Preparation of electrophoretic bath solution: Add 53 g of the modified acrylic resin emulsion prepared in step S1, 0.5 g of BYK-024 defoamer, 0.6 g of BYK-348 leveling agent, and 0.2 g of dimercaptoethanol into a stirring kettle, mix them evenly, then add 8 g of titanium dioxide, 3 g of color pigment, and 2.5 g of the modified nanocomposite prepared in step S2, and stir at 2000 rpm for 32 min; adjust the solid content of the material to 15% with deionized water; then adjust the rotation speed to 500 rpm / min and mix at 55 °C for 15 min; then lower the temperature to 25 °C, slowly add 13 g of toluene diisocyanate and 3 g of methyl ethyl ketoxime, stir for 28 min, then adjust the pH to 5.5 with glacial acetic acid, and pass through a 200-mesh filter to obtain the electrophoretic bath solution; in this step, the modified acrylic resin emulsion serves as the film-forming matrix and provides positive charges; the addition of titanium dioxide is mainly for covering and weather resistance; the function of methyl ethyl ketoxime is mainly to prevent the paint from drying and crusting on the inner surface of the container, ensure good fluidity of the paint when the can is opened, and also have the function of inhibiting oxidation; the selection of BYK-348 can significantly reduce the surface tension of the paint, help improve the material wettability and prevent cratering; it can promote the leveling of the coating during the drying process, reduce the orange peel phenomenon, and improve the smoothness and flatness of the coating.

[0040] S4. First, spray the metal surface to be coated with an alkaline degreaser for 3 min, then wash it twice with reverse running water at room temperature for 1 min, immerse it in a zinc-based phosphating solution for 3 min, control the temperature at 40 °C to form a gray phosphating film; control the temperature at 28 °C, the deposition voltage at 150 V, the electrodeposition time at 2 min, the electrode distance at 10 cm. During the deposition process, the charged resin particles migrate to the cathode in the electric field and form a wet film with a thickness of 15 μm through electrodialysis dehydration. Then, ultrafiltration water is circulated for rinsing to recover the poorly adhered paint; in the curing stage, first pre-bake at 80 °C for 10 min, then raise the temperature to 115 °C for 20 min to complete the curing.

[0041] Comparative Example 1: Except that in step S2, it is not ball-milled and methylacryloxy silane is not added, the other steps are the same as those in Example 1.

[0042] Comparative Example 2: Except that in step S2, N-methylpyrrolidone is not added, the other steps are the same as those in Example 1.

[0043] Figure 1 The scanning electron microscope image of the modified nanocomposite prepared in Example 1 of the present invention. From Figure 1It can be seen that SiO2, CeO2, and ZrO2 in the modified nanocomposite prepared by the present invention are uniformly attached to the surface of carbon nanotubes, which can effectively improve the scratch resistance and ultraviolet resistance of the coating, enhance the conductivity of the composite material, improve the deposition uniformity, and also improve the mechanical strength of the coating. The nanocomposite material obtained by nanosizing and surface modification as a functional filler significantly enhances the hardness, wear resistance, and weather resistance of the coating. Figure 2 , Scanning electron microscope image of the modified nanocomposite prepared in Comparative Example 1 of the present invention. And Figure 1 By comparison, it can be seen that in the modified nanocomposite without ball milling and without adding methacryloxy silane, the particles are obvious, and SiO2, CeO2, and ZrO2 are not well attached to the surface of carbon nanotubes. Figure 3 , Scanning electron microscope image of the modified nanocomposite prepared in Comparative Example 2 of the present invention. The attachment effect of SiO2, CeO2, and ZrO2 is also not as good as that of the modified nanocomposite prepared in Example 1. It can be seen that ball milling is an essential experimental condition, and methacryloxy silane and N-methylpyrrolidone have a synergistic effect in the solution system, assisting the uniformly and firmly attachment of the milled nanoparticles to the surface of carbon nanotubes, thereby improving the scratch resistance and mechanical strength of the material.

[0044] Example 2

[0045] S1. Preparation of modified acrylic resin: After mixing 5.2 g of methyl methacrylate, 2.3 g of butyl acrylate, 1.9 g of styrene and 1.8 g of dimethylaminoethyl methacrylate evenly, 32.5 g of propylene glycol methyl ether and 26.1 g of butanol were continuously added, and stirred for 11 h. Then, 1.3 g of benzoyl peroxide and 0.5 g of sodium dodecyl sulfate were added, and stirred for another 5 h. Then, the above mixture was added into a four-necked flask, purged with Ar gas for 48 min, and then loaded into a microwave reactor. The program was set to raise the temperature to 85 °C within 2 min, and react for 180 min. The microwave power was set to 900 W. After the reaction, it was cooled to 55 °C for standby; The four-necked flask was taken out of the microwave reactor, and then 2.1 g of glacial acetic acid was slowly added, and stirred continuously for 60 min; Then the material was transferred into a high-speed disperser, and deionized water was added dropwise under high-speed shearing at 3000 rpm to form a cationic aqueous dispersion with a solid content of 42%; Finally, the pH was adjusted to 6.0, and impurities were removed by filtration to obtain a modified acrylic resin emulsion. In this step, the purpose of mixing methyl methacrylate, butyl acrylate and styrene in proportion is to adjust the hardness and flexibility of the material. The addition of dimethylaminoethyl methacrylate is mainly used to introduce cationic groups. The purpose of benzoyl peroxide is mainly used as an initiator. The purpose of glacial acetic acid is to act as a neutralizing agent to adjust the pH value of the solution, playing the role of amine neutralization and dispersion. In this process, through the reaction of acid and amine groups (-NH2), cationic groups ( ) are generated, making the resin have a positive charge. The originally hydrophobic resin is transformed into a water-dispersible cationic polymer, ensuring that positively charged particles can stably migrate to the cathode workpiece during electrophoretic coating. In this step, through the strong shearing force generated by high rotation speed, the resin is "torn" into nano-scale fine particles, and at the same time, water molecules are wrapped on the surface of the particles to form a stable emulsion. The oil droplets are evenly dispersed in water. After the resin is neutralized by amine, the surface is positively charged ( ). During water dispersion, the positively charged particles cannot agglomerate due to electrostatic repulsion. This process ensures the electrophoretic efficiency of the coating and the uniformity and weather resistance of the final coating.

[0046] S2. Preparation of modified nanocomposites: Put 1.5 g of SiO2, 1.1 g of CeO2 and 1.4 g of ZrO2 into a ball mill and ball mill for 6 h at a rotation speed of 600 r / min. Take out the ball-milled and mixed powder and add it to a 100 mL polytetrafluoroethylene liner containing 32 mL of ethanol and 23 mL of N-methylpyrrolidone. Then add 3.3 g of carbon nanotubes and 0.8 g of methacryloxy silane. After stirring evenly, set the hydrothermal temperature at 180 °C and the heat preservation time at 12 h. After natural cooling, centrifuge and wash the sample 3 times with ethanol to obtain a black solid material. Transfer it to a ball mill and ball mill for 5 h to obtain the modified nanocomposites. In this step, the functions of nano-SiO2, CeO2 and ZrO2 are to improve scratch resistance and UV resistance. The addition of carbon nanotubes not only enhances the conductivity of the composite material and improves the deposition uniformity, but also can improve the mechanical strength of the coating. In this step, the composite of nano-SiO2, CeO2 and ZrO2 improves the pigment dispersion efficiency and the purity of the coating by virtue of its high hardness and chemical stability. After combining with carbon nanotubes, as a functional filler through nanosizing and surface modification, it significantly enhances the hardness, wear resistance and weather resistance of the coating.

[0047] S3. Preparation of electrophoretic bath solution: Add 68 g of the modified acrylic resin emulsion prepared in step S1, 0.7 g of BYK-024 defoamer, 0.8 g of BYK-348 leveling agent and 0.4 g of dimercaptoethanol into a stirring kettle and mix evenly. Then add 11 g of titanium dioxide, 5 g of color pigment and 3.6 g of the modified nanocomposites prepared in step S2, and stir at 2000 rpm / min for 41 min. Adjust the solid content of the material to 20% with deionized water. Then adjust the rotation speed to 500 rpm / min and mix at 63 °C for 20 min. Then lower the temperature to 30 °C, slowly add 18 g of toluene diisocyanate and 5 g of methyl ethyl ketoxime, stir for 36 min, and then adjust the pH to 6.0 with glacial acetic acid and pass through a 200-mesh filter to obtain the electrophoretic bath solution. In this step, the modified acrylic resin emulsion serves as a film-forming matrix to provide positive charges. The addition of titanium dioxide is mainly for covering and weather resistance. The function of methyl ethyl ketoxime is mainly to prevent the coating from drying and crusting on the inner surface of the container, ensure good fluidity of the coating when the can is opened, and also has the function of inhibiting oxidation. The selection of BYK-348 can significantly reduce the surface tension of the coating, help improve the material wettability and prevent shrinkage pores. It can promote the leveling of the coating during the drying process, reduce orange peel phenomenon, and improve the smoothness and flatness of the coating.

[0048] S4. First, spray the metal surface to be coated with an alkaline degreaser for 5 min, then wash it twice with reverse running water at room temperature for 2 min, immerse it in a zinc-based phosphating solution for 5 min at a temperature controlled at 50 °C to form a gray phosphating film; control the temperature at 30 °C, the deposition voltage at 250 V, the electrodeposition time at 3 min, and the distance between the two electrodes at 20 cm. During the deposition process, charged resin particles migrate towards the cathode in the electric field and form a wet film with a thickness of 25 μm through electrodialysis dehydration. Then, rinse it with an ultrafiltration water cycle to recover the poorly adhered coating; in the curing stage, first pre-bake it at 80 °C for 10 min, and then raise the temperature to 120 °C for 20 min to complete the curing.

[0049] Comparative Example 3: Except for not adding SiO2 in step S2, the other steps are the same as those in Example 2.

[0050] Comparative Example 4: Except for not adding CeO2 in step S2, the other steps are the same as those in Example 2.

[0051] Comparative Example 5: Except for not adding ZrO2 in step S2, the other steps are the same as those in Example 2.

[0052] Comparative Example 6: Except for not adding the modified nanocomposite material in step S4, the other steps are the same as those in Example 2.

[0053] Table 1 shows the number of MEK and ethanol wipes of the coatings prepared in Example 2 and Comparative Examples 3 - 6 of the present invention according to the standard of GB / T 23989. The results show that the number of wipes of the coating prepared in Example 2 is much higher than that of the coatings prepared in Comparative Examples 3 - 6. Comparing Example 2 and Comparative Example 6, it can be seen that the modified nanocomposite material prepared in the present invention plays a crucial role in the coating. The addition of the modified nanocomposite material can greatly improve the number of wipes of the coating. Comparing Example 2 with Comparative Examples 3 - 4, it can be seen that there is a strong interaction between SiO2, CeO2, and ZrO2 in the modified nanocomposite material, and none of them can be missing for the contribution to the wipe resistance of the material. It is speculated that the reason is that there is a certain synergistic effect during the coupling of nanoscale SiO2, CeO2, and ZrO2 with carbon nanotubes, which greatly enhances the effect of the modified nanocomposite material. Table 1 Test results of the number of MEK and ethanol wipes of the prepared coatings

[0054]

[0055] Example 3

[0056] S1. Preparation of modified acrylic resin: After mixing 4.1 g of methyl methacrylate, 2.1 g of butyl acrylate, 1.6 g of styrene and 1.7 g of dimethylaminoethyl methacrylate evenly, 29.8 g of propylene glycol methyl ether and 19.1 g of butanol were continuously added, and stirring was continued for 8 h. 0.9 g of benzoyl peroxide and 0.4 g of sodium dodecyl sulfate were added, and stirring was continued for 4 h. Then the above mixed solution was added to a four-necked flask, and N2 gas was passed through for 40 min. Then it was loaded into a microwave reactor, the program was set, the temperature was raised to 79 °C within 1.5 min, and the reaction was carried out for 120 min. The microwave power was set at 700 W. After the reaction, it was cooled to 44 °C and reserved; The four-necked flask was taken out of the microwave reactor, and then 1.9 g of glacial acetic acid was slowly added, and stirring was continued for 55 min; Then the material was transferred to a high-speed disperser, and deionized water was added dropwise under high-speed shearing at 2000 rpm to form a cationic aqueous dispersion with a solid content of 38%; Finally, the pH was adjusted to 5.8, and impurities were removed by filtration to obtain a modified acrylic resin emulsion. In this step, the purpose of mixing methyl methacrylate, butyl acrylate and styrene in proportion is to adjust the hardness and flexibility of the material. The addition of dimethylaminoethyl methacrylate is mainly used to introduce cationic groups. The purpose of benzoyl peroxide is mainly used as an initiator. The purpose of glacial acetic acid is to act as a neutralizing agent to adjust the pH value of the solution, playing the role of amine neutralization and dispersion. In this process, through the reaction of acid and amine groups (-NH2), cationic groups ( are generated, making the resin have a positive charge. The originally hydrophobic resin is transformed into a water-dispersible cationic polymer, ensuring that positively charged particles can stably migrate to the cathode workpiece during electrophoretic coating. In this step, through the strong shearing force generated by high rotation speed, the resin is "torn" into nano-scale fine particles, and at the same time, water molecules are wrapped on the surface of the particles to form a stable emulsion. The oil droplets are evenly dispersed in water. After the resin is neutralized by amine, the surface is positively charged ( ). During water dispersion, the positively charged particles cannot agglomerate due to electrostatic repulsion. This process ensures the electrophoretic efficiency of the coating and the uniformity and weather resistance of the final coating.

[0057] S2. Preparation of modified nanocomposite: Put 1.4 g of SiO2, 0.9 g of CeO2 and 1.3 g of ZrO2 into a ball mill and ball mill at a rotation speed of 300 r / min for 5 h. Take out the ball-milled and mixed powder and add it to a 100 mL polytetrafluoroethylene liner containing 32 mL of ethanol and 23 mL of N-methylpyrrolidone. Then add 2.4 g of carbon nanotubes and 0.7 g of methacryloxy silane. After stirring evenly, set the hydrothermal temperature at 170 °C and the heat preservation time at 11 h. After natural cooling, centrifuge and wash the sample 3 times with ethanol to obtain a black solid material. Transfer it to a ball mill and ball mill for 4 h to obtain the modified nanocomposite. In this step, the functions of nano-SiO2, CeO2 and ZrO2 are to improve scratch resistance and ultraviolet resistance. The addition of carbon nanotubes not only enhances the conductivity of the composite material and improves the deposition uniformity, but also can improve the mechanical strength of the coating. In this step, the composite of nano-SiO2, CeO2 and ZrO2 improves the pigment dispersion efficiency and the purity of the coating by virtue of its high hardness and chemical stability. After combining with carbon nanotubes, as a functional filler through nanosizing and surface modification, it significantly enhances the hardness, wear resistance and weather resistance of the coating.

[0058] S3. Preparation of electrophoretic bath solution: Add 58 g of the modified acrylic resin emulsion prepared in step S1, 0.8 g of BYK-024 defoamer, 0.7 g of BYK-348 leveling agent and 0.3 g of dimercaptoethanol into a stirring kettle and mix evenly. Then add 9 g of titanium dioxide, 4 g of colored pigment and 3.2 g of the modified nanocomposite prepared in step S2 and stir at 2000 rpm / min for 38 min. Adjust the solid content of the material to 17% with deionized water. Then adjust the rotation speed to 500 rpm / min and mix at 59 °C for 17 min. Then lower the temperature to 25 °C, slowly add 17 g of toluene diisocyanate and 4 g of methyl ethyl ketoxime, stir for 32 min, and then adjust the pH to 5.8 with glacial acetic acid and pass through a 200-mesh filter to obtain the electrophoretic bath solution. In this step, the modified acrylic resin emulsion serves as a film-forming matrix to provide positive charges. The addition of titanium dioxide is mainly for covering and weather resistance. The function of methyl ethyl ketoxime is mainly to prevent the coating from drying and crusting on the inner surface of the container, ensure that the coating maintains good fluidity when the can is opened, and also has the function of inhibiting oxidation. The selection of BYK-348 can significantly reduce the surface tension of the coating, help improve the material wettability and prevent cratering. It can promote the leveling of the coating during the drying process, reduce orange peel phenomenon and improve the smoothness and flatness of the coating.

[0059] S4. First, spray the metal surface to be coated with an alkaline degreaser for 4 min, then wash it twice with reverse running water at room temperature for 2 min, dip it in a zinc-based phosphating solution for 4 min with the temperature controlled at 47 °C to form a gray phosphating film; control the temperature at 30 °C, the deposition voltage at 180 V, the electrodeposition time at 3 min, and the electrode spacing at 18 cm. During the deposition process, the charged resin particles migrate towards the cathode in the electric field and form a wet film through electrodialysis dehydration with a thickness of 19 μm. Then, rinse it with an ultrafiltration water cycle to recover the poorly adhered coating; in the curing stage, first pre-bake it at 80 °C for 10 min, and then raise the temperature to 115 °C for 20 min to complete the curing.

[0060] Comparative Example 7: Except for not adding sodium dodecyl sulfate in step S1, the other steps are the same as those in Example 3.

[0061] Comparative Example 8: Except for not adding butyl acrylate in step S1, the other steps are the same as those in Example 3.

[0062] Comparative Example 9: Except for not adding 2-mercaptoethanol in step S3, the other steps are the same as those in Example 3.

[0063] The appearance and pencil hardness of the coatings prepared in Example 3 of the present invention and Comparative Examples 7 - 9 were tested. The test results showed that the pencil hardness of the coating prepared in Example 3 was the best. Comparing with Comparative Example 7, it can be seen that the addition of sodium dodecyl sulfate helps to improve the hardness of the final coating. Comparing with Comparative Example 8, it can be seen that butyl acrylate is indispensable in the preparation of the modified acrylic resin and helps to improve the pencil hardness of the coating. In Comparative Example 9, 2-mercaptoethanol was not added in step S3, and the pencil hardness was very poor. It is speculated that the reason is that mercaptoethanol plays a role in cross-linking and consolidating the coating in the electrophoresis bath solution. Table 2 Appearance and pencil hardness test of the prepared coatings

[0064]

[0065] Example 4

[0066] S1. Preparation of modified acrylic resin: After mixing 4.1 g of methyl methacrylate, 2.1 g of butyl acrylate, 1.6 g of styrene and 1.7 g of dimethylaminoethyl methacrylate evenly, continue to add 29.6 g of propylene glycol methyl ether and 25.4 g of butanol, and continuously stir for 9 h. Then add 1.2 g of benzoyl peroxide and 0.4 g of sodium dodecyl sulfate, and continue to stir for 4 h. Then add the above mixed solution into a four-necked flask, purge with Ar gas for 42 min, and then place it in a microwave reactor. Set the program to raise the temperature to 81 °C within 2 min, react for 160 min, set the microwave power to 700 W, cool to 50 °C after the reaction, and set aside; Take out the four-necked flask from the microwave reactor, and then slowly add 1.9 g of glacial acetic acid, and continuously stir for 51 min; Then transfer the material to a high-speed disperser, and dropwise add deionized water under high-speed shearing at 3000 rpm to form a cationic aqueous dispersion with a solid content of 39%; Finally, adjust the pH to 6.0, filter to remove impurities, and the modified acrylic resin emulsion can be obtained. In this step, the purpose of mixing methyl methacrylate, butyl acrylate and styrene in proportion is to adjust the hardness and flexibility of the material. The addition of dimethylaminoethyl methacrylate is mainly used to introduce cationic groups. The purpose of benzoyl peroxide is mainly used as an initiator. The purpose of glacial acetic acid is to act as a neutralizing agent to adjust the pH value of the solution, playing the role of amine neutralization and dispersion. In this process, through the reaction of acid and amine groups (-NH2), cationic groups ( ) are generated, making the resin carry a positive charge. The originally hydrophobic resin is transformed into a water-dispersible cationic polymer, ensuring that positively charged particles can stably migrate to the cathode workpiece during electrophoretic coating. In this step, through the strong shearing force generated by high rotation speed, the resin is "torn" into nano-scale fine particles, and at the same time, water molecules are wrapped on the surface of the particles to form a stable emulsion. The oil droplets are evenly dispersed in water. After the resin is neutralized by amine, the surface is positively charged ( ). During water dispersion, the positively charged particles cannot agglomerate due to electrostatic repulsion. This process ensures the electrophoretic efficiency of the coating and the uniformity and weather resistance of the final coating.

[0067] S2. Preparation of modified nanocomposites: Put 1.4 g of SiO2, 1.0 g of CeO2 and 1.3 g of ZrO2 into a ball mill and ball mill at a rotation speed of 600 r / min for 5 h. Take out the ball-milled and mixed powder and add it to a 100 mL polytetrafluoroethylene liner containing 32 mL of ethanol and 23 mL of N-methylpyrrolidone. Then add 2.9 g of carbon nanotubes and 0.7 g of methacryloxy silane. After stirring evenly, set the hydrothermal temperature at 170 °C and the heat preservation time at 11 h. After natural cooling, centrifuge and wash the sample with ethanol 3 times to obtain a black solid material. Transfer it to a ball mill and ball mill for 4 h to obtain the modified nanocomposites. In this step, the functions of nano-SiO2, CeO2 and ZrO2 are to improve scratch resistance and UV resistance. The addition of carbon nanotubes not only enhances the conductivity of the composite material and improves the deposition uniformity, but also can improve the mechanical strength of the coating. In this step, the composite of nano-SiO2, CeO2 and ZrO2 improves the pigment dispersion efficiency and the purity of the coating by virtue of its high hardness and chemical stability. After combining with carbon nanotubes, as a functional filler through nanosizing and surface modification, it significantly enhances the hardness, wear resistance and weather resistance of the coating.

[0068] S3. Preparation of electrophoretic bath solution: Add 53 - 68 g of the modified acrylic resin emulsion prepared in step S1, 0.6 g of BYK-024 defoamer, 0.7 g of BYK-348 leveling agent and 0.3 g of dimercaptoethanol into a stirring kettle and mix evenly. Then add 9 g of titanium dioxide, 4 g of color pigment and 2.8 g of the modified nanocomposites prepared in step S2 and stir at 2000 rpm / min for 37 min. Adjust the solid content of the material to 19% with deionized water. Then adjust the rotation speed to 500 rpm / min and mix at 57 °C for 18 min. Then lower the temperature to 28 °C and slowly add 17 g of toluene diisocyanate and 4 g of methyl ethyl ketoxime, stir for 32 min, and then adjust the pH to 6.0 with glacial acetic acid and pass through a 200-mesh filter to obtain the electrophoretic bath solution. In this step, the modified acrylic resin emulsion serves as a film-forming matrix to provide positive charges. The addition of titanium dioxide is mainly for covering and weather resistance. The function of methyl ethyl ketoxime is mainly to prevent the coating from drying and crusting on the inner surface of the container, ensure good fluidity of the coating when the can is opened, and also has the function of inhibiting oxidation. The selection of BYK-348 can significantly reduce the surface tension of the coating, help improve the material wettability and prevent cratering. It can promote the leveling of the coating during the drying process, reduce orange peel phenomenon and improve the smoothness and flatness of the coating.

[0069] S4. First, spray the metal surface to be coated with an alkaline degreaser for 4 min, then wash it twice with reverse running water at room temperature for 2 min, immerse it in a zinc-based phosphating solution for 3 min at a temperature controlled at 47 °C to form a gray phosphating film; control the temperature at 29 °C, the deposition voltage at 210 V, the electrodeposition time at 3 min, and the electrode spacing at 17 cm. During the deposition process, the charged resin particles migrate towards the cathode in the electric field and form a wet film through electrodialysis dehydration with a thickness of 19 μm. Then, rinse it with an ultrafiltration water cycle to recover the poorly adhered coating; in the curing stage, first pre-bake it at 80 °C for 10 min, and then raise the temperature to 116 °C for 20 min to complete the curing.

[0070] The above-described embodiments only represent the specific implementation manners of the present invention, and the description is relatively specific and detailed, but it should not be construed as a limitation on the protection scope of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A low-temperature curable acrylic cathodic electrophoretic coating, characterized in that: The specific preparation method is as follows: S1. Prepare a modified acrylic resin emulsion: Mix 3.8 - 5.2 g of methyl methacrylate, 1.8 - 2.3 g of butyl acrylate, 1.1 - 1.9 g of styrene, and 1.6 - 1.8 g of dimethylaminoethyl methacrylate evenly. Then, continue to add 25.8 - 32.5 g of propylene glycol methyl ether and 18.3 - 26.1 g of butanol, and continuously stir for 6 - 11 h. Add 0.8 - 1.3 g of benzoyl peroxide and 0.3 - 0.5 g of sodium dodecyl sulfate, and continue to stir for 3 - 5 h. Then, add the above mixture into a four-necked flask, introduce N2 or Ar gas for 35 - 48 min, and then transfer it to a microwave reactor. Set the program to raise the temperature to 75 - 85 °C within 1 - 2 min, react for 90 - 180 min, and set the microwave power to 600 - 900 W. After the reaction, cool it to 40 - 55 °C for standby; Take out the four-necked flask from the microwave reactor, and then slowly add 1.5 - 2.1 g of glacial acetic acid, and continuously stir for 45 - 60 min; Then, transfer the material into a high-speed disperser, and dropwise add deionized water under high-speed shearing at 2000 - 3000 rpm to form a cationic aqueous dispersion with a solid content of 35 - 42%; Finally, adjust the pH to 5.5 - 6.0, filter to remove impurities, and the modified acrylic resin emulsion can be obtained; S2. Prepare a modified nanocomposite: Put 1.3 - 1.5 g of SiO2, 0.8 - 1.1 g of CeO2, and 0.9 - 1.4 g of ZrO2 into a ball mill, and ball mill at a rotation speed of 300 - 600 r / min for 4 - 6 h. Take out the ball-milled and mixed powder and add it to a 100 mL polytetrafluoroethylene liner containing 32 mL of ethanol and 23 mL of N-methylpyrrolidone. Then, add 2.1 - 3.3 g of carbon nanotubes and 0.6 - 0.8 g of methacryloxysilane, stir evenly, set the hydrothermal temperature to 150 - 180 °C, and keep the temperature for 10 - 12 h. After natural cooling, centrifuge and wash the sample with ethanol 3 times to obtain a black solid material. Transfer it to a ball mill and ball mill for 3 - 5 h to obtain the modified nanocomposite; S3. Prepare the electrophoretic bath solution: Add 53 - 68 g of the modified acrylic resin emulsion prepared in step S1, 0.5 - 0.7 g of BYK-024 defoamer, 0.6 - 0.8 g of BYK-348 leveling agent, and 0.2 - 0.4 g of dimercaptoethanol into a stirring kettle and mix evenly. Then add 8 - 11 g of titanium dioxide, 3 - 5 g of colored pigment, and 2.5 - 3.6 g of the modified nanocomposite prepared in step S2, and stir at 2000 rpm for 32 - 41 min; Adjust the solid content of the material to 15 - 20% with deionized water; Then adjust the rotation speed to 500 rpm / min and mix at 55 - 63 °C for 15 - 20 min; Then lower the temperature to 25 - 30 °C, slowly add 13 - 18 g of toluene diisocyanate and 3 - 5 g of methyl ethyl ketoxime, stir for 28 - 36 min, and then adjust the pH to 5.5 - 6.0 with glacial acetic acid, and pass through a 200-mesh filter to obtain the electrophoretic bath solution; S4. Clean and treat the metal surface to be coated, removing impurities such as oil stains and oxide layers; Then use the pretreated workpiece as the cathode, put it into the electrophoretic bath prepared in step S3, deposit the coating under the action of a direct current electric field, and remove the un-deposited coating particles; Finally, completely cure the coating by heating to form the final protective film.

2. The low-temperature curable acrylic cathodic electrophoretic coating according to claim 1, wherein: In step S1, after mixing 3.8 g of methyl methacrylate, 1.8 g of butyl acrylate, 1.1 g of styrene, and 1.6 g of dimethylaminoethyl methacrylate evenly, continue to add 25.8 g of propylene glycol methyl ether and 18.3 g of butanol, and continuously stir for 6 h. Add 0.8 g of benzoyl peroxide and 0.3 g of sodium dodecyl sulfate, and continue to stir for 3 h. Then add the above mixture into a four-necked flask, pass N2 gas for 35 min, and then install it into a microwave reactor. Set the program to raise the temperature to 75 °C within 1 min and react for 90 min. Set the microwave power to 600 W. After the reaction, cool to 40 °C and set aside; Take out the four-necked flask from the microwave reactor, and then slowly add 1.5 g of glacial acetic acid and continuously stir for 45 min; Then transfer the material into a high-speed disperser and dropwise add deionized water under high-speed shearing at 2000 rpm to form a cationic aqueous dispersion with a solid content of 35%; Finally, adjust the pH to 5.5 and filter to remove impurities to obtain the modified acrylic resin emulsion.

3. A low-temperature curable acrylic cathodic electrophoretic coating according to claim 1 or 2, characterized in that: In step S1, 5.2 g of methyl methacrylate, 2.3 g of butyl acrylate, 1.9 g of styrene and 1.8 g of dimethylaminoethyl methacrylate are mixed evenly, and then 32.5 g of propylene glycol methyl ether and 26.1 g of butanol are added continuously. Stir for 11 h, add 1.3 g of benzoyl peroxide and 0.5 g of sodium dodecyl sulfate, and continue to stir for 5 h. Then the above mixture is added into a four-necked flask, and Ar gas is passed through for 48 min. Then it is loaded into a microwave reactor, the program is set, the temperature is raised to 85 °C within 2 min, and the reaction is carried out for 180 min. The microwave power is set to 900 W. After the reaction, it is cooled to 55 °C for standby; the four-necked flask is taken out of the microwave reactor, and then 2.1 g of glacial acetic acid is slowly added, and stirred continuously for 60 min; then the material is transferred into a high-speed disperser, and deionized water is added dropwise under high-speed shearing at 3000 rpm to form a cationic aqueous dispersion with a solid content of 42%; finally, the pH is adjusted to 6.0, and impurities are removed by filtration to obtain the modified acrylic resin emulsion.

4. A low-temperature curable acrylic cathodic electrophoretic coating according to claim 1, characterized in that: In step S2, 1.3 g of SiO2, 0.8 g of CeO2 and 0.9 g of ZrO2 are put into a ball mill and ball milled at a rotation speed of 300 r / min for 4 h. The ball-milled and mixed powder is taken out and added into a 100 mL polytetrafluoroethylene inner liner containing 32 mL of ethanol and 23 mL of N-methylpyrrolidone. Then 2.1 g of carbon nanotubes and 0.6 g of methacryloxy silane are added. After stirring evenly, the hydrothermal temperature is set to 150 °C and the heat preservation time is 10 h. After natural cooling, centrifugal separation is carried out, and the sample is washed 3 times with ethanol to obtain a black solid material, which is transferred into a ball mill and ball milled for 3 - 5 h to obtain the modified nanocomposite material.

5. A low-temperature curable acrylic cathodic electrophoretic coating according to claim 1 or 4, characterized in that: In step S2, 1.5 g of SiO2, 1.1 g of CeO2 and 1.4 g of ZrO2 are put into a ball mill and ball milled at a rotation speed of 600 r / min for 6 h. The ball-milled and mixed powder is taken out and added into a 100 mL polytetrafluoroethylene inner liner containing 32 mL of ethanol and 23 mL of N-methylpyrrolidone. Then 3.3 g of carbon nanotubes and 0.8 g of methacryloxy silane are added. After stirring evenly, the hydrothermal temperature is set to 180 °C and the heat preservation time is 12 h. After natural cooling, centrifugal separation is carried out, and the sample is washed 3 times with ethanol to obtain a black solid material, which is transferred into a ball mill and ball milled for 5 h to obtain the modified nanocomposite material.

6. A low-temperature curable acrylic cathodic electrophoretic coating according to claim 1, characterized in that: In step S3, 53 g of the modified acrylic resin emulsion prepared in step S1, 0.5 g of BYK-024 defoamer, 0.6 g of BYK-348 leveling agent, and 0.2 g of dimercaptoethanol were added to a stirring kettle and mixed evenly. Then, 8 g of titanium dioxide, 3 g of color pigment, and 2.5 g of the modified nanocomposite prepared in step S2 were added, and the mixture was stirred at 2000 rpm for 32 min. The solid content of the material was adjusted to 15% with deionized water. Then, the rotation speed was adjusted to 500 rpm, and the mixture was mixed at 55 °C for 15 min. After that, the temperature was lowered to 25 °C, 13 g of toluene diisocyanate and 3 g of methyl ethyl ketoxime were slowly added, and the mixture was stirred for 28 min. Then, the pH was adjusted to 5.5 with glacial acetic acid, and the mixture was passed through a 200-mesh filter to obtain the electrophoretic bath solution.

7. A low-temperature curable acrylic cathodic electrophoretic coating according to claim 1 or 6, characterized in that: In step S3, 68 g of the modified acrylic resin emulsion prepared in step S1, 0.7 g of BYK-024 defoamer, 0.8 g of BYK-348 leveling agent, and 0.4 g of dimercaptoethanol were added to a stirring kettle and mixed evenly. Then, 11 g of titanium dioxide, 5 g of color pigment, and 3.6 g of the modified nanocomposite prepared in step S2 were added, and the mixture was stirred at 2000 rpm for 41 min. The solid content of the material was adjusted to 20% with deionized water. Then, the rotation speed was adjusted to 500 rpm, and the mixture was mixed at 63 °C for 20 min. After that, the temperature was lowered to 30 °C, 18 g of toluene diisocyanate and 5 g of methyl ethyl ketoxime were slowly added, and the mixture was stirred for 36 min. Then, the pH was adjusted to 6.0 with glacial acetic acid, and the mixture was passed through a 200-mesh filter to obtain the electrophoretic bath solution.

8. A low-temperature curable acrylic cathodic electrophoretic coating according to claim 1, characterized in that: In step S4, the surface of the metal to be coated was sprayed with an alkaline degreaser for 3 min, then washed twice with reverse running water at room temperature for 1 min, and immersed in a zinc-based phosphating solution for 3 min at a temperature controlled at 40 °C to form a gray phosphating film. The temperature was controlled at 28 °C, the deposition voltage was 150 V, the electro-deposition time was 2 min, and the distance between the two electrodes was 10 cm. During the deposition process, the charged resin particles migrated towards the cathode in the electric field and formed a wet film with a thickness of 15 μm through electrodialysis dehydration. Then, the ultrafiltration water was circulated for rinsing to recover the poorly adhered coating. In the curing stage, it was first pre-baked at 80 °C for 10 min, and then the temperature was raised to 115 °C for 20 min to complete the curing.

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