A cathodic electrophoretic paint, its preparation method and use

Abstract

The application provides a cathode electrophoretic paint as well as a preparation method and application thereof. The cathode electrophoretic paint comprises a modified polyimide polymer, a blocked isocyanate, an acid, an organic solvent and water; at least one end of the modified polyimide polymer is a polyimide molecular chain, and the end of the polyimide molecular chain has a hydroxyalkyl-substituted amide group. The cathode electrophoretic paint has good high-temperature resistance, and the formed electrophoretic coating has high density and good thermal stability, which is beneficial to long-term and effective protection of the coating to metal workpieces.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrophoretic coating, in particular to a cathode electrophoretic paint, a preparation method and application thereof. BACKGROUND

[0002] Electrophoretic coating has the advantages of uniform film forming and environmental protection, and thus has a wide application scenario in the processing field of irregular metal workpieces such as industrial equipment and automobile bodies. Electrophoretic coating can be divided into anodic electrophoretic coating and cathodic electrophoretic coating. In anodic electrophoretic coating, the coated object is used as an anode. However, the anode metal is easy to lose electrons to form ionic contamination of the paint film, resulting in uneven color of the paint film, reduced physical and mechanical properties of the paint film, and reduced corrosion resistance. Compared with anodic electrophoretic coating, in cathodic electrophoretic coating, the coated object is used as a cathode, and the metal will not dissolve during electrophoresis, thereby ensuring that the paint forms a uniform film on the metal.

[0003] At present, existing cathode electrophoretic paint is mainly designed based on components such as polyurethane, polybutadiene resin, and acrylic resin. However, the above resins have poor high-temperature resistance, which is not conducive to the thermal stability of the coating, thereby limiting the application scenario of electrophoretic coating. SUMMARY

[0004] Therefore, the present application provides a new cathode electrophoretic paint. The cathode electrophoretic paint has good high-temperature resistance, and the electrophoretic coating formed thereby has high density and good thermal stability, which is conducive to long-term and effective protection of the coating on the metal workpiece.

[0005] The first aspect of the present application provides a cathode electrophoretic paint, comprising a modified polyimide polymer, a blocked isocyanate, an acid, an organic solvent, and water; at least one end of the modified polyimide polymer is a polyimide molecular chain, and the end of the polyimide molecular chain has a hydroxyalkyl-substituted amide group.

[0006] In the cathode electrophoretic paint provided by the present application, the hydroxyalkyl-substituted amide group in the modified polyimide polymer can form a quaternary ammonium cation under the action of the acid. The positively charged modified polyimide polymer enables the paint to be applied in cathodic electrophoretic coating. The hydroxyl group in the modified polyimide polymer can react with the blocked isocyanate to form a crosslinked network structure, thereby obtaining a coating with high density, ensuring that the coating has good insulating properties. In addition, since the polyimide has good high-temperature resistance, the coating can have good thermal stability.

[0007] Optionally, the number of hydroxyl groups in the hydroxyalkyl-substituted amide group is greater than or equal to 2.

[0008] Optionally, the end of the polyimide molecular chain also has a carboxyl group.

[0009] Optionally, the modified polyimide-based polymer is composed of polyimide molecular chains, and the molecular weight of the modified polyimide-based polymer is 3000 g / mol to 100000 g / mol.

[0010] Optionally, the modified polyimide-based polymer further comprises epoxy resin molecular chains, and the epoxy resin molecular chains are connected to the polyimide molecular chains through carbon-oxygen bonds.

[0011] Optionally, the epoxy resin molecular chains comprise one or more of glycidyl structures and non-glycidyl structures.

[0012] Optionally, the epoxy resin molecular chains comprise glycidyl ether structures.

[0013] Optionally, in the modified polyimide-based polymer, the mass ratio of the polyimide molecular chains to the epoxy resin molecular chains is (1-3):1.

[0014] Optionally, the cathode electrophoretic paint comprises the following components by weight: 15 parts to 35 parts of the modified polyimide-based polymer; 2 parts to 15 parts of blocked isocyanate; 0.2 parts to 2.0 parts of acid; 20 parts to 50 parts of organic solvent; and 25 parts to 60 parts of water.

[0015] Optionally, the acidity coefficient of the acid is less than or equal to 5.

[0016] Optionally, the acid comprises an organic acid, and the organic acid comprises one or more of formic acid, acetic acid, lactic acid, sulfamic acid, and guanidino acetic acid.

[0017] The molar ratio of the hydroxyl alkyl-substituted amide group in the modified polyimide-based polymer to the acid is 1:(0.1-0.8).

[0018] Optionally, the molar ratio of the hydroxyl group in the modified polyimide-based polymer to the blocked isocyanate is 1:(0.1-0.6).

[0019] Optionally, the blocked isocyanate is prepared from a blocking agent and an isocyanate, the blocking agent comprises one or more of lactam compounds, oxime compounds, phenolic compounds, aliphatic alcohol compounds, or ether alcohol compounds, and the isocyanate comprises one or more of aromatic polyisocyanate compounds and modified bodies thereof, and alicyclic polyisocyanate compounds and modified bodies thereof.

[0020] Optionally, the organic solvent comprises a water-soluble organic solvent, and the water-soluble organic solvent comprises one or more of N-methyl pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, γ-butyrolactone, γ-valerolactone, and sulfolane.

[0021] Secondly, this application provides a method for preparing a cathodic electrophoretic coating, comprising: reacting an anhydride-terminated polyimide with an organic amine to obtain a solution containing a modified polyimide polymer, wherein the organic amine includes one or more of primary or secondary amines, and the organic amine contains a hydroxyl substituent; and mixing the solution containing the modified polyimide polymer with a blocked isocyanate, an acid, and water to obtain the cathodic electrophoretic coating.

[0022] Optionally, the organic amine includes one or more of diethanolamine, N-ethylethanolamine, and N-methylethanolamine.

[0023] Optionally, the molar ratio of the anhydride group to the organic amine in the anhydride-terminated polyimide is 1:(0.2-1).

[0024] Optionally, the molecular weight of the anhydride-terminated polyimide is 3000 g / mol to 100000 g / mol.

[0025] Optionally, the anhydride-terminated polyimide is reacted with an organic amine to obtain a modified polyimide, wherein the modified polyimide has a carboxyl group at its end; the modified polyimide is then reacted with an epoxy resin to obtain a solution containing a modified polyimide polymer.

[0026] Optionally, the epoxy equivalent of the epoxy resin is 200 g / eq to 2500 g / eq.

[0027] Optionally, the molar ratio of the epoxy resin to the modified polyimide is 1:(0.4 to 1.2).

[0028] Thirdly, an application of a cathodic electrophoretic coating in electrophoretic coating involves placing a metal workpiece in a cathodic electrophoretic coating as described in the first aspect or in a cathodic electrophoretic coating prepared by the method described in the second aspect, with the metal workpiece serving as the cathode, and the cathodic electrophoretic coating forming an electrophoretic coating on the surface of the metal workpiece after electrodeposition and curing.

[0029] Optionally, the electrodeposition voltage is 40V to 120V, the electrodeposition time is 1min to 5min, the curing temperature is 50℃ to 200℃, and the curing time is 20min to 100min.

[0030] Optionally, the curing includes a first curing and a second curing, wherein the temperature of the first curing is 50℃~100℃ and the time of the first curing is 10min~50min; the temperature of the second curing is 100℃~200℃ and the time of the second curing is 10min~50min. Attached Figure Description

[0031] Figure 1 A flowchart illustrating the preparation process of a cathodic electrophoretic coating provided in one embodiment of this application;

[0032] Figure 2 A flowchart illustrating the preparation process of a cathodic electrophoretic coating provided in one embodiment of this application;

[0033] Figure 3 This is a synthetic route diagram for the anhydride-terminated polyimide provided in Example 1 of this application;

[0034] Figure 4 This is a synthetic route diagram of the modified polyimide polymer provided in Example 1 of this application;

[0035] Figure 5 This is a synthetic route diagram of the fully enclosed isocyanate provided in Example 1 of this application;

[0036] Figure 6 This is a synthetic route diagram of the modified polyimide polymer provided in Example 2 of this application;

[0037] Figure 7 Thermogravimetric analysis (TGA) curve of the amination-modified polyimide provided in Example 2 of this application;

[0038] Figure 8 Thermogravimetric analysis (TGA) diagram of E-51 epoxy resin provided in Example 2 of this application;

[0039] Figure 9 Thermogravimetric analysis (TGA) diagram of the polyimide-modified epoxy resin provided in Example 2 of this application. Detailed Implementation

[0040] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0041] Cathodic electrophoretic coating involves connecting a metal workpiece to the negative terminal of a power source, which becomes the electrophoretic cathode, and immersing it in an aqueous electrophoretic coating. Simultaneously, the electrophoretic anode, connected to the positive terminal of the power source, is also immersed in the coating and positioned directly opposite the metal workpiece. Under the influence of the electric field, positively charged coating particles migrate towards the cathode, gain electrons, and are deposited on the surface of the metal workpiece, thus forming a coating. The coating is then baked and cured to obtain a uniform, dense, and smooth finish. Currently, existing cathodic electrophoretic coatings primarily use polyurethane, polybutadiene resin, and acrylic resin as coating particles. However, these resins cannot withstand temperatures above 200°C for extended periods, thus limiting the application of cathodic electrophoretic coating in high-temperature applications.

[0042] This application provides a novel electrophoretic coating comprising the following components in parts by weight: 15 to 35 parts of modified polyimide polymer; 2 to 15 parts of blocked isocyanate; 0.2 to 2.0 parts of acid; 20 to 50 parts of organic solvent; and 25 to 60 parts of water. The modified polyimide polymer exhibits good high-temperature resistance, resulting in a coating with good thermal stability, which is beneficial for expanding the application scenarios of electrophoretic coating. The specific parts by weight of the modified polyimide polymer in the electrophoretic coating may be, but are not limited to, 15, 20, 25, 30, or 30 parts.

[0043] In the embodiments of this application, at least one end of the modified polyimide polymer is a polyimide molecular chain, and the end of the polyimide molecular chain has a hydroxyalkyl-substituted amide group. In some embodiments, the general structural formula of the polyimide molecular chain located on one side of the modified polyimide polymer is shown in formula (1-1) or formula (1-2):

[0044]

[0045]

[0046] In formula (1-1), at least one of R1 and R2 is a hydroxyl-substituted alkyl group, and the hydroxyl-substituted alkyl group has 1-10 carbon atoms; R3 can be a carboxyl group or an amide group. In formula (1-2), R1 is a hydroxyl-substituted alkyl group, and the hydroxyl-substituted alkyl group has 1-10 carbon atoms; R3 can be a carboxyl group or an amide group. In formulas (1-1) and (1-2), This represents a repeating unit of imide. Polyimide molecular chains are obtained by reacting diamines and dianhydrides. This is the structure after the reaction of dianhydrides. This is the structure after the reaction of diamine.

[0047] In this application, the polyimide molecular chain is located at at least one end of the modified polyimide polymer, and the end of the polyimide molecular chain has a hydroxyalkyl-substituted amide group. This amide group can form a quaternary ammonium cation under the action of acid, making the modified polyimide polymer positively charged, thus enabling its application in cathodic electrophoresis. The polyimide molecular chain in the modified polyimide polymer has excellent heat resistance (stable at 300℃), cold resistance (maintaining a certain degree of flexibility at -196℃), solvent resistance, radiation resistance, mechanical properties, and insulation properties. Therefore, the coating formed by the polymer can have good high and low temperature resistance, solvent resistance, radiation resistance, mechanical properties, and electrical insulation effect.

[0048] In some embodiments of this application, the nitrogen atom in the amide group is further connected to a hydroxyl-substituted alkyl group. The hydroxyl group can, on the one hand, improve the water solubility of the modified polyimide polymer, promoting the formation of a stable coating; on the other hand, the hydroxyl group and the blocked isocyanate can react to form a dense cross-linked network structure, which is beneficial for improving the insulation performance of the coating. In some embodiments of this application, the number of hydroxyl groups in the hydroxyl-alkyl substituted amide group is greater than or equal to 2, that is, the number of hydroxyl groups in R1 and R2 is greater than or equal to 2. A higher content of hydroxyl groups in the modified polyimide polymer is beneficial for increasing the water solubility of the modified polyimide polymer, thereby obtaining a uniform and stable electrophoretic emulsion. Furthermore, the modified polyimide polymer can also fully react with the blocked isocyanate to form a coating with good insulation properties. In some embodiments of this application, the ends of the polyimide molecular chains also have carboxyl groups. Carboxyl groups can also enhance the water solubility of the modified polyimide polymer, allowing the modified polyimide polymer to be uniformly dispersed in the coating, thereby forming a uniform and relatively dense electrophoretic coating.

[0049] In this application, the modified polyimide polymer is composed of polyimide molecular chains, and the modified polyimide polymer is a copolymer of polyimide. In some embodiments, the general structural formula of the modified polyimide polymer is shown in formula (1-3) or formula (1-4):

[0050]

[0051]

[0052] In some embodiments of this application, the molecular weight of the modified polyimide polymer composed of polyimide molecular chains is 3000 g / mol to 100000 g / mol. Specifically, the molecular weight of the modified polyimide polymer composed of polyimide molecular chains can be, but is not limited to, 3000 g / mol, 5000 g / mol, 8000 g / mol, 10000 g / mol, 30000 g / mol, 50000 g / mol, 80000 g / mol, or 100000 g / mol. In some embodiments, the molecular weight of the modified polyimide polymer composed of polyimide molecular chains is 5000 g / mol to 50000 g / mol. Controlling the molecular weight of the modified polyimide polymer allows the polymer to have moderate hydrophilicity, which on the one hand allows for stable dispersion in the coating, and on the other hand allows the polymer particles to separate from water during electrodeposition, thereby uniformly depositing on the workpiece surface to form a smooth surface and a coating of appropriate thickness, ensuring good insulation properties of the coating.

[0053] In some embodiments of this application, when the modified polyimide polymer is composed of polyimide molecular chains, the acid content in the coating can be appropriately reduced. In some embodiments, the cathodic electrophoretic coating includes the following components in the following mass percentages: 15 to 35 parts of modified polyimide polymer; 2 to 15 parts of blocked isocyanate; 0.2 to 1.0 parts of acid; 20 to 50 parts of organic solvent; and 25 to 60 parts of water.

[0054] In some embodiments of this application, the modified polyimide polymer further includes epoxy resin molecular chains, i.e., the modified polyimide polymer is a block copolymer of epoxy resin and polyimide. Although polyimide has excellent high-temperature resistance, it may separate from the substrate under environments with large temperature differences. Epoxy resin, on the other hand, has advantages such as high bonding strength, wide bonding surface, and good processing performance. Combining it with polyimide can achieve complementary performance, which is beneficial for obtaining a coating with strong adhesion. Moreover, epoxy resin has a lower cost, and its combination with polyimide can also reduce product costs. In some embodiments, the general structural formula of the modified polyimide polymer containing epoxy resin molecular chains is shown in formula (2-1) or formula (2-2):

[0055]

[0056]

[0057] In the embodiments of this application, the epoxy groups in the epoxy resin react with the carboxyl groups in the polyimide to form a block polymer, as shown in formulas (2-1) and (2-2). This indicates the structure of epoxy resin after reaction. In modified polyimide polymers, epoxy resin molecular chains are connected to each other through carbon-oxygen bonds.

[0058] In some embodiments of this application, the epoxy resin molecular chain includes one or more of a glycidyl structure and a non-glycidyl structure. In some embodiments, the epoxy resin molecular chain includes a glycidyl structure; further, the epoxy resin molecular chain includes a glycidyl ether structure. The ether bonds in the epoxy resin molecular chain segments with a glycidyl ether structure can undergo intramolecular rotation, thus the polymer chain has high flexibility, which is beneficial to improving the flexibility of the coating.

[0059] In some embodiments of this application, the mass percentage of epoxy resin molecular chains in the modified polyimide polymer is 25% to 50%. In some embodiments of this application, the mass ratio of polyimide molecular chains to epoxy resin molecular chains in the modified polyimide polymer is (1 to 3):1. The specific mass ratio of polyimide molecular chains to epoxy resin molecular chains can be, but is not limited to, 1:1, 1.5:1, 2:1, 2.5:1, or 3:1. Controlling the mass ratio of polyimide molecular chains to epoxy resin molecular chains is beneficial for obtaining modified polyimide polymers that possess both good high-temperature resistance and high adhesive strength, thereby ensuring that the coating can provide strong protection for the workpiece.

[0060] In some embodiments of this application, when the modified polyimide polymer is a block copolymer of epoxy resin and polyimide, the acid content in the coating will be relatively high. In some embodiments, the cathodic electrophoretic coating includes the following components in the following mass percentages: 15 to 35 parts of modified polyimide polymer; 2 to 15 parts of blocked isocyanate; 0.5 to 2.0 parts of acid; 20 to 50 parts of organic solvent; and 25 to 60 parts of water.

[0061] In this application, the blocked isocyanate is prepared from a blocking agent and an isocyanate. The blocked isocyanate can react with modified polyimide polymers to form a dense cross-linked network structure, which is beneficial to improving the insulation performance of the coating. In the cathodic electrophoretic coating, the weight parts of the blocked isocyanate can be, but are not limited to, 2 parts, 5 parts, 8 parts, 10 parts, 12 parts, or 15 parts. In some embodiments of this application, the blocking agent includes one or more of lactam compounds, oxime compounds, phenolic compounds, aliphatic alcohols, or ether alcohols; the isocyanate includes one or more of aromatic polyisocyanate compounds and their modified forms, and alicyclic polyisocyanate compounds and their modified forms. In some embodiments, the polyisocyanate includes aromatic polyisocyanate compounds and their modified forms. Aromatic polyisocyanate compounds contain aromatic rings and have high stability, which is beneficial to improving the high-temperature resistance of the coating. In some embodiments, the blocking agent includes one or more of diethylene glycol ethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, isooctanol, methyl ethyl ketone oxime, methyl pentanone oxime, acetone oxime, 3,5-dimethylpyrazole, p-tert-butylphenol, cresol, ε-caprolactam, and γ-butyrolactam; the polyisocyanate includes one or more of toluene diisocyanate, diphenylmethane-4,4'-diisocyanate, phenylenediamine diisocyanate, naphthalene-1,5-diisocyanate, and tetramethylphenylenediamine diisocyanate.

[0062] In some embodiments of this application, the molar ratio of hydroxyl groups to blocked isocyanate in the modified polyimide polymer is 1:(0.1–0.6). Specifically, this molar ratio can be, but is not limited to, 1:0.1, 1:0.3, 1:0.4, 1:0.5, or 1:0.6. In some embodiments, the molar ratio of hydroxyl alkyl-substituted amide groups to acid in the modified polyimide polymer is 1:(0.2–0.4). Controlling the relative content of blocked isocyanate to hydroxyl groups in the modified polyimide polymer is beneficial for the formation of a dense network structure in the polymer, thereby improving the uniformity and insulation of the coating.

[0063] In this embodiment, the acid in the electrophoretic coating can be any one of inorganic or organic acids. The modified polyimide polymer can form quaternary ammonium cations under the action of the acid. Under the action of an electric field, the positively charged modified polyimide polymer can be deposited at the cathode to obtain a coating. In some embodiments, the acidity coefficient is less than or equal to 5. In some embodiments, the acid includes an organic acid, including one or more of formic acid, acetic acid, lactic acid, aminosulfonic acid, and guanidinoacetic acid. In some embodiments, the acid includes lactic acid. In the cathodic electrophoretic coating, the weight parts of the acid can specifically be, but are not limited to, 0.2 parts, 0.3 parts, 0.5 parts, 0.8 parts, 1 part, 1.5 parts, 1.8 parts, or 2.0 parts.

[0064] In some embodiments of this application, the molar ratio of hydroxyalkyl-substituted amide groups to acid in the modified polyimide polymer is 1:(0.1–0.8). Specifically, this molar ratio can be, but is not limited to, 1:0.1, 1:0.3, 1:0.5, 1:0.6, or 1:0.8. In some embodiments, the molar ratio of hydroxyalkyl-substituted amide groups to acid in the modified polyimide polymer is 1:(0.4–0.6). Controlling the acid content relative to the hydroxyalkyl-substituted amide groups in the modified polyimide polymer ensures that the polymer particles in the coating have appropriate electrical properties, can be uniformly deposited on the workpiece surface under an electric field, and that the coating also exhibits good stability.

[0065] The cathodic electrophoretic coating provided in this application has the advantages of high penetration, high safety, and low volatile organic compounds and harmful air pollutants. The electrophoretic coating formed by it is uniform and dense, with excellent high temperature resistance and insulation properties, which is conducive to expanding the application of cathodic electrophoretic coating technology in the field of high temperature resistance.

[0066] This application also provides a method for preparing a cathodic electrophoretic coating; please refer to [link to relevant documentation]. Figure 1 , Figure 1 This is a flowchart illustrating the preparation process of a cathodic electrophoretic coating according to an embodiment of this application. The preparation method of the cathodic electrophoretic coating includes:

[0067] Step 100: React the anhydride-terminated polyimide with an organic amine to obtain a solution containing a modified polyimide polymer;

[0068] Step 200: The solution containing the modified polyimide polymer is mixed with the blocked isocyanate, acid and water to obtain the cathodic electrophoretic coating.

[0069] In step 100 of this application, since tertiary amines cannot react with anhydride-terminated polyimides (a-PI), the organic amine can be either a primary or secondary amine. In some embodiments, the organic amine is a secondary amine. Because primary amines still have exposed sites that can continuously react with anhydride-terminated polyimides, this leads to continuous intermolecular polymerization, resulting in an excessively large molecular weight of the product and reduced coating stability. Therefore, it is preferable for secondary amines to react with anhydride-terminated polyimides. In some embodiments, the organic amine includes one or more of diethanolamine, N-ethylethanolamine, and N-methylethanolamine. In some embodiments, the organic amine is a secondary amine containing two or more hydroxyl groups, such as diethanolamine. In the embodiments of this application, the organic amine contains hydroxyl substituents. Organic amines containing hydroxyl groups can, on the one hand, improve the water solubility of the polymer and form a stable coating; on the other hand, the hydroxyl groups and the blocked isocyanate react to form a dense cross-linked network structure, thereby improving the insulation performance of the coating.

[0070] In this application, when anhydride-terminated polyimide reacts with an organic amine, the anhydride in the polyimide undergoes a nucleophilic substitution reaction with the organic amine to generate an amide bond and simultaneously produce a carboxyl group. Specifically, the amine group acts as a nucleophile, attacking a carbonyl carbon atom in the anhydride, causing the anhydride to open its ring and form an amide bond, while simultaneously generating a carboxyl group. In some embodiments of this application, the reaction temperature of the anhydride-terminated polyimide with the organic amine is 20°C to 40°C, and the reaction time is 4 h to 10 h, yielding anamine-modified polyimide (d-PI) after the reaction. In some embodiments of this application, the molar ratio of the anhydride to the organic amine in the anhydride-terminated polyimide is 1:(0.2 to 1), and the molar ratio of the anhydride to the organic amine in the anhydride-terminated polyimide is 1:(0.5 to 0.9).

[0071] In some embodiments of this application, the anhydride-terminated polyimide can be prepared by the following methods:

[0072] (1) Add dianhydride monomer and diamine monomer to an organic solvent and react at 5℃~30℃ for 4~12h to obtain an anhydride-terminated polyamic acid (a-PAA) solution;

[0073] (2) Add an azeotropic solvent to the anhydride-terminated polyamic acid (a-PAA) solution, reflux at 170-180°C to remove water, and react for 2-8 hours to obtain an anhydride-terminated polyimide (a-PI) solution.

[0074] In step (1), the reaction atmosphere is an inert gas atmosphere, such as nitrogen; the organic solvent includes a water-soluble polar solvent, and in some embodiments, the organic solvent includes one or more of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, γ-butyrolactone, γ-valerolactone, and sulfolane. In embodiments of this application, a dianhydride monomer and a diamine monomer are added to the organic solvent to obtain a mixed solution, wherein the mass percentage of the organic solvent in the mixed solution is 70% to 90%, and further, the mass percentage of the organic solvent in the mixed solution is 75% to 85%.

[0075] In some embodiments of this application, the diamine monomer includes one or more of aliphatic compounds containing two primary amino groups and aromatic compounds containing two primary amino groups. Further, the diamine monomer includes aromatic compounds containing two primary amino groups. In some embodiments, the diamine includes m-phenylenediamine, p-phenylenediamine, 2,4-diaminotoluene, 4,4'-diamino-3,3'-dimethyl-1,1'-biphenyl, 4,4'-diamino-3,3'-dihydroxy-1,1'-biphenyl, 3,4'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfide, 2,2'-bis(4-aminophenyl)propane, 2,2'-bis(4-aminophenyl)hexafluoropropane, 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 4,4 One or more of '-bis(4-aminophenoxy)biphenyl, 1,3-bis[2-(4-aminophenyl)-2-propyl]benzene, 1,4-bis[2-(4-aminophenyl)-2-propyl]benzene, 2,2'-bis[4-(4-aminophenoxy)phenyl]propane, 2,2'-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, bis[4-(3-aminophenoxy)phenyl]sulfone, bis[4-(4-aminophenoxy)phenyl]sulfone, 2,6-diaminopyridine, 2,6-diamino-4-methylpyridine, 4,4'-(9-fluoreneyl)diphenylamine, and α,α-bis(4-aminophenyl)-1,3-diisopropylbenzene. In some embodiments of this application, the dianhydride monomer includes one or more of an aliphatic compound containing two anhydrides in its molecule and an aromatic compound containing two anhydrides in its molecule. Further, the dianhydride monomer includes an aromatic compound containing two anhydrides in its molecule. In some embodiments, the dianhydride includes one or more of pyromellitic dianhydride, 3,3',4,4'-biphenyltetrafluoropropane dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, 2,2'-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride, and 3,3',4,4'-biphenyl sulfone tetracarboxylic dianhydride.

[0076] In some embodiments of this application, the molar ratio of dianhydride monomer to diamine monomer is 1:(0.6-0.95), and further, the molar ratio is 1:(0.75-0.85). Controlling the relative content of diamine and dianhydride ensures sufficient reaction between the diamine and dianhydride, and enables the formation of anhydride-terminated structures. In some embodiments of this application, the molecular weight of the anhydride-terminated polyamic acid obtained after the reaction of the dianhydride monomer and diamine monomer is 3000 g / mol to 100000 g / mol, and further, the molecular weight of the anhydride-terminated polyamic acid is 5000 g / mol to 50000 g / mol.

[0077] In step (2), the azeotropic solvent includes one or more of toluene and xylene. The azeotropic solvent is mixed with the anhydride-terminated polyamic acid solution to obtain a mixture. The mass percentage of the azeotropic solvent in the mixture is 5% to 50%, and further, the mass percentage of the azeotropic solvent in the mixture is 20% to 40%. During the reflux water separation process, the reaction is terminated when no water increases in the water separator, resulting in an anhydride-terminated polyimide solution with a certain molecular weight. In some embodiments of this application, the molecular weight of the anhydride-terminated polyimide is 3000 g / mol to 100000 g / mol, and further, the molecular weight of the anhydride-terminated polyimide is 5000 g / mol to 50000 g / mol.

[0078] In step 200, the conditions for mixing the modified polyimide polymer solution with the blocked isocyanate, acid, and water include: stirring each component at 20–40°C for 1–3 hours to obtain a cathodic electrophoretic coating. In embodiments of this application, the blocked isocyanate is formed by reacting a blocking agent with an isocyanate. In some embodiments of this application, the method for preparing the blocked isocyanate includes: adding a blocking agent to the isocyanate and reacting at 50–100°C for 4–10 hours to obtain a blocked isocyanate curing agent. In some embodiments, the blocking agent is a small molecule compound containing active hydrogen, such as alcohol ethers, oximes, phenols, or lactams. Further, the blocking agent includes one or more of diethylene glycol ethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, isooctyl alcohol, methyl ethyl ketone oxime, methyl pentanone oxime, acetone oxime, 3,5-dimethylpyrazole, p-tert-butylphenol, cresol, ε-caprolactam, and γ-butyrolactam. In some embodiments, the isocyanate includes aliphatic and / or aromatic diisocyanates, triisocyanates, and modified forms thereof. In some embodiments, one or more of toluene diisocyanate, diphenylmethane-4,4'-diisocyanate, phenylenediamine diisocyanate, naphthalene-1,5-diisocyanate, and tetramethylphenylenediamine diisocyanate are used.

[0079] In some embodiments of this application, the modified polyimide polymer in the cathodic electrophoretic coating also contains epoxy resin molecular chains. Please refer to [link to relevant documentation]. Figure 2 , Figure 2 This is a flowchart illustrating the preparation process of a cathodic electrophoretic coating according to an embodiment of this application. The preparation method of the cathodic electrophoretic coating includes:

[0080] Step 100: React the anhydride-terminated polyimide with an organic amine to obtain a solution containing modified polyimide;

[0081] Step 200: Add epoxy resin to the solution containing modified polyimide, and after the reaction, obtain a solution containing modified polyimide polymer.

[0082] Step 300: The solution containing the modified polyimide polymer is mixed with the blocked isocyanate, acid and water to obtain the cathodic electrophoretic coating.

[0083] In step 200, the reaction temperature between the modified polyimide and the epoxy resin is 10°C to 50°C, and the reaction time is 4 hours to 10 hours. In some embodiments of this application, the epoxy resin includes one or more of glycidyl ether epoxy resins and non-glycidyl ether epoxy resins. In some embodiments, the epoxy resin includes glycidyl ether epoxy resins. In some embodiments, the epoxy resin includes one or more of bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, and hydrogenated bisphenol A type epoxy resin. In some embodiments of this application, the epoxy equivalent of the epoxy resin is 200 g / eq to 2500 g / eq, and more specifically, the epoxy equivalent of the epoxy resin is 200 g / eq to 1000 g / eq. The higher the epoxy equivalent, the larger the molecular weight of the epoxy resin. Therefore, the epoxy equivalent of the epoxy resin affects the molecular weight of the modified polyimide polymer. Moreover, epoxy resin contains hydrophobic groups (such as phenyl, ester, alkyl, etc.). Increasing the epoxy equivalent will increase the hydrophobicity of the modified polyimide polymer, and the particle size of the electrophoretic emulsion will also increase. Therefore, controlling the epoxy equivalent of the epoxy resin can adjust the particle size of the polymer in the electrophoretic coating, which is beneficial to improving the storage stability of the polymer in the coating. This results in a coating with high stability, moderate hardness, good insulation properties, good adhesion to metal workpieces, and high density.

[0084] In some embodiments of this application, the molar ratio of epoxy resin to modified polyimide is 1:(0.4 to 1.2), and the molar ratio of epoxy resin to modified polyimide may specifically be, but is not limited to, 1:0.4, 1:0.8, 1:1, or 1:1.2. In some embodiments, the molar ratio of epoxy resin to modified polyimide is 1:(0.6 to 1).

[0085] The method for preparing the cathodic electrophoretic coating provided in this application firstly synthesizes anhydride-terminated polyimide using dianhydride monomers and diamine monomers, then modifies it with organic amines to obtain modified polyimide, and then adds blocked isocyanate, acid and water and stirs to obtain a stable cathodic electrophoretic coating. This coating has good uniformity and fluidity, and when applied to electrophoretic coating, it can prepare a coating with good thermal stability.

[0086] This application also provides the application of the above-mentioned cathodic electrophoretic coating in electrophoretic coating, which includes: surface degreasing, alkaline washing, acid washing, and pure water washing of the metal workpiece; then placing the metal workpiece in the cathodic electrophoretic coating, with the metal workpiece as the cathode and another metal plate connected to the positive terminal of the power supply as the anode; electrophoresis at 40V to 120V for 1 min to 5 min; after electrophoresis, removing the metal workpiece, washing away the residual electrophoretic coating on the coating surface with water, and then curing the coating to obtain the coating; wherein the electrophoresis temperature is 25 to 30℃. In some embodiments of this application, the curing temperature is 50℃ to 200℃, and the curing time is 20 min to 100 min. The curing temperature can be, but is not limited to, 50℃, 80℃, 100℃, 120℃, 150℃, or 200℃. During the curing process, the blocked isocyanate reacts with the hydroxyl groups in the modified polyimide polymer to form a dense coating. In some embodiments of this application, the coating is subjected to a two-stage curing process: the first curing temperature is 50°C to 100°C and the first curing time is 10 min to 50 min; the second curing temperature is 100°C to 200°C and the second curing time is 10 min to 50 min.

[0087] The cathodic electrophoretic coating provided in this application can be used to prepare electrophoretic coatings with high crosslinking degree and density, as well as good thermal stability and insulation properties, and has broad application prospects.

[0088] The technical solution of this application will be further described below with reference to several embodiments.

[0089] Example 1

[0090] A method for preparing a cathodic electrophoretic coating, comprising:

[0091] 1. Preparation of anhydride-terminated polyimides:

[0092] Under a nitrogen atmosphere, 17.45 g (80 mmol) of pyromellitic dianhydride (PDMA) and 15.02 g (75 mmol) of 4,4'-diaminodiphenyl ether (ODA) were added to 100 g of N-methylpyrrolidone solvent, and the mixture was reacted in an ice-water bath (0 °C) for 6 hours to obtain a number-average molecular weight M. n It is a polyamic acid (a-PAA) solution with an anhydride end capping at 12700 g / mol.

[0093] Add 20 g of xylene to an anhydride-terminated polyamic acid (a-PAA) solution, reflux at 175°C for 4 hours until no more water is added in the separator, yielding an anhydride-terminated polyimide (a-PI) solution. (See [link to relevant documentation]). Figure 3 , Figure 3This is a synthetic route diagram for the anhydride-terminated polyimide provided in Example 1 of this application.

[0094] 2. Preparation of modified polyimide polymers:

[0095] Add 0.54 g (5.11 mmol) of diethanolamine to an anhydride-terminated polyimide (a-PI) solution and react at 25 °C for 4 hours to obtain anamine-modified polyimide (d-PI) solution, which is a modified polyimide polymer solution. (See also: [link to relevant documentation]) Figure 4 , Figure 4 This is a synthetic route diagram for the modified polyimide polymer provided in Example 1 of this application.

[0096] 3. Preparation of blocked isocyanates:

[0097] Add 47.86 g (405 mmol) of ethylene glycol monobutyl ether to 50 g (200 mmol) of diphenylmethane-4,4'-diisocyanate, and react at 100 °C for 4 hours to obtain a fully enclosed isocyanate curing agent (e-MDI). (See also...) Figure 5 , Figure 5 This is a synthetic route diagram for the fully enclosed isocyanate provided in Example 1 of this application.

[0098] 4. Preparation of cathodic electrophoretic coating:

[0099] Take 1.01 g (2.04 mmol) of fully enclosed isocyanate curing agent (e-MDI), add it to the modified polyimide polymer solution and stir for 30 min. Then add 0.14 g (1.54 mmol) of lactic acid and 36 g of deionized water, and stir at 25 °C for 1 hour to obtain a stable cathodic electrophoretic coating with a solid content of 20%.

[0100] A method for preparing an electrophoretic coating, comprising:

[0101] Metal workpieces, after surface degreasing, alkaline washing, acid washing, and pure water washing, are immersed in the aforementioned cathodic electrophoretic coating. One metal plate is connected to the negative terminal of the power supply as the cathode, and another metal plate is connected to the positive terminal as the anode. The electrophoresis temperature is maintained at 25°C. Electrophoresis begins with the power applied, and the voltage is controlled between 40V and 120V. The electrophoresis time is 1–3 minutes (specific electrophoresis conditions are shown in Table 1). After this process, a uniform and dense coating is formed on the surface of the metal workpiece. After electrophoretic deposition, the residual electrophoretic coating is washed away with pure water. The coating then undergoes a two-stage heating and curing process: maintaining the temperature at 80°C for 30 minutes, and then raising the temperature to 180°C and maintaining it for 30 minutes, resulting in the electrophoretic coating.

[0102] Example 2

[0103] A method for preparing a cathodic electrophoretic coating, comprising:

[0104] 1. Preparation of anhydride-terminated polyimides:

[0105] Under a nitrogen atmosphere, 17.45 g (80 mmol) of pyromellitic dianhydride (PDMA) and 12.01 g (60 mmol) of 4,4'-diaminodiphenyl ether (ODA) were added to 100 g of N-methylpyrrolidone solvent, and the mixture was reacted in an ice-water bath (0 °C) for 6 hours to obtain a number-average molecular weight M. n It is a 3700 g / mol solution of anhydride-terminated polyamic acid (a-PAA).

[0106] Add 20g of xylene to an anhydride-terminated polyamic acid (a-PAA) solution, reflux at 180℃ to separate water, and react for 4 hours until no water increases in the separator to obtain an anhydride-terminated polyimide (a-PI) solution.

[0107] 2. Preparation of modified polyimide polymers:

[0108] Add 1.67 g (15.92 mmol) of diethanolamine to an anhydride-terminated polyimide (a-PI) solution and react at 25 °C for 4 hours to obtain an amination-modified polyimide (d-PI) solution. Add 4.96 g (26.53 mmol) of E51 epoxy resin (epoxy equivalent 184–190 g / eq) to the amination-modified polyimide (d-PI) solution and react at 50 °C for 4 hours to obtain a polyimide-modified epoxy resin (PI-b-EP) solution. This yields a modified polyimide polymer solution. Please refer to [link to relevant documentation]. Figure 6 , Figure 6 This is a synthetic route diagram for the modified polyimide polymer provided in Example 2 of this application.

[0109] A fully enclosed isocyanate curing agent (e-MDI) was prepared using the same method as in Example 1. 3.08 g (6.36 mmol) of the fully enclosed isocyanate curing agent (e-MDI) was added to a modified polyimide polymer solution and stirred for 30 min. Then, 0.86 g (9.55 mmol) of lactic acid and 45 g of deionized water were added, and the mixture was stirred at 25 °C for 1 hour to obtain a stable cathodic electrophoretic coating with a solid content of 20%.

[0110] The cathodic electrophoretic coating obtained above was used to prepare an electrophoretic coating in the same way as in Example 1.

[0111] Example 3

[0112] A method for preparing a cathodic electrophoretic coating, comprising:

[0113] 1. Preparation of anhydride-terminated polyimides:

[0114] Under a nitrogen atmosphere, 17.45 g (80 mmol) of pyromellitic dianhydride (PDMA) and 15.02 g (75 mmol) of 4,4'-diaminodiphenyl ether (ODA) were added to 100 g of N-methylpyrrolidone solvent, and the mixture was reacted in an ice-water bath (0 °C) for 6 hours to obtain a number-average molecular weight M. n It is a polyamic acid (a-PAA) solution with an anhydride end capping at 12700 g / mol.

[0115] Add 20g of xylene to an anhydride-terminated polyamic acid (a-PAA) solution, reflux at 180℃ to separate water, and react for 4 hours until no water increases in the separator to obtain an anhydride-terminated polyimide (a-PI) solution.

[0116] 2. Preparation of modified polyimide polymers:

[0117] 0.54 g (5.11 mmol) of diethanolamine was added to an anhydride-terminated polyimide (a-PI) solution, and the reaction was carried out at 25 °C for 4 hours to obtain an amination-modified polyimide (d-PI) solution. 1.59 g (8.52 mmol) of E51 epoxy resin (epoxy equivalent 184-190 g / eq) was added to the amination-modified polyimide (d-PI) solution, and the reaction was carried out at 50 °C for 4 hours to obtain a polyimide-modified epoxy resin (PI-b-EP) solution, which is a modified polyimide polymer solution.

[0118] 3. Preparation of cathodic electrophoretic coating:

[0119] The fully enclosed isocyanate curing agent was prepared using the same method as in Example 1. 1.01 g (2.04 mmol) of the fully enclosed isocyanate curing agent (e-MDI) was added to the modified polyimide polymer solution and stirred for 30 min. Then, 0.28 g (3.07 mmol) of lactic acid and 38 g of deionized water were added, and the mixture was stirred at 25 °C for 1 hour to obtain a stable cathodic electrophoretic coating with a solid content of 20%.

[0120] The cathodic electrophoretic coating obtained above was used to prepare an electrophoretic coating in the same way as in Example 1.

[0121] Example 4

[0122] 1. Preparation of modified polyimide polymers:

[0123] An amination-modified polyimide (d-PI) solution was prepared using the same method as in Example 1. 12.60 g (26.53 mmol) of E21 epoxy resin (epoxy equivalent 450-500 g / eq) was added to the amination-modified polyimide (d-PI) solution, and the reaction was carried out at 50 °C for 4 hours to obtain a polyimide-modified epoxy resin (PI-b-EP) solution, which is a modified polyimide polymer solution.

[0124] 2. Preparation of cathodic electrophoretic coating:

[0125] The fully enclosed isocyanate curing agent was prepared using the same method as in Example 1. 3.08 g (6.36 mmol) of the fully enclosed isocyanate curing agent (e-MDI) was added to the modified polyimide polymer solution and stirred for 30 min. Then, 0.86 g (9.55 mmol) of lactic acid and 45 g of deionized water were added, and the mixture was stirred at 25 °C for 1 hour to obtain a stable cathodic electrophoretic coating with a solid content of 20%.

[0126] The cathodic electrophoretic coating obtained above was used to prepare an electrophoretic coating in the same way as in Example 1.

[0127] Example 5

[0128] A method for preparing a cathodic electrophoretic coating, comprising:

[0129] 1. Preparation of anhydride-terminated polyimides:

[0130] Under a nitrogen atmosphere, 25.78 g (80 mmol) of 3,3',4,4'-benzophenone tetracarboxylic dianhydride and 12.01 g (60 mmol) of 4,4'-diaminodiphenyl ether were added to 100 g of N-methylpyrrolidone solvent, and the mixture was reacted in an ice-water bath (0 °C) for 6 hours to obtain a number-average molecular weight M. n It is a polyamic acid (a-PAA) solution with an anhydride end-capped concentration of 4500 g / mol.

[0131] Add 20g of xylene to an anhydride-terminated polyamic acid (a-PAA) solution, reflux at 180℃ to separate water, and react for 4 hours until no water increases in the separator to obtain an anhydride-terminated polyimide (a-PI) solution.

[0132] 2. Preparation of modified polyimide polymers:

[0133] 1.76 g (16.79 mmol) of diethanolamine was added to an anhydride-terminated polyimide (a-PI) solution, and the reaction was carried out at 25 °C for 4 hours to obtain an amination-modified polyimide (d-PI) solution. 5.23 g (27.98 mmol) of E51 epoxy resin (epoxy equivalent 184–190 g / eq) was added to the amination-modified polyimide (d-PI) solution, and the reaction was carried out at 50 °C for 4 hours to obtain a polyimide-modified epoxy resin (PI-b-EP) solution, thus obtaining a modified polyimide polymer solution.

[0134] 3. Preparation of cathodic electrophoretic coating:

[0135] The fully enclosed isocyanate curing agent was prepared using the same method as in Example 1. 3.28 g (6.72 mmol) of the fully enclosed isocyanate curing agent (e-MDI) was added to the modified polyimide polymer solution and stirred for 30 min. Then, 0.91 g (10.07 mmol) of lactic acid and 45 g of deionized water were added, and the mixture was stirred at 25 °C for 1 hour to obtain a stable cathodic electrophoretic coating with a solid content of 20%.

[0136] The cathodic electrophoretic coating obtained above was used to prepare an electrophoretic coating in the same way as in Example 1.

[0137] Example 6

[0138] 1. Preparation of modified polyimide polymers:

[0139] An anhydride-terminated polyimide (a-PI) solution was prepared using the same method as in Example 3. 0.46 g (5.11 mmol) of N-ethylethanolamine was added to the anhydride-terminated polyimide (a-PI) solution, and the reaction was carried out at 25°C for 4 hours to obtain an amination-modified polyimide (d-PI) solution. 1.59 g (8.52 mmol) of E51 epoxy resin (epoxy equivalent 184–190 g / eq) was added to the amination-modified polyimide (d-PI) solution, and the reaction was carried out at 50°C for 4 hours to obtain a polyimide-modified epoxy resin (PI-b-EP) solution, thus obtaining a modified polyimide polymer solution.

[0140] 2. Preparation of cathodic electrophoretic coating:

[0141] Preparation of cathodic electrophoretic coating:

[0142] The fully enclosed isocyanate curing agent was prepared using the same method as in Example 1. 3.08 g (6.36 mmol) of the fully enclosed isocyanate curing agent (e-MDI) was added to the modified polyimide polymer solution and stirred for 30 min. Then, 0.86 g (9.55 mmol) of lactic acid and 45 g of deionized water were added, and the mixture was stirred at 25 °C for 1 hour to obtain a stable cathodic electrophoretic coating with a solid content of 20%.

[0143] The cathodic electrophoretic coating obtained above was used to prepare an electrophoretic coating in the same way as in Example 1.

[0144] Comparative Example 1

[0145] 187 g (1 mol) of E-51 epoxy resin, 63.46 g (0.278 mol) of bisphenol A, and 52.89 g (0.200 mol) of dodecylphenol were added sequentially to a flask. The mixture was heated to 120 °C, and after the bisphenol A was completely dissolved, 0.27 g (0.002 mol) of N,N-dimethylbenzylamine catalyst was added, and the temperature was raised to 150 °C. The reaction system released a large amount of heat and was heated to 180 °C. The system was then slowly cooled to 150 °C, and the epoxy value was measured until it reached 1000 g / eq.

[0146] The system was rapidly cooled to below 110°C, 100g of propylene glycol monobutyl ether was added, and the mixture was stirred for 30 minutes. After the chain-extended epoxy resin was completely dissolved, the temperature was lowered to 90°C, 25.51g (0.243mol) of diethanolamine was added, and the mixture was kept at this temperature for 4 hours to allow the epoxy resin to fully open its ring.

[0147] The temperature was lowered to 60℃, the amine value was measured, and then 8.75g (0.097mol) of lactic acid was added and stirred for 30min. Then 47.34g (0.097mol) of fully enclosed isocyanate curing agent (e-MDI, prepared in the same way as in Example 1) was added and stirred for 30min. Under stirring at 1500r / min, 1220g of deionized water was slowly added to the cationic epoxy resin prepared above, stirred for 30min, allowed to stand to defoam and filtered to obtain a self-emulsifying cationic epoxy emulsion with a solid content of 20%.

[0148] Metal workpieces, after surface degreasing, alkaline washing, acid washing, and pure water rinsing, are immersed in the aforementioned electrophoretic coating. One metal plate is connected to the negative terminal of the power supply as the cathode, and another metal plate is connected to the positive terminal as the anode. The electrophoresis temperature is maintained at 25–30°C. Electrophoresis begins with the power applied, and the voltage is controlled at 50V–200V. After 5 minutes of electrophoresis, a uniform and dense coating is formed on the metal surface. After electrophoretic deposition, the residual electrophoretic coating is washed away with pure water. The coating then undergoes a two-stage curing process: 80°C for 30 minutes, followed by a temperature increase to 180°C and a 30-minute curing period.

[0149] Effect Example

[0150] To verify the performance of the cathodic electrophoretic coating and electrophoretic coating prepared in this application, this application also provides effective examples.

[0151] 1) The amination-modified polyimide, E-51 epoxy resin, and polyimide-modified epoxy resin in Example 2 were characterized using differential scanning calorimetry and thermogravimetric analysis. The testing standard was GB / T 13464-2008. Please refer to [link / reference needed]. Figures 7-9 , Figure 7 This is a thermogravimetric analysis (TGA) chart of the amination-modified polyimide provided in Example 2 of this application. Figure 8 The thermogravimetric analysis (TGA) diagram of E-51 epoxy resin provided in Example 2 of this application is shown. Figure 9 This is a thermogravimetric analysis (TG) curve of the polyimide-modified epoxy resin provided in Example 2 of this application, where the light-colored curve represents the TG (thermogravimetric) curve and the dark-colored curve represents the DSC (thermo-current coefficient) curve. Figure 7 It can be seen that the amination-modified polyimide prepared in Example 2 begins to decompose at around 500℃, and reaches a pyrolysis rate of 50% at 560℃. Figure 8 It can be seen that E-51 epoxy resin begins to decompose at around 230℃, and reaches a pyrolysis rate of 50% at 340℃. Figure 9 It can be seen that the polyimide-modified epoxy resin prepared in Example 2 begins to decompose at around 250°C and reaches a pyrolysis rate of 50% at 400°C. This shows that combining amination-modified polyimide with epoxy resin can significantly improve the thermal stability of epoxy resin polymers.

[0152] 2) Film thickness test:

[0153] The film thickness data of different regions in the electrophoretic coatings obtained in Examples 1-6 and Comparative Example 1 were measured using a film thickness gauge. At least eight different regions were selected for the same coating. The preparation conditions and test results of the electrophoretic coatings are shown in Table 1.

[0154] Table 1. Preparation and structural parameters of the electrophoretic coatings in each embodiment and comparative example.

[0155]

[0156] As shown in Table 1, when preparing electrophoretic coatings of the same thickness, compared with the epoxy resin electrophoretic coatings in the comparative examples, the amination-modified polyimide electrophoretic coating obtained in Example 1 of this application and the polyimide-modified epoxy resin electrophoretic coatings in Examples 2-6 require lower electrophoretic voltages, shorter times, and are more energy-efficient and safer. Furthermore, the coatings in the examples obtained by the electrophoretic coating process have uniform thicknesses, resulting in a uniform and dense coating.

[0157] 3) Adhesion test: The adhesion of the electrophoretic coatings obtained in Examples 1-6 and Comparative Example 1 was tested according to GB / T 9286 "Cross-cut test of paint and varnish film". The test results are shown in Table 2.

[0158] Room temperature insulation test: The electrophoretic coatings obtained in Examples 1-6 and Comparative Example 1 were subjected to withstand voltage tests according to GB / T 1408.1-2006 "Test Methods for Electrical Strength of Insulating Materials Part 1: Tests at Power Frequency". The requirements were: withstand voltage for 60 seconds at 1.5kV AC with a leakage current less than 10mA; withstand voltage for 60 seconds at 3kV DC with a leakage current less than 10mA. The test results are shown in Table 2.

[0159] Table 2. Adhesion and room temperature insulation properties of the electrophoretic coatings in each embodiment and comparative example.

[0160]

[0161] High temperature resistance test: The electrophoretic coatings obtained in Examples 1-6 and Comparative Example 1 were tested for high temperature resistance. The test method was as follows: the product was stored at 500℃ for 10 minutes and the appearance of the product was observed for peeling or flaking. The test results are shown in Table 3.

[0162] High-Temperature Coating Insulation Test: After the high-temperature test, a withstand voltage test was conducted according to GB / T 1408.1-2006 "Test Methods for Electrical Strength of Insulating Materials Part 1: Tests at Power Frequency". The requirements were: withstand voltage for 60 seconds at AC 1.5kV with a leakage current less than 10mA; withstand voltage for 60 seconds at DC 3kV with a leakage current less than 10mA. The test results are shown in Table 3.

[0163] Table 3. High-temperature resistance and post-high-temperature insulation properties of the electrophoretic coatings in each embodiment and comparative example.

[0164]

[0165] As shown in Tables 2 and 3, the epoxy resin electrophoretic coating of Comparative Example 1 exhibits good adhesion and stable insulation performance at room temperature. The leakage current at 1.5kV AC is between 230-310μA, and at 3kV DC is only a few microamps. However, the coating is damaged at high temperatures, losing its insulating properties and failing to meet insulation requirements under high-temperature conditions. The electrophoretic coating of Example 1 demonstrates good insulation performance at both room and high temperatures, but its coefficient of thermal expansion differs significantly from that of the substrate, making it prone to separation and exhibiting poor adhesion. The polyimide-modified epoxy resin electrophoretic coatings obtained in Examples 2-5 exhibited good adhesion after electrophoretic coating, maintaining a good appearance even at high temperatures. Furthermore, the resulting electrophoretic coatings demonstrated stable insulation performance at both room and high temperatures, with leakage currents ranging from 250-280 μA at room temperature (AC 1.5kV) and 310-340 μA at high temperature (AC 1.5kV). The leakage current at both room temperature and high temperature (DC 3kV) was only a few microamps. The polyimide-modified epoxy resin electrophoretic coating obtained in Example 6 showed good adhesion after electrophoretic coating, but its insulation performance was slightly inferior to the coatings of Examples 2-5. This is because the organic amine hydroxyl content in Example 6 was lower, resulting in reduced emulsion stability and slightly lower density of the cured coating, leading to slightly poorer insulation performance. Therefore, the electrophoretic coatings of Examples 2-5 of this invention exhibited the most superior overall performance at high temperatures.

[0166] The above description represents the preferred embodiments of this application, but should not be construed as limiting the scope of this application. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of this application, and these improvements and modifications are also considered to be within the scope of protection of this application.

Claims

1. A cathodic electrophoretic coating, characterized in that, The mixture includes a modified polyimide polymer, a blocked isocyanate, an acid, an organic solvent, and water; at least one end of the modified polyimide polymer is a polyimide molecular chain, and the end of the polyimide molecular chain has a hydroxyalkyl-substituted amide group; the modified polyimide polymer further includes an epoxy resin molecular chain, the epoxy resin molecular chain and the polyimide molecular chain being connected by a carbon-oxygen bond; in the modified polyimide polymer, the mass ratio of the polyimide molecular chain to the epoxy resin molecular chain is (1~3):

1.

2. The cathodic electrophoretic coating as described in claim 1, characterized in that, In the hydroxyalkyl-substituted amide group, the number of hydroxyl groups is greater than or equal to 2.

3. The cathodic electrophoretic coating as described in claim 1 or 2, characterized in that, The modified polyimide polymer is composed of polyimide molecular chains, and the molecular weight of the modified polyimide polymer is 3000 g / mol to 100000 g / mol.

4. The cathodic electrophoretic coating as described in claim 3, characterized in that, The epoxy resin molecular chain includes one or more of a glycidyl structure and a non-glycidyl structure.

5. The cathodic electrophoretic coating as described in claim 1 or 2, characterized in that, The cathodic electrophoretic coating comprises the following components in parts by weight: 15 to 35 parts of modified polyimide polymer; 2 to 15 parts of blocked isocyanate; and 0.2 to 2.0 parts of acid. Organic solvent 20-50 parts; water 25-60 parts.

6. The cathodic electrophoretic coating as described in claim 1 or 2, characterized in that, The acidity coefficient of the acid is less than or equal to 5.

7. The cathodic electrophoretic coating as described in claim 1 or 2, characterized in that, The molar ratio of the hydroxyalkyl-substituted amide group in the modified polyimide polymer to the acid is 1:(0.1~0.8).

8. The cathodic electrophoretic coating as described in claim 1 or 2, characterized in that, The molar ratio of hydroxyl groups in the modified polyimide polymer to the blocked isocyanate is 1:(0.1~0.6).

9. A method for preparing a cathodic electrophoretic coating as described in any one of claims 1-8, characterized in that, include: A modified polyimide is obtained by reacting an anhydride-terminated polyimide solution with an organic amine, wherein the modified polyimide has a carboxyl group at its end. The modified polyimide is reacted with an epoxy resin to obtain a solution containing a modified polyimide polymer, wherein the organic amine includes one or more of primary or secondary amines, the organic amine contains a hydroxyl substituent, and the anhydride-terminated polyimide solution includes an organic solvent. The solution containing the modified polyimide polymer is mixed with a blocked isocyanate, an acid, and water to obtain a cathodic electrophoretic coating.

10. The method for preparing the cathodic electrophoretic coating as described in claim 9, characterized in that, The organic amine includes one or more of diethanolamine, N-ethylethanolamine, and N-methylethanolamine.

11. The method for preparing the cathodic electrophoretic coating as described in claim 9 or 10, characterized in that, The molar ratio of the anhydride group to the organic amine in the anhydride-terminated polyimide is 1:(0.2~1).

12. The method for preparing the cathodic electrophoretic coating as described in claim 9 or 10, characterized in that, The anhydride-terminated polyimide has a molecular weight of 3000 g / mol to 100000 g / mol.

13. The method for preparing the cathodic electrophoretic coating as described in claim 12, characterized in that, The epoxy equivalent of the epoxy resin is 200 g / eq to 2500 g / eq.

14. The method for preparing the cathodic electrophoretic coating as described in claim 12, characterized in that, The molar ratio of the epoxy resin to the modified polyimide is 1:(0.4~1.2).

15. An application of a cathodic electrophoretic coating in electrophoretic coating, characterized in that, A metal workpiece is placed in a cathodic electrophoretic coating as described in any one of claims 1-8 or a cathodic electrophoretic coating prepared by the method described in any one of claims 9-14, with the metal workpiece as the cathode, and the cathodic electrophoretic coating forms an electrophoretic coating on the surface of the metal workpiece after electrodeposition and curing.

16. The application as described in claim 15, characterized in that, The electrodeposition voltage is 40V~120V, and the electrodeposition time is 1min~5min; the curing temperature is 50℃~200℃, and the curing time is 20min~100min.

17. The application as described in claim 15 or 16, characterized in that, The curing process includes a first curing and a second curing. The temperature of the first curing is 50℃~100℃ and the time of the first curing is 10min~50min. The temperature of the second curing is 100℃~200℃ and the time of the second curing is 10min~50min.

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