Preparation method of primer-topcoat type cathode electrophoretic coating

Abstract

The invention discloses a preparation method of a primer-topcoat type cathode electrophoretic coating. The method comprises the following steps: preparing a curing agent, preparing epoxy resin without benzene rings, preparing a matrix resin solution, preparing pigment dispersion resin, preparing emulsion, preparing color paste, and preparing the primer-topcoat type cathode electrophoretic coating. In the step of preparing the matrix resin solution, the mineral filler is composed of kaolin and carbon black, the particle size of the kaolin is 400 meshes or above, the pH value of the carbon black is 8, the carbon black has medium and high chromaticity, the adding proportion of the kaolin accounts for 2%-10% of the total amount of the coating, the adding proportion of the carbon black accounts for 0.5%-5% of the total amount of the coating, the kaolin has good filling performance, and the filling performance is good. And the carbon black mainly provides coloring and covering power, so that when the two are combined for use, the durability and the protection performance of the coating can be further improved, the thickness of a coating film can be further reduced, and meanwhile, the weather resistance and the salt fog resistance of the coating film can be improved.

Description

Technical Field

[0001] The invention relates to the technical field of electrophoretic resin coatings, and in particular to a method for preparing a bottom-and-top integrated cathode electrophoretic coating. Background Art

[0002] The resin contained in the cathodic electrophoretic coating has alkaline groups, which are soluble in water after being neutralized by acid and become salts. When direct current is applied, the negative ions of the acid radical move toward the anode, and the resin ions and the pigment particles wrapped in them move toward the cathode with positive charges and are deposited on the cathode. This is the basic principle of electrophoretic coating.

[0003] The two most commonly used resins for cathodic electrophoretic coatings are epoxy resin and acrylic resin. Epoxy resin has the advantages of high modulus, high strength, good adhesion, and excellent corrosion resistance, but poor weather resistance. Acrylic resin has the advantages of excellent weather resistance, good color and gloss retention, high gloss, and good decorative properties, but poor corrosion resistance. At present, people have found that by adjusting the surface tension, epoxy resin with high surface tension is deposited on the lower layer, and acrylic resin with low surface tension is deposited on the upper layer, so that a coating has both the high adhesion and high corrosion resistance of epoxy resin, and the high decorativeness and high weather resistance of acrylic resin, and forms a film at one time, with the effects of both primer and topcoat, which can simplify the coating process and save costs. CN113861823A discloses a high weather-resistant cathode electrophoretic paint with integrated bottom and top surfaces and a preparation method thereof. By adding hydrogenated bisphenol A and epoxy chain extender (epoxy equivalent is above 2200), 0.1~0.15% light stabilizer, and HDI curing agent, neutral salt spray and aging resistance are improved. However, the coating thickness required to meet the performance standards is relatively high (≥50μm), the material cost is high, and the coating applicability is poor.

[0004] Therefore, the prior art needs to provide a method for preparing a bottom-and-top integrated cathodic electrophoretic coating to improve the salt spray resistance and weather resistance of the electrophoretic coating while maintaining a medium film thickness (the film thickness is required to be between 10μm and 50μm, and the optimal film thickness is 20~30μm). Summary of the invention

[0005] The object of the present invention is to provide a method for preparing a bottom-and-top integrated cathodic electrophoretic coating, so as to solve the problem that it is difficult to reduce the thickness of the coating film after improving the salt spray resistance and weather resistance of the coating film in the bottom-and-top integrated cathodic electrophoretic coating prepared with epoxy resin and acrylic resin as the main raw materials.

[0006] In order to solve the above technical problems, the present invention specifically provides the following technical solutions: The present invention provides a method for preparing a bottom-and-top integrated cathode electrophoretic coating, comprising the following steps: Q1: Preparation of curing agent Heat the isocyanate monomer to 40 °C, and dropwise add a mixture of trimethylolpropane and an alcohol ether solvent to obtain a curing agent; Q2: Preparation of epoxy resin without benzene ring Add a chain extender, methylhexahydrophthalic anhydride and neopentyl glycol, to the hydrogenated epoxy resin to obtain an epoxy resin with an epoxy equivalent of 1100 - 1200 and without a benzene ring, and then dilute it with methyl isobutyl ketone to form an epoxy resin solution; Q3: Preparation of the main resin solution In a reaction kettle, premix acrylic monomers, a cationic group introducing agent and an initiator, add 1 / 4 of the premix to the epoxy resin solution in Q2 to initiate the monomer pre-polymerization reaction; after the monomer pre-polymerization reaction stage is completed, continue to dropwise add the remaining acrylic monomer premix to complete the polymerization reaction to obtain an epoxy-acrylic resin solution; The obtained epoxy-acrylic resin solution is dissolved together in a mixed solvent of methyl isobutyl ketone and ethylene glycol monobutyl ether, and a crosslinking agent is added to form the main resin solution; Q4: Preparation of pigment dispersion resin In a three-necked flask, dissolve epoxy resin in ethylene glycol monobutyl ether to obtain product A, then dissolve acrylate monomers and initiator azobisisobutyronitrile in solvent n-butanol to obtain product B, drop product B into product A to obtain product C, add a quaternarizing agent to product C, and after cooling, obtain the pigment dispersion resin. Then add the obtained pigment dispersion resin to the main resin solution, and at the same time add mineral fillers; Q5: Preparation of emulsion By weight, sequentially add 30 - 40 parts of the main resin solution, 1 - 2 parts of an alcohol ether co-solvent, 0.5 - 1 part of an organic acid or inorganic acid to an emulsifier, stir and mix evenly, control the temperature below 90 °C, and then add 40 - 50 parts of deionized water to the emulsifier in two or more times for high-speed emulsification for 1 hour to obtain a crude emulsion. Among them, the emulsification temperature is controlled below 40 °C. Heat the crude emulsion to 50 - 70 °C and then perform extraction to extract the solvent in the crude emulsion. Then add an equal amount of deionized water to the extracted emulsion as the extracted solvent, adjust the pH value of the emulsion to 6 by adding a weak acid, and finally filter with a filter to obtain an emulsion with excellent stability; Q6: Preparation of color paste By weight, sequentially add 40 - 50 parts of pigment dispersion resin, 38 - 45 parts of color powder, 3 - 5 parts of dryer, 3 - 5 parts of high-boiling alcohol ether film-forming co-solvent and 10 - 15 parts of deionized water to a container, stir and mix evenly, then disperse at high speed, and grind with a sand mill to a fineness ≤ 15 μm, and filter with a bag filter to obtain a color paste; Q7: Preparation of bottom and top combined cathodic electrophoretic coating Mix the color paste, emulsion and deionized water in a molar ratio of 1:4 - 7:6 - 7 to obtain the bottom and surface integrated cathodic electrophoretic coating.

[0007] Preferably, in Q1, the isocyanate is a blocked isocyanate. The preparation steps of the blocked isocyanate are as follows: aliphatic isocyanate hexamethylene diisocyanate (HDI) and alcohol blocking agent methyl ethyl ketoxime are mixed in a molar ratio of 1:1 - 10, and the reaction temperature is between room temperature and 80 °C.

[0008] Preferably, in Q1, trimethylolpropane and alcohol ether solvent are mixed and heated. Before the temperature reaches 60 °C, the blocked isocyanate is added dropwise, and then kept at 60 °C for 1 - 2 hours, and then heated to 70 °C and kept at 70 °C for 2 hours to obtain the curing agent; among them, the molar ratio of the blocked isocyanate, alcohol ether solvent, and trimethylolpropane is 2 - 2.5:1 - 2:1 - 1.5. After the reaction is completed, the curing agent is obtained through separation and purification.

[0009] Preferably, in Q2, epoxy resin and / or hydrogenated epoxy resin: methyl hexahydrophthalic anhydride (chain extender): neopentyl glycol (chain extender), the molar ratio is 1:0.3 - 0.7:0.05 - 0.2, and the chain extension is carried out to an epoxy equivalent of 1100 - 1200; among them, the epoxy equivalent of the epoxy resin is 188, and the epoxy equivalent of the hydrogenated epoxy resin is between 780 and 850.

[0010] Preferably, in Q3, the mineral filler includes kaolin and carbon black.

[0011] Preferably, the particle size of the kaolin is above 400 mesh, and the pH value of the carbon black is 8 and it has medium to high chroma.

[0012] Preferably, the addition ratio of the kaolin is 2% - 10% of the total amount of the coating, and the addition ratio of the carbon black is 0.5% - 5% of the total amount of the coating.

[0013] Preferably, in Q3, the acrylic monomer is acrylate and divinylbenzene, the cationic group introducing agent is dimethyldiallylammonium chloride (DMDAAC), and the initiator is tert-butyl peroxy-2-ethylhexanoate; during the polymerization reaction, a dispersant and an antifoaming agent are added. After the reaction is completed, the reaction mixture is cooled; then the polymer particles are separated by filtration, and the polymer particles are washed with ethanol to remove unreacted monomers and by-products.

[0014] Preferably, the preparation steps of the pigment dispersion resin in Q4 include: S1. Under the protection of nitrogen, weigh 200 - 300 g of epoxy resin and put it into a three-necked flask. Add 100 - 180 mL of ethylene glycol monobutyl ether. Under a constant temperature water bath at 70 - 80 °C, stir until the epoxy resin is completely dissolved to obtain product A; S2. Then dissolve 30 - 60 g of acrylate monomer and 0.05 - 0.2 g of initiator azobisisobutyronitrile in an appropriate amount of solvent n-butanol. Heat up to 85 - 105 °C. After complete dissolution, obtain product B and drop it into product A in the three-necked flask, and control to finish dropping within 2 hours to obtain product C; S3. Then add a quaternizing agent to product C, keep the temperature for reaction for 4 - 6 hours, then cool down and discharge to obtain the pigment-dispersing resin.

[0015] Preferably, in Q4, the preparation steps of the quaternizing agent include: S1. Add 85 - 90 g of xylylene diisocyanate and an appropriate amount of solvent into a four-necked flask equipped with a stirrer, a thermometer and a reflux condenser. Control the reaction temperature at 25 - 35 °C, and drop 46 - 53 g of a blocking agent within 2 - 3 hours. Then heat up to 55 - 65 °C and keep the temperature for reaction until the blocking agent completely reacts to obtain product 1; S2. Heat up to 45 - 55 °C. Take 52 - 56 g of dimethylethanolamine and dissolve it in an appropriate amount of solvent, transfer it to a dropping funnel, and drop it into product 1 within 15 - 20 minutes. After dropping, heat up to 55 - 65 °C and keep the temperature for reaction for 1 - 1.5 hours, then heat up to 80 - 90 °C and keep the temperature for reaction for 30 - 60 minutes to obtain product 2; S3. Add acetic acid solution to product 2 to adjust the pH to neutral, and continue to keep the temperature for reaction for 30 - 40 minutes to obtain the quaternizing agent.

[0016] The present invention has the following beneficial effects compared with the prior art: 1. The present invention obtains modified epoxy resin by a debenzene ring method, which can improve the compatibility of epoxy resin and acrylic resin, thereby improving the interlayer adhesion. The modified epoxy resin has a higher molecular weight and a more stable chemical structure, which helps to improve the overall stability of the coating and reduce performance changes caused by environmental factors (such as temperature, humidity, etc.).

[0017] 2. By using a blocked aliphatic isocyanate curing agent, the present invention utilizes its property that it is inactive at room temperature but can release active isocyanate groups during the coating process through heating or contact with a catalyst. These groups can react with hydroxyl groups or other active groups in epoxy resins and acrylic resins to form stronger chemical bonds, thereby improving the adhesion of the coating and solving the problem that the coating still shows delamination or peeling during long-term use when a high proportion of acrylic resin exists after debenzylation. At the same time, good weather resistance is obtained, and a denser and more uniform-thickness coating is formed, which helps to improve the mechanical properties and protective properties of the coating, such as abrasion resistance and corrosion resistance.

[0018] 3. In the step of preparing the main resin solution of the present invention, through the use of a mineral filler composed of a mixture of kaolin and carbon black, the particle size of the kaolin is above 400 mesh, and the pH value of the carbon black is 8 and it has medium to high chromaticity. Kaolin itself has good filling properties, while carbon black mainly provides coloring and covering power. When the two are used in combination, the covering power and filling properties of the coating can be further improved, making the coating smoother and denser. On the other hand, carbon black can improve the weather resistance and chemical resistance of the coating, while kaolin can enhance the mechanical properties of the coating. Therefore, when the two are used in combination, the durability and protective properties of the coating can be further improved, which also helps to further reduce the coating film thickness, and at the same time can improve the weather resistance and salt spray resistance of the coating film. Detailed implementation mode

[0019] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0020] Example 1: This example provides a preparation method of a bottom-and-top-in-one cathodic electrophoretic coating, including the following steps: Q1: Prepare the curing agent Heat the isocyanate monomer to 40°C, and dropwise add a mixture of trimethylolpropane and alcohol ether solvents to obtain the curing agent; Q2: Prepare the epoxy resin without benzene ring Add a chain extender, methylhexahydrophthalic anhydride, and neopentyl glycol to the hydrogenated epoxy resin to obtain an epoxy resin with an epoxy equivalent of 1100 - 1200 and without a benzene ring, and then dilute it with methyl isobutyl ketone into an epoxy resin solution; Q3: Prepare the main resin solution In a reaction kettle, acrylic monomers, cationic group introducing agents, and initiators are premixed. 1 / 4 of the premix is added to the epoxy resin solution in Q2 to initiate the monomer prepolymerization reaction. After the monomer prepolymerization reaction stage is completed, the remaining acrylic monomer premix is continuously added dropwise to complete the polymerization reaction, obtaining an epoxy-acrylic resin solution. The obtained epoxy-acrylic resin solution is dissolved together in a mixed solvent of methyl isobutyl ketone and ethylene glycol monobutyl ether, and a crosslinking agent is added to form a main resin solution. Q4: Preparation of pigment dispersion resin In a three-necked flask, epoxy resin is taken and dissolved in ethylene glycol monobutyl ether to obtain product A. Then, acrylate monomers and initiator azobisisobutyronitrile are dissolved in solvent n-butanol to obtain product B. Product B is added dropwise to product A to obtain product C. A quaternizing agent is added to product C. After cooling, a pigment dispersion resin is obtained. Then, the obtained pigment dispersion resin is added to the main resin solution, and at the same time, mineral fillers are added. Q5: Preparation of emulsion By weight, 30 - 40 parts of the main resin solution, 1 - 2 parts of an alcohol ether cosolvent, and 0.5 - 1 part of an organic acid or inorganic acid are sequentially added to an emulsifier and stirred and mixed evenly. The temperature is controlled below 90°C. Then, 40 - 50 parts of deionized water are added to the emulsifier in two or more times for high-speed emulsification for 1 hour to obtain a crude emulsion. Among them, the emulsification temperature is controlled below 40°C. The crude emulsion is heated to 50 - 70°C and then extracted to extract the solvent in the crude emulsion. Then, the same amount of deionized water as the extracted solvent is added to the extracted emulsion. The pH value of the emulsion is adjusted to 6 by adding a weak acid. Finally, it is filtered through a filter to obtain an emulsion with excellent stability. Q6: Preparation of color paste By weight, 40 - 50 parts of pigment dispersion resin, 38 - 45 parts of color powder, 3 - 5 parts of drier, 3 - 5 parts of high-boiling-point alcohol ether film-forming cosolvent, and 10 - 15 parts of deionized water are sequentially added to a container, stirred and mixed evenly, then dispersed at high speed, and ground with a sand mill until the fineness is ≤15 μm, and then filtered through a bag filter to obtain a color paste. Q7: Preparation of one-coat cathodic electrophoretic coating The color paste, emulsion, and deionized water are compounded in a molar ratio of 1:4 - 7:6 - 7 to obtain the one-coat cathodic electrophoretic coating.

[0021] A preparation method of a bottom-surface integrated cathodic electrophoretic coating provided by the present invention. In step Q1, the isocyanate is a blocked isocyanate. The preparation steps of the blocked isocyanate are as follows: The aliphatic isocyanate hexamethylene diisocyanate (HDI) and the alcohol blocking agent methyl ethyl ketoxime are mixed in a molar ratio of 1:1 to 10, the reaction temperature is between room temperature and 80 °C, and then separation and purification are carried out to separate the blocked isocyanate from the unreacted blocking agent and other by-products. In this example, the alcohol blocking agent methyl ethyl ketoxime needs to be added in excess to ensure that all of the isocyanate hexamethylene diisocyanate (HDI) can participate in the reaction to form blocked hexamethylene diisocyanate. As the addition amount of methyl ethyl ketoxime increases, the performance of the blocked hexamethylene diisocyanate becomes more stable, and the required reaction temperature during preparation also increases accordingly. Preferably, the molar ratio of hexamethylene diisocyanate (HDI) to methyl ethyl ketoxime is 1:6, and the reaction temperature is 60 °C.

[0022] In this example, in step Q1, trimethylolpropane and an alcohol ether solvent are mixed and heated again. Before the temperature reaches 60 °C, the blocked isocyanate is added dropwise, and then after holding at 60 °C for 1 to 2 hours, it is heated to 70 °C and held at 70 °C for 2 hours to obtain a curing agent; wherein, the molar ratio of the blocked isocyanate, the alcohol ether solvent, and trimethylolpropane is 2 to 2.5:1 to 2:1 to 1.5, and after the reaction is completed, the curing agent is obtained through separation and purification.

[0023] A preparation method of a bottom-surface integrated cathodic electrophoretic coating provided by the present invention. In step Q2, epoxy resin and / or hydrogenated epoxy resin: methylhexahydrophthalic anhydride (chain extender): neopentyl glycol (chain extender), the molar ratio is: 1:0.3 to 0.7:0.05 to 0.2, and the chain extension is carried out to an epoxy equivalent of 1100 - 1200; wherein, the epoxy equivalent of the epoxy resin is 188, and the epoxy equivalent of the hydrogenated epoxy resin is between 780 and 850. After the reaction is completed, the epoxy resin solution is neutralized to adjust the pH value of the solution, and then the debenzylated epoxy resin solution is diluted to an appropriate concentration.

[0024] In this embodiment, the modified epoxy resin is obtained by debenzocyclization, which can improve the compatibility between the epoxy resin and the acrylic resin, thereby enhancing the interlayer adhesion. The modified epoxy resin has a higher molecular weight and a more stable chemical structure, which helps to improve the overall stability of the coating and reduce the performance changes caused by environmental factors (such as temperature, humidity, etc.). However, although debenzocyclization can improve the compatibility between the epoxy resin and the acrylic resin, this improvement may be limited, especially in the presence of a high proportion of acrylic resin. The limited improvement in compatibility may lead to problems such as delamination or peeling of the coating during long-term use. In this embodiment, the blocked aliphatic isocyanate curing agent is inactive at room temperature but can release active isocyanate groups during the coating process by heating or contacting with a catalyst. These groups can react with hydroxyl groups or other active groups in the epoxy resin and the acrylic resin to form stronger chemical bonds, thereby improving the adhesion of the coating and helping to solve the problems of delamination or peeling of the coating during long-term use. On the other hand, the coating cured by the blocked aliphatic isocyanate curing agent has good weather resistance and can resist the erosion of environmental factors such as ultraviolet rays and moisture, which helps to improve the long-term stability and service life of the coating. On the other hand, the blocked aliphatic isocyanate curing agent can react with the epoxy resin and the acrylic resin to form a denser and more uniform-thickness coating, which helps to improve the mechanical properties and protective properties of the coating, such as abrasion resistance and corrosion resistance.

[0025] In the preparation method of a bottom-combined cathodic electrophoretic coating provided by the present invention, the mineral filler includes kaolin, carbon black, etc. The addition ratio of the kaolin is 2% - 10% of the total amount of the coating, and the addition ratio of the carbon black is 0.5% - 5% of the total amount of the coating. Mineral fillers usually have a large volume. Adding an appropriate amount of mineral fillers to the coating can significantly increase the total volume of the coating. This means that when using a coating containing mineral fillers, the same weight of the coating can cover a larger area, thereby reducing the coating consumption per unit area. Mineral fillers can improve the rheological properties of the coating, making it easier to apply. During the construction process, the coating can be more evenly distributed on the surface, thereby reducing coating waste and reducing the local film thickness. Mineral fillers can improve the durability of the coating, making it more weather-resistant, chemical-resistant, and wear-resistant. The coating can be thinner while still maintaining sufficient protection performance. Mineral fillers can increase the hardness and wear resistance of the coating. The coating can be thinner while still maintaining sufficient strength and durability. In the present invention, through the use of a mineral filler composed of a mixture of kaolin and carbon black, the particle size of the kaolin is above 400 mesh, and the pH value of the carbon black is 8 and it has medium to high chroma. Kaolin itself has good filling properties, while carbon black mainly provides coloring and covering power. When the two are used in combination, the covering power and filling properties of the coating can be further improved, making the coating more uniform and smooth. On the other hand, carbon black can improve the weather resistance and chemical resistance of the coating, while kaolin can enhance the mechanical properties of the coating. Therefore, when the two are used in combination, the durability and protection performance of the coating can be further improved, and it can also help to further reduce the film thickness, while improving the weather resistance and salt spray resistance of the coating film.

[0026] In this embodiment, in step Q4, the preparation steps of the pigment-dispersing resin include: S1. Under the protection of nitrogen, weigh 200 - 300 g of epoxy resin and put it into a three-necked flask, add 100 - 180 mL of ethylene glycol monobutyl ether, and stir at a constant water bath temperature of 70 - 80 °C until the epoxy resin is completely dissolved to obtain product A; S2. Then dissolve 30 - 60 g of acrylate monomer and 0.05 - 0.2 g of initiator azobisisobutyronitrile in an appropriate amount of solvent n-butanol, heat up to 85 - 105 °C, and after complete dissolution, obtain product B, and drop it into product A in the three-necked flask, and control to finish dropping within 2 hours to obtain product C; S3. Then add a quaternizing agent to product C, keep the temperature for reaction for 4 - 6 hours, then cool down and discharge to obtain the pigment-dispersing resin.

[0027] In this embodiment, in step Q4, the preparation steps of the quaternizing agent include: S1. Add 85 - 90 g of xylene diisocyanate and an appropriate amount of solvent into a four - necked flask equipped with a stirrer, a thermometer and a reflux condenser. Control the reaction temperature at 25 - 35 °C, and dropwise add 46 - 53 g of a blocking agent within 2 - 3 hours. Then raise the temperature to 55 - 65 °C and keep the reaction until the blocking agent completely reacts to obtain Product 1; S2. Raise the temperature to 45 - 55 °C. Dissolve 52 - 56 g of dimethylethanolamine in an appropriate amount of solvent, transfer it to a dropping funnel, and dropwise add it to Product 1 within 15 - 20 minutes. After the addition, raise the temperature to 55 - 65 °C and keep the reaction for 1 - 1.5 hours, then raise the temperature to 80 - 90 °C and keep the reaction for 30 - 60 minutes to obtain Product 2; S3. Add acetic acid solution to Product 2 to adjust the pH to neutral, and continue to keep the reaction for 30 - 40 minutes to obtain a quaternizing agent.

[0028] In this example, in step Q3, the acrylic monomer is acrylate and divinylbenzene, the cationic group introducing agent is dimethyldiallylammonium chloride (DMDAAC), and the initiator is tert - butyl peroxy - 2 - ethylhexanoate; during the polymerization reaction, a dispersant and an antifoaming agent are added. After the reaction is completed, the reaction mixture is cooled; then the polymer particles are separated by filtration, and the polymer particles are washed with ethanol to remove unreacted monomers and by - products.

[0029] Comparative Example 1: In the preparation method of the bottom - surface - integrated cathodic electrophoretic coating provided in this comparative example, during the preparation of the curing agent, an open - type isocyanate is used, and other preparation processes are the same as those in Example 1.

[0030] Comparative Example 2: In the preparation method of the bottom - surface - integrated cathodic electrophoretic coating provided in this comparative example, a main resin solution is prepared using an epoxy resin solution containing a benzene ring, and other preparation processes are the same as those in Example 1.

[0031] Comparative Example 3: In the preparation method of the bottom - surface - integrated cathodic electrophoretic coating provided in this comparative example, mineral fillers are not used during the preparation of the main resin solution, and other preparation processes are the same as those in Example 1.

[0032] Comparative Example 4: In the preparation method of the bottom - surface - integrated cathodic electrophoretic coating provided in this comparative example, one of kaolin and carbon black is used as a mineral filler during the preparation of the main resin solution, and other preparation processes are the same as those in Example 1.

[0033] As shown in the test content of the following table, the formulations in Examples 1-3 improved the salt spray resistance and weather resistance of the coating film compared to the formulations in Comparative Examples 1-4, and the coating film thickness could be reduced by adjusting the mixing ratio and mesh number of kaolin and carbon black without affecting other properties of the coating film.

[0034] Category Weather resistance (h) Salt spray resistance (h) Film thickness (μm) Example 1 ≥700 ≥700 20-30 Comparative Example 1 400~450 ≥500 ≥50 Comparative Example 2 400~500 ≥600 ≥50 Comparative Example 3 ≥500 300~350 ≥50 Comparative Example 4 ≥500 ≥500 45-50 The above embodiments are only exemplary embodiments of the present application and are not used to limit the present application. The protection scope of the present application is defined by the claims. Those skilled in the art can make various modifications or equivalent replacements within the essence and protection scope of the present application, and such modifications or equivalent replacements should also be regarded as falling within the protection scope of the present application.

Claims

1. A method for preparing a bottom-to-top integrated cathode electrophoretic coating, comprising the following steps: Q1: Preparation of curing agent The isocyanate monomer is heated to 40°C, and a mixture of trimethylolpropane and an alcohol ether solvent is added dropwise to prepare a curing agent; Q2: Preparation of epoxy resin without benzene ring Adding chain extenders methyl hexahydrophthalic anhydride and neopentyl glycol to hydrogenated epoxy resin to obtain epoxy resin with an epoxy equivalent of 1100-1200 and no benzene ring, and then diluting with methyl isobutyl ketone to obtain epoxy resin solution; Q3: Preparation of main resin solution In a reaction kettle, acrylic monomer, cationic group introducing agent and initiator are premixed, and 1 / 4 of the premix is ​​added to the epoxy resin solution of Q2 to start the monomer prepolymerization reaction; after the monomer prepolymerization reaction stage is completed, the remaining acrylic monomer premix is ​​continued to be added dropwise to continue the polymerization reaction to obtain an epoxy-acrylic resin solution; The obtained epoxy-acrylic resin solution is dissolved in a mixed solvent of methyl isobutyl ketone and ethylene glycol butyl ether, and a cross-linking agent is added to form a main resin solution; Q4: Preparation of pigment dispersion resin In a three-necked flask, epoxy resin is dissolved in ethylene glycol butyl ether to obtain product A, and then acrylate monomer and initiator azobisisobutyronitrile are dissolved in solvent n-butanol to obtain product B, and product B is added dropwise to product A to obtain product C, and a quaternizing agent is added to product C. After cooling, a pigment dispersion resin is obtained, and then the obtained pigment dispersion resin is added to the main resin solution, and mineral filler is added at the same time; Q5: Preparation of emulsion In an emulsifier, 30-40 parts of the main resin solution, 1-2 parts of alcohol ether cosolvent, and 0.5-1 part of an organic acid or an inorganic acid are added in sequence by weight, and the mixture is stirred and mixed evenly. The temperature is controlled below 90° C., and then 40-50 parts of deionized water are added twice or more to the emulsifier for high-speed emulsification for 1 hour to obtain a crude emulsion, wherein the emulsification temperature is controlled below 40° C., the crude emulsion is heated to 50-70° C. and then extracted to extract the solvent in the crude emulsion, and then deionized water in an amount equal to the extracted solvent is added to the extracted emulsion, and the pH value of the emulsion is adjusted to about 6 by adding a weak acid, and finally filtered to obtain an emulsion with excellent stability; Q6: Preparation of color paste Add 40-50 parts of pigment dispersing resin, 38-45 parts of color powder, 3-5 parts of drying agent, 3-5 parts of high boiling point alcohol ether film-forming co-solvent and 10-15 parts of deionized water in a container in order by weight, stir and mix evenly, disperse at high speed, grind with a sand mill to a fineness of ≤15μm, and filter with a bag filter to obtain a color paste; Q7: Preparation of bottom-to-top integrated cathodic electrophoretic coating The base-to-surface cathode electrophoretic coating can be obtained by compounding the color paste, emulsion and deionized water in a molar ratio of 1:4-7:6-7.

2. The method for preparing a bottom-to-top integrated cathode electrophoretic coating according to claim 1, characterized in that: In Q1, the isocyanate is a blocked isocyanate, and the preparation steps of the blocked isocyanate are as follows: aliphatic isocyanate hexamethylene diisocyanate (HDI) and alcohol blocking agent methyl ethyl ketoxime are mixed in a molar ratio of 1:1 to 10, and the reaction temperature is between room temperature and 80°C.

3. The method for preparing a bottom-to-top integrated cathode electrophoretic coating according to claim 2, characterized in that: In the Q1, trimethylolpropane and an alcohol ether solvent are mixed and heated, and a blocked isocyanate is added dropwise before the temperature reaches 60°C. After the mixture is kept at 60°C for 1 to 2 hours, the mixture is heated to 70°C and kept at 70°C for 2 hours to obtain a curing agent. The molar ratio of the blocked isocyanate, the alcohol ether solvent and the trimethylolpropane is 2-2.5:1-2:1-1.

5. After the reaction is completed, the curing agent is obtained by separation and purification.

4. The method for preparing a bottom-to-top integrated cathode electrophoretic coating according to claim 3, characterized in that: In Q2, the epoxy resin and / or hydrogenated epoxy resin: methyl hexahydrophthalic anhydride (chain extender): neopentyl glycol (chain extender) are chain extended in a molar ratio of 1:0.3~0.7:0.05~0.2 to an epoxy equivalent of 1100-1200; wherein the epoxy equivalent of the epoxy resin is 188, and the epoxy equivalent of the hydrogenated epoxy resin is between 780 and 850.

5. The method for preparing a bottom-to-top integrated cathode electrophoretic coating according to claim 4, characterized in that: In Q3, the mineral filler comprises kaolin and carbon black.

6. The method for preparing a bottom-to-top integrated cathode electrophoretic coating according to claim 5, characterized in that: The particle size of the kaolin is above 400 meshes, and the pH value of the carbon black is 8 and the chroma is medium to high.

7. The method for preparing a bottom-to-top integrated cathode electrophoretic coating according to claim 6, characterized in that: The addition ratio of the kaolin is 2% to 10% of the total amount of the coating, and the addition ratio of the carbon black is 0.5% to 5% of the total amount of the coating.

8. The method for preparing a bottom-to-top integrated cathodic electrophoretic coating according to claim 7, characterized in that: In Q3, the acrylic monomers are acrylic acid ester and divinylbenzene, the cationic group introducing agent is dimethyldiallylammonium chloride (DMDAAC), and the initiator is tert-butyl peroxide-2-ethylhexanoate; during the polymerization reaction, a dispersant and a defoaming agent are added, and after the reaction is completed, the reaction mixture is cooled; the polymer particles are then separated by filtration and washed with ethanol to remove unreacted monomers and by-products.

9. The method for preparing a bottom-to-top integrated cathode electrophoretic coating according to claim 8, characterized in that: In the Q4, the preparation steps of the pigment dispersion resin include: S1. Under the protection of nitrogen, weigh 200-300 g of epoxy resin into a three-necked flask, add 100-180 mL of ethylene glycol butyl ether, and stir in a constant temperature water bath at 70-80° C. until the epoxy resin is completely dissolved to obtain product A; S2, dissolving 30-60g of acrylate monomer and 0.05-0.2g of initiator azobisisobutyronitrile with an appropriate amount of n-butanol solvent, heating to 85-105°C, and obtaining product B after complete dissolution, and adding the product B dropwise to product A in the three-necked flask, and controlling the dripping to be completed within 2 hours, to obtain product C; S3. Add a quaternizing agent to the product C, keep the temperature to react for 4 to 6 hours, then cool and discharge the product to obtain a pigment dispersion resin.

10. The method for preparing a bottom-to-top integrated cathode electrophoretic coating according to claim 8, characterized in that: In the Q4, the preparation steps of the quaternizing agent include: S1. Add 85-90 g of xylene diisocyanate and an appropriate amount of solvent into a four-necked flask equipped with a stirrer, a thermometer and a reflux condenser, control the reaction temperature at 25-35° C., dropwise add 46-53 g of a blocking agent within 2-3 hours, then raise the temperature to 55-65° C. and keep the temperature to react until the blocking agent is completely reacted to obtain product 1; S2, heating to 45-55°C, dissolving 52-56 g of dimethylethanolamine in an appropriate amount of solvent, transferring to a dropping funnel, and dropping to product 1 within 15-20 minutes. After the dropping is complete, heating to 55-65°C and keeping the temperature for reaction for 1-1.5 hours, then heating to 80-90°C and keeping the temperature for reaction for 30-60 minutes to obtain product 2; S3. Add acetic acid solution to product 2 to adjust the pH to neutral, and continue to keep warm and react for 30 to 40 minutes to obtain a quaternizing agent.