Electrodepositable cationic emulsion and preparation method thereof

Abstract

The invention discloses an electrodepositable cationic emulsion which comprises the following components in percentage by mass: 16%-19% of epoxy resin, 4%-6% of polyether resin, 0.02%-0.08% of a catalyst, 10%-12% of a cross-linking agent, 2%-6% of an acrylic copolymer, 2.8%-3.6% of an amination reagent, 2%-4% of an auxiliary agent, 0.2%-2% of a first solvent, 0.1%-0.4% of a neutralizer and the balance of pure water. According to the present invention, the emulsion and the pigment slurry are combined to form the electrophoretic paint working solution, and the electrophoretic paint working solution can significantly reduce the generation of the flow mark during the coating construction process, such that the coating defect caused by the flow mark and the subsequent additional polishing and re-coating process are eliminated, the production quality can be substantially improved, and the production time and the production cost can be substantially reduced.

Description

Technical Field

[0001] This invention belongs to the field of cathodic electrophoretic coating technology, and particularly relates to an electrodeposable cationic emulsion and its preparation method. Background Technology

[0002] Cathodic electrophoretic coatings are favored for their good corrosion resistance, high penetration, and low environmental pollution. Currently, almost all automotive body electrophoretic coatings in China use cathodic electrophoretic coatings.

[0003] However, on electrophoretic coating lines, due to the complex structure of the car body and components, residual electrophoretic coating solution in the gaps and cavities cannot usually be eliminated by normal draining and blowing processes. During baking, the residual solution continuously accumulates, overflowing from the gaps or cavities onto the workpiece surface, forming flow marks and other defects in the paint film. Subsequent processes require additional grinding, polishing, or even recoating, which damages the electrophoretic paint film, reduces its corrosion resistance, increases production labor costs, and poses potential quality risks. Therefore, there is a need for an emulsion for electrophoretic paint that can reduce flow marks during the coating process. Summary of the Invention

[0004] In view of this, the present invention provides an emulsion for electrodepositable electrophoretic paint and a method for preparing the same. The emulsion is combined with pigment paste to form the working solution of the electrophoretic paint. This working solution can significantly reduce the generation of flow marks during the coating process, thereby eliminating coating defects caused by flow marks and the need for additional sanding and recoating afterward. This can greatly improve production quality and reduce production time and costs.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: The first aspect of the present invention provides an electrodeposable cationic emulsion comprising, by weight percentage: 16%-19% epoxy resin, 4%-6% polyether resin, 10-12% crosslinking agent, 2-6.5% acrylic copolymer, 2.8-3.6% amination reagent, 2-4% additives, 0.2-2% first solvent, 0.1-0.4% neutralizing agent, and the balance being pure water.

[0006] Preferably, the acrylic copolymer is obtained by free radical polymerization, and the acrylic copolymer comprises the following components by mass percentage: 28-32% second solvent, 3-15% styrene, 9-12% hydroxypropyl methacrylate, 23-35% isooctyl methacrylate, 19-23% glycidyl methacrylate, and 1-2% initiator.

[0007] Preferably, the second solvent is selected from any one or a combination of several of propylene glycol methyl ether, methyl isobutyl ketone, or ethylene glycol butyl ether; The initiator is selected from any one or a combination of several of benzoyl peroxide, di-tert-butyl peroxide, or di-tert-pentyl peroxide.

[0008] Preferably, the glass transition temperature of the acrylic copolymer is 15-30℃. The glass transition temperature of the acrylic copolymer needs to be strictly controlled. If the glass transition temperature of the acrylic copolymer is too high, the appearance of the subsequent electrophoretic paint film will be poor. If the glass transition temperature of the acrylic copolymer is too low, the effect of reducing flow marks during the coating process of the electrophoretic paint working liquid will not be obvious. Within the required range, the flow marks of the electrophoretic paint working liquid during the coating process are significantly reduced, and the appearance of the subsequent paint film is also more beautiful.

[0009] Preferably, the mass percentage of the acrylic copolymer is 2-4.03% or 4.03-6.45%. The mass percentage of the acrylic copolymer needs to be strictly controlled. If the amount of acrylic copolymer added is less than 2%, the flow mark problem during the coating process cannot be solved. If the amount added exceeds 6.5%, the flow mark problem cannot be significantly reduced.

[0010] Preferably, the polyether resin is a bisphenol A polyoxyethylene ether polymer.

[0011] Preferably, the epoxy resin is obtained by chain extension of a small molecule epoxy resin and bisphenol A at a temperature of 140-160°C under the action of a catalyst. The small molecule epoxy resin is, for example, E-51 epoxy resin. The mass fraction of the small molecule epoxy resin in the electrodepositable cationic emulsion is 16%-19%, the mass fraction of bisphenol A in the electrodepositable cationic emulsion is 4%-6%, and the mass fraction of the catalyst in the electrodepositable cationic emulsion is 0.02-0.08%.

[0012] Preferably, the catalyst is selected from any one or more of N,N-dimethylbenzylamine, tetrabutylammonium bromide, tetraethylammonium bromide, or triphenylphosphine.

[0013] Preferably, the crosslinking agent is a fully closed addition reaction product of a polyisocyanate.

[0014] Preferably, the additive is selected from any one or more of the following: polyether polyol leveling aids, polyacetal film-forming aids, microgel anti-cratering aids, or rosin acid drying agents. The amination reagent is selected from any one or more of diethanolamine, methylmonoethanolamine, dimethylaminopropylamine, diethylenetriamine, or ketoimines of polyethylenepolyamines; The first solvent is any one or a combination of several of ethylene glycol butyl ether, diethylene glycol butyl ether, ethylene glycol monohexyl ether, or ethylene glycol isooctyl ether. The neutralizing agent is selected from any one or more of lactic acid, aminosulfonic acid, formic acid, methanesulfonic acid, or salicylic acid.

[0015] A second aspect of the present invention provides a method for preparing an electrodepositable cationic emulsion, comprising: A: Add E-51 epoxy resin, bisphenol A, and polyether resin to a reaction flask. Under nitrogen protection, heat to 130-150℃ with stirring, then add catalyst and react at 140-160℃ for 2-4 hours. When the epoxy equivalent reaches 1140-1160, add acrylic copolymer and crosslinking agent. After cooling to 110℃, quickly add amination reagent and react at 115℃ for 2.5 hours. B: Mix the neutralizing agent and 1 / 5 pure water to prepare an acidic base solution for later use; C: After the reaction in step A is completed, a solvent is added to cool the material in the reaction flask. After the temperature is cooled to below 100°C, acidic water base is added and stirred evenly. Then, the remaining 4 / 5 of pure water is added to the reaction flask under stirring to form a cationic emulsion that can be electrodeposited.

[0016] Preferably, the preparation method of the crosslinking agent is as follows: polyisocyanate monomer and methyl isobutyl ketone are put into a reaction flask, and the temperature is raised to 55-65°C under nitrogen protection and stirring. Dibutyltin dilaurate catalyst is added, and the temperature is stabilized at 60±2°C. Ethylene glycol butyl ether is slowly added dropwise using a dropping funnel to dissolve the blocking agent into a transparent mixed solution. The addition is completed in 2-3 hours. After the addition is completed, the temperature is kept for 3 hours, and then trimethylolpropane is added. The temperature is raised to 90°C and kept until the NCO groups disappear. The crosslinking agent is then cooled and prepared for use.

[0017] Preferably, the preparation method of the acrylic copolymer is as follows: Styrene monomer, hydroxypropyl methacrylate, isooctyl methacrylate, glycidyl methacrylate, and 99.3% initiator are mixed uniformly to obtain a mixture; then, half of the second solvent is added to the reaction flask, and under nitrogen protection, the temperature is raised to 120-130°C. The mixture is added dropwise using a dropping funnel, and the addition is completed in 3-4 hours. After the addition is completed, the temperature is maintained for 2.5-3.5 hours. Then, the remaining mixture of the second solvent and initiator is added all at once, and the temperature is maintained at 120-130°C for another 1.5-2.5 hours. The mixture is then cooled to obtain an acrylic copolymer with a solid content of 65-75%.

[0018] Because the present invention adopts the above technical solution, it has the following advantages and positive effects compared with the prior art: The electrodepositionable cationic emulsion provided by this invention modifies the main resin by introducing an epoxy-containing acrylic copolymer during the amination stage, allowing the epoxy-containing acrylic copolymer to undergo a grafting reaction with the chain-extended epoxy resin. A stable aqueous emulsion is obtained through neutralization and dispersion. This emulsion, when mixed with pigment paste to form an electrophoretic paint, solves the problem of paint film flow marks caused by residual plaster in the overlap gaps and cavities of coated parts on actual electrophoretic paint coating lines, significantly improving the quality and production efficiency of vehicle body coating. Attached Figure Description

[0019] Appendix Figure 1 This is a schematic diagram of the electrophoretic sample of the working solution for cathodic electrophoretic coating in an application example of the present invention. Detailed Implementation

[0020] The present invention provides a detailed description of an electrodeposable cationic emulsion and its preparation method, with reference to specific embodiments. The advantages and features of the present invention will become clearer from the following description.

[0021] The present invention provides an electrodepositable cationic emulsion comprising, by mass percentage: 16%-19% epoxy resin; 4%-6% polyether resin; 10-12% crosslinking agent; 2-6.5% acrylic copolymer; 2.8-3.6% amination reagent; 2-4% additives; 0.2-2% first solvent; 0.1-0.4% neutralizing agent; and the balance being pure water.

[0022] The epoxy resin is a macromolecular epoxy resin obtained by chain extension of small molecule E-51 epoxy resin and bisphenol A at 140-160℃ under the action of a catalyst. The catalyst is any one or more of N,N-dimethylbenzylamine, tetrabutylammonium bromide, tetraethylammonium bromide or triphenylphosphine.

[0023] The polyether resin is a bisphenol A polyoxyethylene ether polymer. The crosslinking agent is a product of a fully blocked addition reaction of a polyisocyanate.

[0024] Preferably, the additive is selected from any one or more of the following: polyether polyol leveling aids, polyacetal film-forming aids, microgel anti-cratering aids, or rosin acid drying agents. The amination reagent is selected from any one or more of diethanolamine, methylmonoethanolamine, dimethylaminopropylamine, diethylenetriamine, or ketoimines of polyethylenepolyamines; The first solvent is any one or a combination of several of ethylene glycol butyl ether, diethylene glycol butyl ether, ethylene glycol monohexyl ether, or ethylene glycol isooctyl ether. The neutralizing agent is selected from any one or more of lactic acid, aminosulfonic acid, formic acid, methanesulfonic acid, or salicylic acid.

[0025] Preferably, the acrylic copolymer is obtained by free radical polymerization, and the acrylic copolymer comprises the following components by mass percentage: 28-32% second solvent, 3-15% styrene, 9-12% hydroxypropyl methacrylate, 23-35% isooctyl methacrylate, 19-23% glycidyl methacrylate, and 1-2% initiator. Specifically, styrene monomer, hydroxypropyl methacrylate, isooctyl methacrylate, glycidyl methacrylate, and 99.3 wt% initiator (99.3 wt% here refers to 99.3 wt% of the initiator used in the acrylic copolymer reaction) are mixed evenly to obtain a mixture. Then, 1 / 2 weight of the second solvent (1 / 2 weight here refers to 1 / 2 of the weight of the second solvent used in the acrylic copolymer reaction) is added to a four-necked reaction flask equipped with a stirrer, thermometer, and reflux condenser. Under nitrogen protection, the temperature is slowly raised to 125°C, and the above mixture is slowly added dropwise using a dropping funnel. The addition is completed in 3-4 hours. After the addition is completed, the temperature is maintained for 3 hours. Then, the remaining mixture of the second solvent and initiator is added at once, and the temperature is maintained at 125°C for another 2 hours. After cooling, an acrylic composition with a solid content of 65-75% is obtained for later use.

[0026] The glass transition temperature (Tg) of the acrylic copolymer is 15-30°C, such as 15°C, 23°C, or 30°C, and the mass percentage of the acrylic copolymer in the electrodeposable cationic emulsion is any value within the range of 2%, 2.06%, 4.03%, 6%, 6.45%, or 6.5%.

[0027] Preferably, the second solvent is selected from any one or a combination of several of propylene glycol methyl ether, methyl isobutyl ketone, or ethylene glycol butyl ether; The initiator is selected from any one or a combination of several of benzoyl peroxide, di-tert-butyl peroxide, or di-tert-pentyl peroxide.

[0028] Preferably, the crosslinking agent is a fully enclosed addition reaction product of a polyisocyanate; its preparation method is to react the polyisocyanate monomer with the enclosing agent at 60-100℃ until the characteristic peak of the -NCO group in the infrared spectrum completely disappears. Specifically, liquefied polyisocyanate monomer and enclosing agent are added to a four-necked reaction flask equipped with a stirrer, thermometer, and reflux condenser. Under nitrogen protection, the temperature is slowly raised to 60℃, dibutyltin dilaurate catalyst is added, and the temperature is stabilized at 60±2℃. Ethylene glycol butyl ether is slowly added dropwise to dissolve caprolactam into a transparent mixed solution using a dropping funnel. The addition is completed in 2-3 hours. After the addition is completed, the temperature is maintained for 3 hours, and then trimethylolpropane is added. The temperature is raised to 90-100℃ and maintained until the NCO group disappears. The crosslinking agent is then cooled for later use.

[0029] A second aspect of the present invention provides a method for preparing an electrodepositable cationic emulsion, comprising: A: Add E-51 epoxy resin, bisphenol A, and polyether resin to a reaction flask. Under nitrogen protection, heat to 130-150℃ with stirring, then add catalyst and react at 140-160℃ for 2-4 hours. When the epoxy equivalent reaches 1140-1160, add acrylic copolymer and crosslinking agent. After cooling to 110℃, quickly add amination reagent and react at 115℃ for 2.5 hours. B: Mix the neutralizing agent and 1 / 5 of the weight of pure water (here, 1 / 5 of the weight refers to 1 / 5 of the weight of pure water used in the electrodeposition cationic emulsion, the same below) to form an acidic base material for later use; C: After the reaction in step A is completed, a solvent is added to cool the material in the reaction flask. After the temperature is cooled to below 100°C, acidic water base is added and stirred evenly. Then, the remaining 4 / 5 of the weight of pure water is added to the reaction flask while stirring to form a cationic emulsion that can be electrodeposited.

[0030] The following detailed description is based on specific embodiments. All examples mentioned below refer to mass percentages.

[0031] Example 1: Preparation of acrylic polymer: Table 1 Raw materials for acrylic polymers A, B, and C Preparation method: 1. Mix the monomers styrene, hydroxypropyl methacrylate, isooctyl methacrylate, glycidyl methacrylate, and di-tert-amyl peroxide evenly beforehand; 2. Add 1 / 2 the weight of propylene glycol methyl ether to a four-necked flask equipped with a stirrer, thermometer, and reflux condenser. Under nitrogen protection, slowly heat to 125°C. Slowly add the above mixture dropwise using a dropping funnel. The addition is completed in 3-4 hours. After the addition is completed, keep the temperature for 3 hours. Then, add the remaining propylene glycol methyl ether and di-tert-amyl peroxide II mixture all at once. Continue to maintain the temperature at 125°C for 2 hours. Cool down to obtain an acrylic acid composition with a solid content of approximately 70% for later use.

[0032] Example 2: Preparation of crosslinking agent: Table 2 Raw materials for crosslinking agent preparation Preparation method: 45.5 parts by mass of liquefied diphenylmethane diisocyanate (MDI) and 20.0 parts by mass of methyl isobutyl ketone were added to a four-necked flask equipped with a stirrer, thermometer, and reflux condenser. Under nitrogen protection, the temperature was slowly raised to 60°C. 0.1 parts by mass of dibutyltin dilaurate were added, and the temperature was stabilized at 60±2°C. 24.5 parts by mass of ethylene glycol butyl ether was slowly added dropwise to dissolve 6.3 parts of caprolactam into a clear mixed solution. The addition was completed in 2-3 hours. After the addition was completed, the temperature was maintained for 3 hours, and the NCO content was measured until it reached the design value and was deemed acceptable. After the NCO content was deemed acceptable, 3.7 parts by mass of trimethylolpropane were added, and the temperature was raised to 90°C. The temperature was maintained until the NCO groups disappeared. The mixture was then cooled to obtain a crosslinking agent with approximately 80% solids for later use.

[0033] Example 3: Preparation of Electrodepositable Cationic Emulsions: I. Emulsions were prepared using acrylic copolymers with different glass transition temperatures obtained in Example 1, and blank control emulsions were prepared simultaneously. Table 3. Raw material composition of cationic emulsions Preparation method: E-51 epoxy resin, bisphenol A, and polyether resin are added to a four-necked flask equipped with a stirrer, thermometer, reflux condenser, and thermometer. Under nitrogen protection, the temperature is raised to 140℃ with stirring, a catalyst is added, and the reaction is carried out at 140-160℃ for 2-4 hours. After the epoxy equivalent reaches 1140-1160, acrylic polymer and crosslinking agent are added. The temperature is lowered to 110℃, and ketimine and methyl ethanolamine are quickly added. The reaction is carried out at 115℃ for 2.5 hours. During this time, neutralizing agent and 1 / 5 water are mixed evenly according to the mass ratio to form an acidic water base for later use. After amination at 115℃, the material in the reaction flask is cooled with ethylene glycol monohexyl ether. After the temperature in the reaction flask drops below 100℃, the prepared acidic water is added and stirred evenly. The remaining water is added to the flask with stirring to form a cationic emulsion that can be electrodeposited. The solid content of the prepared emulsion is between 30-38%.

[0034] II. Using the acrylic copolymer C from Example 1, cationic emulsions containing different amounts of the acrylic copolymer were prepared. Table 4. Raw material composition of cationic emulsions The preparation method is the same as above.

[0035] Application example: The cationic emulsion prepared above will be mixed with pigments to prepare an electrophoresis working solution. Pigment paste preparation: Pigment paste is taken from commercially available products produced in the workshop. The specific production process can be referenced as follows: 27.04 parts of dispersion resin and 39.29 parts of deionized water were premixed evenly. Then, 24.5 parts of titanium dioxide, 6.74 parts of kaolin, 0.6 parts of carbon black, and 1.83 parts of dibutyltin oxide were added and mixed at low speed for 30 minutes in a grinding mill until homogeneous. The mixture was then ground in a high-speed sand mill for 2-4 hours until the fineness was less than 15 μm. After the fineness was qualified, the solid content of the slurry was adjusted to 52-60% with deionized water.

[0036] Prepare the cathodic electrophoretic coating working solution according to the mass fractions listed in Table 5.

[0037] Table 5 Working solution for cathodic electrophoretic coating The above working solution was aged at 30°C for 48 hours before use.

[0038] The electrophoretic paint flow marks were evaluated in the laboratory using the following method. Overlap two metal plates, each 0.5mm thick, 7.5cm long, and 15cm wide. Use 2cm long and 0.8cm wide shims to support the left and right edges of the overlap, then secure them with conductive paperclips (e.g., Figure 1 (as shown in the sample), the sample was electrophoresed, immersed in pure water and shaken back and forth 5 times, placed for 2 min, 5 min and 10 min respectively as specified, and then baked at 180℃ for 20 min and the flow mark state was observed. Evaluation criteria: Recorded as ○ – Visually, the paint film shows no spots, flow marks, or defects that affect the coating quality; Recorded as □ - Visually, the paint film has a few spots and minor flow marks. Slight sanding is needed, which has little impact on the coating quality. Recorded as △ - There are many spots on the paint film, as well as many flow marks and defects, which need to be sanded and will affect the coating quality; Recorded as × — Visually, the paint film has many spots, runs, and other defects that are completely unacceptable and require sanding and recoating, which seriously affects the quality of the coating.

[0039] Evaluation results Experimental results show that: comparing working solutions BD with working solutions A and E, it can be seen that strictly controlling the glass transition temperature of the acrylic copolymer can significantly improve the flow marks on the paint film, especially when using working solution C with Tg=23℃; comparing working solutions C1-C4 with working solution F, it can be seen that strictly controlling the amount of acrylic copolymer added can also significantly improve the flow marks on the paint film, especially when the amount of acrylic copolymer added is 4.03wt%.

[0040] The electrophoretic paint working solution using the emulsion combination pigment paste of the present invention can significantly reduce the generation of flow marks, thereby eliminating coating defects caused by flow marks.

[0041] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the above embodiments. Even if various changes are made to the present invention, if these changes fall within the scope of the claims of the present invention and their equivalents, they shall still fall within the protection scope of the present invention.

Claims

1. An electrodeposable cationic emulsion, characterized in that, The composition, calculated by weight percentage, includes: epoxy resin 16%-19%, polyether resin 4%-6%, crosslinking agent 10-12%, acrylic copolymer 2-6.5%, amination agent 2.8-3.6%, additives 2-4%, primary solvent 0.2-2%, neutralizing agent 0.1-0.4%, and pure water as the balance.

2. The electrodeposable cationic emulsion according to claim 1, characterized in that, The acrylic copolymer is obtained by free radical polymerization, and the acrylic copolymer comprises the following components by mass percentage: 28-32% second solvent, 3-15% styrene, 9-12% hydroxypropyl methacrylate, 23-35% isooctyl methacrylate, 19-23% glycidyl methacrylate, and 1-2% initiator.

3. The electrodeposable cationic emulsion according to claim 2, characterized in that, The second solvent is any one or a combination of several of propylene glycol methyl ether, methyl isobutyl ketone, or ethylene glycol butyl ether; The initiator is selected from any one or a combination of several of benzoyl peroxide, di-tert-butyl peroxide, or di-tert-pentyl peroxide.

4. The electrodeposable cationic emulsion according to claim 1 or 2, characterized in that, The glass transition temperature of the acrylic copolymer is 15-30℃.

5. The electrodeposable cationic emulsion according to claim 1 or 2, characterized in that, The mass percentage of the acrylic copolymer is 2-4.03% or 4.03-6.45%.

6. The electrodeposable cationic emulsion according to claim 1, characterized in that, The polyether resin is a bisphenol A polyoxyethylene ether polymer; The epoxy resin is obtained by chain extension of a small molecule epoxy resin and bisphenol A at a temperature of 140-160°C under the action of a catalyst. The mass fraction of the small molecule epoxy resin in the electrodepositable cationic emulsion is 16%-19%, the mass fraction of bisphenol A in the electrodepositable cationic emulsion is 4%-6%, and the mass fraction of the catalyst in the electrodepositable cationic emulsion is 0.02-0.08%. The catalyst is selected from any one or more of N,N-dimethylbenzylamine, tetrabutylammonium bromide, tetraethylammonium bromide, or triphenylphosphine.

7. The electrodeposable cationic emulsion according to claim 1, characterized in that, The crosslinking agent is a fully enclosed addition reaction product of a polyisocyanate; The additives are selected from any one or more of the following: polyether polyol leveling agents, polyacetal film-forming agents, microgel anti-cratering agents, or rosin acid drying agents. The amination reagent is selected from any one or more of diethanolamine, methylmonoethanolamine, dimethylaminopropylamine, diethylenetriamine, or ketoimines of polyethylenepolyamines; The first solvent is any one or a combination of several of ethylene glycol butyl ether, diethylene glycol butyl ether, ethylene glycol monohexyl ether, or ethylene glycol isooctyl ether. The neutralizing agent is selected from any one or more of lactic acid, aminosulfonic acid, formic acid, methanesulfonic acid, or salicylic acid.

8. A method for preparing an electrodeposable cationic emulsion as described in any one of claims 6-7, characterized in that, include: A: Add epoxy resin and polyether resin to a reaction flask, heat to 130-150℃ under nitrogen protection and stirring, then add catalyst, react at 140-160℃ for 2-4 hours, and check that the epoxy equivalent reaches 1140-1160. Then add acrylic copolymer and crosslinking agent, cool to 100-120℃ and add amination reagent, react at 100-130℃ for 2-3 hours. B: Mix the neutralizing agent and 1 / 5 pure water to prepare an acidic base solution for later use; C: After the reaction in step A is completed, a solvent is added to cool the material in the reaction flask. After the temperature is cooled to below 100°C, acidic water base is added and stirred evenly. Then, the remaining 4 / 5 of pure water is added to the reaction flask under stirring to form a cationic emulsion that can be electrodeposited.

9. The method for preparing an electrodepositable cationic emulsion according to claim 8, characterized in that, The preparation method of the crosslinking agent is as follows: Polyisocyanate monomer and methyl isobutyl ketone are put into a reaction flask, and the temperature is raised to 55-65℃ under nitrogen protection and stirring. Dibutyltin dilaurate catalyst is added, and the temperature is stabilized at 60±2℃. Ethylene glycol butyl ether is slowly added dropwise using a dropping funnel to dissolve the blocking agent into a transparent mixed solution. The addition is completed in 2-3 hours. After the addition is completed, the temperature is kept for 3 hours, and then trimethylolpropane is added. The temperature is raised to 90-100℃ and kept until the NCO groups disappear. The crosslinking agent is then cooled and prepared for use.

10. The method for preparing an electrodepositable cationic emulsion according to claim 8, characterized in that, The specific preparation method of the acrylic copolymer is as follows: Styrene monomer, hydroxypropyl methacrylate, isooctyl methacrylate, glycidyl methacrylate, and 99.3% initiator are mixed uniformly to obtain a mixture; then, half of the second solvent is added to the reaction flask, and under nitrogen protection, the temperature is raised to 120-130℃. The mixture is added dropwise using a dropping funnel, and the addition is completed in 3-4 hours. After the addition is completed, the temperature is maintained for 2.5-3.5 hours. Then, the remaining mixture of the second solvent and initiator is added all at once, and the temperature is maintained at 120-130℃ for another 1.5-2.5 hours. After cooling, an acrylic copolymer with a solid content of 65-75% is obtained.

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