Preparation method of bottom-top integrated cathode electrophoretic coating

Abstract

The present invention discloses a method for preparing a bottom-to-top integrated cathodic electrophoretic coating. The method comprises the following steps: preparing a curing agent, preparing an epoxy resin without a benzene ring, preparing a main resin solution, preparing a pigment dispersion resin, preparing an emulsion, preparing a color paste, and preparing the bottom-to-top integrated cathodic electrophoretic coating. In the step of preparing the main resin solution, the mineral filler comprises kaolin and carbon black. The kaolin has a particle size of 400 mesh or more, the carbon black has a pH of 8 and a medium-to-high chroma, the kaolin is added in an amount of 2% to 10% of the total coating, and the carbon black is added in an amount of 0.5% to 5% of the total coating. The kaolin itself has good filling properties, while the carbon black mainly provides coloring and hiding power. When the two are used in combination, the durability and protective properties of the coating can be further improved, the coating film thickness can be further reduced, and the weather resistance and salt spray resistance of the coating 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 cathodic electrophoretic coatings has alkaline groups that, after acid neutralization, form salts that dissolve in water. When a direct current is applied, the negative acid ions migrate toward the anode, while the resin ions and the pigment particles they contain carry a positive charge and migrate toward the cathode, where they are deposited. 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. Currently, people have discovered that by regulating 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. In addition, the film is formed in one step, and it has the effects of both primer and topcoat, which can simplify the coating process and save costs. CN113861823A discloses a highly weather-resistant cathode electrophoretic paint with a combined bottom and top surface, and a preparation method thereof. By combining hydrogenated bisphenol A and epoxy chain extenders (epoxy equivalent of 2200 or above), 0.1-0.15% of a light stabilizer, and an HDI curing agent, the neutral salt spray and aging resistance properties 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 purpose 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 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, it is difficult to reduce the thickness of the coating film.

[0006] In order to solve the above technical problems, the present invention specifically provides the following technical solutions:

[0007] The present invention provides a method for preparing a bottom-and-top integrated cathode electrophoretic coating, comprising the following steps:

[0008] Q1: Preparation of curing agent

[0009] 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;

[0010] Q2: Preparation of epoxy resin without benzene ring

[0011] Adding chain extenders methyl hexahydrophthalic anhydride and neopentyl glycol to the hydrogenated epoxy resin to obtain an epoxy resin with an epoxy equivalent weight of 1100-1200 and no benzene ring, and then diluting it with methyl isobutyl ketone to form an epoxy resin solution;

[0012] Q3: Preparation of main resin solution

[0013] In a reaction kettle, acrylic acid monomer, a cationic group introducing agent, and an initiator are premixed, and 1 / 4 of the premix is ​​added to the epoxy resin solution in Q2 to initiate a monomer prepolymerization reaction; after the monomer prepolymerization reaction stage is completed, the remaining acrylic acid monomer premix is ​​continuously added dropwise to continue the polymerization reaction to obtain an epoxy-acrylic resin solution;

[0014] 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;

[0015] Q4: Preparation of pigment dispersion resin

[0016] 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. A quaternizing agent is added to product C, and after cooling, a pigment dispersion resin is obtained. The obtained pigment dispersion resin is then added to the main resin solution, and mineral filler is added at the same time;

[0017] Q5: Preparation of emulsion

[0018] In an emulsifier, 30-40 parts of a main resin solution, 1-2 parts of an alcohol ether cosolvent, and 0.5-1 part of an organic acid or an inorganic acid are sequentially added by weight, and the mixture is stirred and mixed uniformly. 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 from the crude emulsion. 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 6 by adding a weak acid. Finally, the emulsion is filtered to obtain an emulsion with excellent stability.

[0019] Q6: Preparation of color paste

[0020] 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;

[0021] Q7: Preparation of bottom-to-top combined cathodic electrophoretic coating

[0022] The base and top combined 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.

[0023] Preferably, in Q1, the isocyanate is a blocked isocyanate, and the preparation steps of the blocked isocyanate are: aliphatic isocyanate hexamethylene diisocyanate (HDI) and an 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.

[0024] Preferably, in Q1, trimethylolpropane and an alcohol ether solvent are mixed and heated, and a blocked isocyanate is added dropwise before the temperature reaches 60°C, and then kept at 60°C for 1 to 2 hours, heated to 70°C, and kept at 70°C for 2 hours to obtain a curing agent; wherein the molar ratio of the blocked isocyanate, the alcohol ether solvent, and the trimethylolpropane is 2~2.5:1~2:1~1.5, and after the reaction is completed, the curing agent is obtained by separation and purification.

[0025] Preferably, 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.

[0026] Preferably, in Q3, the mineral filler comprises kaolin and carbon black.

[0027] Preferably, the particle size of the kaolin is above 400 mesh, and the pH value of the carbon black is 8 and the chroma is medium to high.

[0028] Preferably, 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.

[0029] Preferably, 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.

[0030] Preferably, in Q4, the step of preparing the pigment dispersion resin comprises:

[0031] S1. Under nitrogen protection, 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;

[0032] S2, dissolving 30-60 g of acrylate monomer and 0.05-0.2 g of initiator azobisisobutyronitrile in an appropriate amount of n-butanol solvent, raising the temperature to 85-105° C., and after complete dissolution, obtaining product B, which was added dropwise to product A in the three-necked flask, and the entire amount of the addition was controlled within 2 hours to obtain product C;

[0033] 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.

[0034] Preferably, in Q4, the preparation step of the quaternizing agent comprises:

[0035] S1. Add 85-90 g of xylene diisocyanate and an appropriate amount of solvent to a four-necked flask equipped with a stirrer, a thermometer, and a reflux condenser. Control the reaction temperature at 25-35°C. Add 46-53 g of a blocking agent dropwise over 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.

[0036] S2, heating to 45-55°C, dissolving 52-56 g of dimethylethanolamine in an appropriate amount of solvent, transferring the mixture to a dropping funnel, and adding the mixture dropwise to product 1 over 15-20 minutes. After the addition is complete, heating to 55-65°C and keeping the mixture warm for 1-1.5 hours, then heating to 80-90°C and keeping the mixture warm for 30-60 minutes to obtain product 2;

[0037] S3. Add acetic acid solution to product 2 to adjust the pH to neutral, and continue to keep the reaction warm for 30 to 40 minutes to obtain a quaternizing agent.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1. The modified epoxy resin obtained by the present invention through debenzenization 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.).

[0040] 2. The present invention uses a blocked aliphatic isocyanate curing agent, which is inactive at room temperature but can release active isocyanate groups by heating or contact with a catalyst during the coating process. These groups can react with hydroxyl groups or other active groups in epoxy resin and acrylic resin to form stronger chemical bonds, thereby improving the adhesion of the coating and improving the problem of delamination or peeling of the coating during long-term use when a high proportion of acrylic resin is present after debenzenization. At the same time, good weather resistance is obtained, forming a denser and more uniformly thick coating, which helps to improve the mechanical properties and protective properties of the coating, such as wear resistance and corrosion resistance.

[0041] 3. In the step of preparing the main resin solution, the present invention uses a mineral filler composed of a mixture of kaolin and carbon black, wherein the kaolin has a particle size of 400 mesh or larger, and the carbon black has a pH of 8 and a medium-to-high chroma. Kaolin itself has good filling properties, while carbon black primarily provides coloring and hiding power. When the two are used in combination, the hiding 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, and it can also help to further reduce the thickness of the coating film, while improving the weather resistance and salt spray resistance of the coating film. DETAILED DESCRIPTION

[0042] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0043] Example 1: This example provides a method for preparing a bottom-and-top integrated cathode electrophoretic coating, comprising the following steps:

[0044] Q1: Preparation of curing agent

[0045] 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;

[0046] Q2: Preparation of epoxy resin without benzene ring

[0047] Adding chain extenders methyl hexahydrophthalic anhydride and neopentyl glycol to the hydrogenated epoxy resin to obtain an epoxy resin with an epoxy equivalent weight of 1100-1200 and no benzene ring, and then diluting it with methyl isobutyl ketone to form an epoxy resin solution;

[0048] Q3: Preparation of main resin solution

[0049] 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;

[0050] 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;

[0051] Q4: Preparation of pigment dispersion resin

[0052] 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. A quaternizing agent is added to product C, and after cooling, a pigment dispersion resin is obtained. The obtained pigment dispersion resin is then added to the main resin solution, and mineral filler is added at the same time;

[0053] Q5: Preparation of emulsion

[0054] In an emulsifier, 30-40 parts of a main resin solution, 1-2 parts of an alcohol ether cosolvent, and 0.5-1 part of an organic acid or an inorganic acid are sequentially added by weight, and the mixture is stirred and mixed uniformly. 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 from the crude emulsion. 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 6 by adding a weak acid. Finally, the emulsion is filtered to obtain an emulsion with excellent stability.

[0055] Q6: Preparation of color paste

[0056] 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;

[0057] Q7: Preparation of bottom-to-top combined cathodic electrophoretic coating

[0058] The base and top combined 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.

[0059] The present invention provides a method for preparing a bottom-to-top integrated cathode electrophoretic coating. In step Q1, the isocyanate is a blocked isocyanate. The blocked isocyanate is prepared by mixing an aliphatic isocyanate hexamethylene diisocyanate (HDI) and an alcohol blocking agent methyl ethyl ketoxime in a molar ratio of 1:1 to 10, reacting at a temperature between room temperature and 80°C, and then performing separation and purification to separate the blocked isocyanate from unreacted blocking agent and other by-products. In this embodiment, the alcohol blocking agent methyl ethyl ketoxime needs to be added in excess to ensure that both the isocyanate hexamethylene diisocyanate (HDI) and the isocyanate hexamethylene diisocyanate (HDI) can participate in the reaction to form the blocked hexamethylene diisocyanate. As the amount of methyl ethyl ketoxime added increases, the performance of the blocked hexamethylene diisocyanate becomes increasingly stable, and the reaction temperature required for preparation also increases accordingly. Preferably, the molar ratio of hexamethylene diisocyanate (HDI):methyl ethyl ketoxime is 1:6, and the reaction temperature is 60°C.

[0060] In step Q1 of this embodiment, trimethylolpropane and an alcohol ether solvent are mixed and heated again, and a blocked isocyanate is added dropwise before the temperature reaches 60° C., and then the mixture is kept at 60° C. for 1 to 2 hours, heated to 70° C., and kept at 70° C. for 2 hours to obtain a curing agent; wherein 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.

[0061] The present invention provides a method for preparing a bottom-and-top integrated cathodic electrophoretic coating. In step Q2, epoxy resin and / or hydrogenated epoxy resin: methyl hexahydrophthalic anhydride (chain extender): neopentyl glycol (chain extender) are chain extended to an epoxy equivalent of 1100-1200 at a molar ratio of 1:0.3-0.7:0.05-0.2; 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 debenzenized epoxy resin solution is diluted to an appropriate concentration.

[0062] In this embodiment, the modified epoxy resin obtained by debenzenization can improve the compatibility of the epoxy resin and the acrylic resin, thereby improving interlayer adhesion. The modified epoxy resin has a higher molecular weight and a more stable chemical structure, which helps improve the overall stability of the coating and reduce performance changes caused by environmental factors (such as temperature and humidity). However, although debenzenization can improve the compatibility of the epoxy resin and the acrylic resin, this improvement may be limited, especially when a high proportion of acrylic resin is present. The limited compatibility improvement may cause the coating to still experience delamination or peeling problems 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 through heating or contact with a catalyst. These groups can react with hydroxyl groups or other active groups in the epoxy resin and acrylic resin to form stronger chemical bonds, thereby improving the adhesion of the coating and helping to alleviate the problem of delamination or peeling of the coating during long-term use. On the other hand, blocked aliphatic isocyanate curing agents can provide coatings with excellent weather resistance, resisting erosion from environmental factors such as ultraviolet rays and moisture, which helps improve the long-term stability and service life of the coating. Furthermore, blocked aliphatic isocyanate curing agents can react with epoxy resins and acrylic resins to form a denser and more uniformly thick coating, which helps improve the mechanical properties and protective properties of the coating, such as wear resistance and corrosion resistance.

[0063] In the preparation method of a bottom-and-top integrated cathodic electrophoretic coating provided by the present invention, the mineral filler comprises kaolin, carbon black, etc. The kaolin is added in an amount of 2% to 10% of the total coating, and the carbon black is added in an amount of 0.5% to 5% of the total coating. Mineral fillers generally have a large volume, and 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 coating can cover a larger area, thereby reducing the amount of coating 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 local film thickness. Mineral fillers can improve the durability of the coating, making it more resistant to weathering, chemical corrosion, and wear, and the coating can be thinner while still maintaining sufficient protective properties. Mineral fillers can increase the hardness and wear resistance of the coating, and the coating can be thinner while still maintaining sufficient strength and durability. In the present invention, a mineral filler composed of a mixture of kaolin and carbon black is used, wherein the kaolin has a particle size of 400 mesh or greater, and the carbon black has a pH of 8 and a medium-to-high chroma. Kaolin itself has good filling properties, while carbon black primarily provides coloring and hiding power. When the two are used in combination, the hiding power and filling properties of the coating can be further improved, making the coating more uniform and smooth. Furthermore, carbon black can improve the coating's weather resistance and chemical resistance, while kaolin can enhance the coating's mechanical properties. Therefore, when the two are used in combination, the durability and protective properties of the coating can be further improved, and it can also help further reduce the coating film thickness while improving the coating's weather resistance and salt spray resistance.

[0064] In step Q4 of this embodiment, the steps of preparing the pigment dispersion resin include:

[0065] S1. Under nitrogen protection, 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;

[0066] S2, dissolving 30-60 g of acrylate monomer and 0.05-0.2 g of initiator azobisisobutyronitrile in an appropriate amount of n-butanol solvent, raising the temperature to 85-105° C., and after complete dissolution, obtaining product B, which was added dropwise to product A in the three-necked flask, and the entire amount of the addition was controlled within 2 hours to obtain product C;

[0067] 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.

[0068] In step Q4 of this embodiment, the preparation steps of the quaternizing agent include:

[0069] S1. Add 85-90 g of xylene diisocyanate and an appropriate amount of solvent to a four-necked flask equipped with a stirrer, a thermometer, and a reflux condenser. Control the reaction temperature at 25-35°C. Add 46-53 g of a blocking agent dropwise over 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.

[0070] S2, heating to 45-55°C, dissolving 52-56 g of dimethylethanolamine in an appropriate amount of solvent, transferring the mixture to a dropping funnel, and adding the mixture dropwise to product 1 over 15-20 minutes. After the addition is complete, heating to 55-65°C and keeping the mixture warm for 1-1.5 hours, then heating to 80-90°C and keeping the mixture warm for 30-60 minutes to obtain product 2;

[0071] S3. Add acetic acid solution to product 2 to adjust the pH to neutral, and continue to keep the reaction warm for 30 to 40 minutes to obtain a quaternizing agent.

[0072] In step Q3 of this embodiment, the acrylic monomers are acrylic acid ester 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 a defoaming agent are added. 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.

[0073] Comparative Example 1:

[0074] In the preparation method of the bottom-top integrated cathode electrophoretic coating provided in this comparative example, open isocyanate is used in the preparation of the curing agent, and the other preparation processes are consistent with those in Example 1.

[0075] Comparative Example 2:

[0076] In the preparation method of the bottom-top integrated cathode electrophoretic coating provided in this comparative example, an epoxy resin solution containing a benzene ring is used to prepare the main resin solution, and the other preparation processes are consistent with those in Example 1.

[0077] Comparative Example 3:

[0078] In the preparation method of the bottom-top integrated cathode electrophoretic coating provided in this comparative example, no mineral filler is used in the preparation process of the main resin solution, and the other preparation processes are consistent with those in Example 1.

[0079] Comparative Example 4:

[0080] In the preparation method of the bottom-to-top integrated cathode 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 the other preparation processes remain consistent with those in Example 1.

[0081] As shown in the test content in the table below, the formulations in Examples 1-3 improve the salt spray resistance and weather resistance of the coating film relative to the formulations in Comparative Examples 1-4, and the coating film thickness can be reduced by adjusting the mixing ratio and mesh size of kaolin and carbon black without affecting other properties of the coating film.

[0082] 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

[0083] The above embodiments are merely exemplary embodiments of the present application and are not intended to limit the scope of the present application. The scope of protection of the present application is defined by the claims. Those skilled in the art may make various modifications or equivalent substitutions to the present application within the essence and scope of protection of the present application, and such modifications or equivalent substitutions shall also be deemed to fall within the scope of protection of the present application.

Claims

1. A method for preparing a bottom-to-top integrated cathodic 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 the hydrogenated epoxy resin to obtain an epoxy resin with an epoxy equivalent weight of 1100-1200 and no benzene ring, and then diluting it with methyl isobutyl ketone to form an epoxy resin solution; Q3: Preparation of main resin solution In a reaction kettle, acrylic acid monomer, a cationic group introducing agent, and an initiator are premixed, and 1 / 4 of the premix is ​​added to the epoxy resin solution in Q2 to initiate a monomer prepolymerization reaction; after the monomer prepolymerization reaction stage is completed, the remaining acrylic acid monomer premix is ​​continuously 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. A quaternizing agent is added to product C, and after cooling, a pigment dispersion resin is obtained. The obtained pigment dispersion resin is then 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 a main resin solution, 1-2 parts of an alcohol ether cosolvent, and 0.5-1 part of an organic acid or an inorganic acid are sequentially added by weight, and the mixture is stirred and mixed uniformly. 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 from the crude emulsion. 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. Finally, filtration is performed 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 combined cathodic electrophoretic coating The base and top combined 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 cathodic 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: aliphatic isocyanate hexamethylene diisocyanate (HDI) and an 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 cathodic 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. The mixture is then kept at 60°C for 1 to 2 hours, 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 cathodic 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 weight of 1100-1200; wherein the epoxy equivalent weight of the epoxy resin is 188, and the epoxy equivalent weight of the hydrogenated epoxy resin is between 780 and 850.

5. The method for preparing a bottom-to-top integrated cathodic 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-and-top integrated cathodic 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-and-top integrated cathodic 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-and-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 peroxy-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 cathodic electrophoretic coating according to claim 8, characterized in that: In the above-mentioned Q4, the steps of preparing the pigment dispersion resin include: S1. Under nitrogen protection, 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 30-60 g of acrylate monomer and 0.05-0.2 g of initiator azobisisobutyronitrile in an appropriate amount of n-butanol solvent, raising the temperature to 85-105° C., and after complete dissolution, obtaining product B, which was added dropwise to product A in the three-necked flask, and the entire amount of the addition was controlled 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-and-top integrated cathodic 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 to a four-necked flask equipped with a stirrer, a thermometer, and a reflux condenser. Control the reaction temperature at 25-35°C. Add 46-53 g of a blocking agent dropwise over 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 the mixture to a dropping funnel, and adding the mixture dropwise to product 1 over 15-20 minutes. After the addition is complete, heating to 55-65°C and keeping the mixture warm for 1-1.5 hours, then heating to 80-90°C and keeping the mixture warm 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 warm for 30 to 40 minutes to obtain a quaternizing agent.