A non-fading cathodic electro-deposition coating with overbake resistance and gloss and a method for preparing the same

Abstract

This invention relates to a cathodic electrophoretic emulsion with gloss retention after baking and its preparation method, belonging to the field of electrophoretic coating technology. The preparation method of the electrophoretic emulsion includes the following steps: S1, reacting an aliphatic or alicyclic diisocyanate with a hydroxyl-containing compound and then blocking it with an oxime blocking agent to obtain a blocked isocyanate crosslinking agent; S2, modifying a bisphenol-type epoxy resin through ring-opening chain extension with mixed amines, fatty acid amidation modification, and crosslinking enhancement with phenolic resin to obtain a multi-modified resin; S3, blending the two and neutralizing with an organic acid; S4, emulsifying and dispersing to obtain the cathodic electrophoretic emulsion. After electrophoretic coating, the obtained emulsion exhibits a gloss retention rate of ≥95% after baking at 200℃ for 30 minutes, while also possessing excellent mechanical properties and corrosion resistance.

Description

Technical Field

[0001] This invention relates to the field of electrophoretic coating technology, and in particular to a cathodic electrophoretic emulsion with gloss that does not decrease after baking and its preparation method. Background Technology

[0002] Cathodic electrophoretic coating has become the mainstream process for anti-corrosion primers on metal workpieces in the automotive and home appliance industries due to its excellent uniformity, high penetration, and high degree of automation. In existing technologies, cathodic electrophoretic coatings have made significant progress in corrosion resistance and edge coverage, with resin system design largely focusing on performance optimization under standard curing conditions. However, in actual production lines, due to thickness differences in different areas of the same workpiece, and even the coexistence of medium-thick and thin plates in the same batch, the oven temperature typically needs to be about 20°C higher than the standard temperature to ensure sufficient curing of the coating film in thick-walled areas. Under these conditions, the coating film in thin-walled areas is over-baked, making it prone to gloss degradation, color inconsistencies, and other appearance deterioration problems due to thermal degradation or excessive cross-linking of the resin system.

[0003] In existing technologies, the industry mainly alleviates the above problems by optimizing the baking curve, improving the uniformity of hot air circulation in the drying tunnel, or performing zoned baking of the workpiece. However, these methods all have drawbacks such as high difficulty in equipment modification, increased energy consumption, or limited production flexibility. Conventional cathodic electrophoretic coatings have a narrow crosslinking reaction window, and the main chain of resin molecules is prone to degradation and chain breakage at high temperatures. At the same time, the crosslinking network tends to become brittle after over-curing, leading to the deterioration of the appearance and mechanical properties of the paint film. In particular, the decrease in gloss directly affects the appearance quality and market competitiveness of the end product.

[0004] Therefore, it is necessary to start with the coating resin system itself and provide a cathodic electrophoretic emulsion that does not reduce gloss after baking, so that it can maintain a high degree of gloss consistency of the coating film within a wide baking window, in order to solve the shortcomings of the existing technology. Summary of the Invention

[0005] The purpose of this invention is to provide a cathodic electrophoretic emulsion with gloss retention under over-baking conditions and its preparation method, thereby solving the technical problems of gloss reduction and inconsistent gloss in different areas of the same workpiece under over-baking conditions in existing cathodic electrophoretic coatings.

[0006] In a first aspect, the present invention provides a method for preparing a cathodic electrophoretic emulsion with gloss retention that does not decrease after baking, comprising the following steps: S1: An aliphatic or alicyclic diisocyanate is reacted with a hydroxyl-containing compound to obtain an NCO-terminated prepolymer, which is then reacted with an oxime blocking agent to obtain a blocked isocyanate crosslinking agent; wherein, the NCO / OH molar ratio of the diisocyanate to the hydroxyl-containing compound is 1.5 to 2.5:1. S2: Bisphenol epoxy resin is subjected to a ring-opening and chain-extending reaction with mixed amines to obtain amination epoxy resin. The amination epoxy resin is then reacted with fatty acids to generate amide bonds and with phenolic resin to introduce multifunctional crosslinking nodes to obtain a multi-modified resin. S3: The blocked isocyanate crosslinking agent is blended with the multiple modified resin and then neutralized with an organic acid to a neutralization degree of 40% to 90% to obtain a cationic resin salt; wherein the molar ratio of the blocked NCO group in the blocked isocyanate crosslinking agent to the amine group in the multiple modified resin is 0.01 to 0.1:1; S4: The cationic resin salt is dispersed in deionized water under shearing action to obtain the cathodic electrophoretic emulsion.

[0007] Further, in step S1, the diisocyanate is selected from isophorone diisocyanate and hexamethylene diisocyanate, or, The hydroxyl-containing compound is selected from any one or more of polycaprolactone diol and polyether diol; and / or, The oxime blocking agent is selected from any one or more of butanone oxime and cyclohexanone oxime, and the molar ratio of the blocking agent to the remaining NCO group is 0.8 to 1.2:1.

[0008] Further, in step S1, the amount of the diisocyanate is 15-26 parts by weight, the amount of the hydroxyl-containing compound is 10-22 parts, and the amount of the blocking agent is 6-30 parts.

[0009] Further, in step S1, the synthesis reaction temperature of the NCO-terminated prepolymer is 30–60°C, and the blocking reaction temperature is 30–60°C.

[0010] Further, in step S2, the bisphenol type epoxy resin is selected from any one or more of bisphenol A type epoxy resin E-20 and bisphenol A type epoxy resin E-51; and / or, The mixed amine includes a polyamine and an alcoholamine, wherein the polyamine is selected from one or more of diethylenetriamine and triethylenetetramine, and the alcoholamine is selected from one or more of methylethanolamine and diethanolamine.

[0011] Further, in step S2, the amount of fatty acid used is 16 to 30 parts by weight, and the amount of phenolic resin used is 18 to 35 parts; the fatty acid is selected from any one or more of oleic acid and linolenic acid, and the phenolic resin is selected from any one or more of cashew phenolic resin and tert-butylphenolic resin.

[0012] Furthermore, step S2 also includes: Before the ring-opening and chain-extending reaction, phenolic resin and epoxy resin are pre-reacted under an inert atmosphere to obtain a modified epoxy component, which then participates in the subsequent reaction together with the bisphenol type epoxy resin.

[0013] Furthermore, the epoxy resin used in the pre-reaction is selected from any one or more of hydrogenated bisphenol A epoxy resin, alicyclic epoxy resin, naphthalene-cyclic epoxy resin, and phosphorus-containing epoxy resin.

[0014] Further, in step S3, the organic acid is selected from any one or more of lactic acid and acetic acid; and / or, In step S4, the cationic resin salt is emulsified by dripping it into deionized water containing a co-solvent in a thin stream. The solid content of the emulsion is adjusted to 10% to 50%, and the pH value is controlled at 5.0 to 7.0. After emulsification, the emulsion is sealed and stirred for 10 to 50 hours at room temperature.

[0015] Secondly, the present invention provides a cathodic electrophoretic emulsion with no gloss reduction after baking, which is prepared by the aforementioned preparation method. After electrophoretic coating, the gloss retention rate of the coating is ≥95% and the gloss at a 60° angle is ≥70 GU under the condition of baking at 200°C for 30 min.

[0016] Through one or more embodiments of the above embodiments of the present invention, at least the following technical effects can be achieved: This invention constructs a triple synergistic system of "phenolic resin multifunctional crosslinking enhancement + closed isocyanate wide temperature window crosslinking + fatty acid amide bond flexibility adjustment", which enables the coating to maintain a high degree of gloss consistency in the range from standard baking temperature to over-baking temperature.

[0017] The NCO / OH molar ratio of 1.5 to 2.5:1 ensures the structural controllability of the prepolymer, the molar ratio of blocked NCO groups to amine groups of 0.01 to 0.1:1 constructs a gradient-adjustable crosslinking density, and the neutralization degree of 40% to 90% ensures the long-term storage stability of the emulsion.

[0018] The multifunctional rigid nodes of phenolic resin endow the coating with high-temperature dimensional stability, the amidation of fatty acid long chains provides an internal plasticizing effect to release over-crosslinking internal stress, and the stepwise deblocking kinetics of oxime blocking agents enable the NCO groups to be released slowly over a wide temperature range rather than reacting abruptly. The three factors work together to effectively solve the industry pain point of gloss differences in different areas of the same product caused by the thickness difference of the workpiece. At the same time, it achieves emulsion storage stability of more than 6 months and good corrosion resistance, which has significant industrial application value. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a product image of the cathodic electrophoretic emulsion; Figure 2 This is a product image of the cathodic electrophoretic emulsion obtained in Example 1 after coating; Figure 3 This is a product image of the cathodic electrophoretic emulsion obtained in Example 2 after coating; Figure 4 This is a product image of the cathodic electrophoretic emulsion obtained in Example 3 after coating; Figure 5 This is a product image of the cathodic electrophoretic emulsion obtained in Example 4 after coating. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0022] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Furthermore, the character " / " in this document, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0023] As described in the background section, in the field of electrophoretic coating, due to the complex geometry and inconsistent wall thickness of the workpiece, the oven temperature often needs to be set at a level approximately 20°C higher than the standard curing temperature to ensure sufficient curing of the paint film in the thick-walled areas. Under this excessive baking condition, the paint film in the thin-walled areas experiences a significant decrease in gloss due to overheating, resulting in inconsistent appearance quality for the same workpiece or batch of products.

[0024] To address the aforementioned issues, this application provides a method for preparing a cathodic electrophoretic emulsion with gloss retention that is resistant to overbaking. This method comprises two parallel synthetic routes consisting of four steps, along with subsequent combination, neutralization, and emulsification processes.

[0025] The first route (S1) is the independent synthesis of a blocked isocyanate crosslinking agent, providing temperature-responsive crosslinking capability; The second route (S2) involves the preparation of a multi-modified resin, which goes through three stages in sequence: amine ring-opening and chain extension, fatty acid amidation modification, and phenolic resin crosslinking enhancement, to construct a resin matrix that combines cationic basis, flexibility, and high crosslinking density.

[0026] The products from the two routes are combined and blended in step S3 and neutralized with acid to impart water dispersibility, and finally emulsified dispersion is completed in step S4.

[0027] Specifically, a method for preparing a cathodic electrophoretic emulsion with gloss retention that does not decrease after baking includes the following steps: S1: An aliphatic or alicyclic diisocyanate is reacted with a hydroxyl-containing compound to obtain an NCO-terminated prepolymer, which is then reacted with an oxime blocking agent to obtain a blocked isocyanate crosslinking agent; wherein the NCO / OH molar ratio of the diisocyanate to the hydroxyl-containing compound is 1.5 to 2.5:1.

[0028] In some embodiments, the hydroxyl-containing compound is selected from one or more of polycaprolactone diols and polyether diols. Polycaprolactone diols can impart moderate flexibility and good hydrolytic stability to the crosslinking agent; polyether diols provide higher chain segment freedom of movement through the flexible segments of the ether bond. The specific type of hydroxyl-containing compound is not limited, as long as it can react with the NCO group in the diisocyanate to form a urethane bond.

[0029] In some embodiments, the NCO / OH molar ratio can be any value among 1.5:1, 1.8:1, 2.0:1, 2.2:1, 2.5:1, or a range between any two values.

[0030] It can be understood that one NCO group in the diisocyanate reacts with one OH group in the hydroxyl-containing compound to form a urethane bond, while the other NCO group is retained as an end group, thereby obtaining a linear NCO-terminated prepolymer with a uniform structure.

[0031] If the ratio is less than 1.5:1, both OH groups of some hydroxyl compounds will participate in the reaction to form chain extension or branched products, and the molecular weight of the prepolymer will increase sharply or even gel. If the ratio is greater than 2.5:1, the content of free diisocyanate monomers will be too high, which will not only increase the amount of blocking agent, but may also cause monomer migration during storage, affecting the stability of the emulsion.

[0032] In some embodiments, the amount of the diisocyanate, by weight, can be any value or a range between any two of 15, 18, 20, 24, or 26 parts; the amount of the hydroxyl-containing compound, by weight, can be any value or a range between any two of 10, 14, 16, 20, or 22 parts; and the amount of the blocking agent, by weight, can be any value or a range between any two of 6, 10, 14, 20, or 30 parts.

[0033] It should be noted that during the synthesis of NCO-terminated prepolymers, an appropriate amount of low-boiling-point inert solvent (such as acetone or butanone) is usually added to adjust the viscosity and control the heat of reaction.

[0034] In some embodiments, the synthesis reaction temperature of the NCO-terminated prepolymer can be 30–60°C, for example, any value or a range between any two of 30, 35, 40, 45, 50, and 60°C. In some embodiments, the synthesis reaction time of the NCO-terminated prepolymer can be 1–3 hours, for example, any value or a range between any two of 1, 1.5, 2, 2.5, and 3 hours.

[0035] The sealing agent must be added slowly after the system has cooled to a lower temperature. In some embodiments, the sealing reaction temperature can be 30–60°C, for example, any value or a range between any two of 30, 35, 40, 42, 45, 50, 55, and 60°C. In some embodiments, the sealing reaction time can be 0.5–2 hours, for example, any value or a range between any two of 0.5, 1, 1.5, and 2 hours.

[0036] The deblocking temperature of oxime-based blocking agents is approximately 150–170°C, which matches well with the conventional baking temperature (170–185°C) of cathodic electrophoretic coatings. Furthermore, the deblocking behavior of oxime-based blocking agents follows a dynamic equilibrium mechanism, with NCO groups being released gradually at a temperature-dependent, gradual rate within the 150–200°C temperature range. The increase in crosslinking density tends to plateau, and the probability of over-crosslinking is extremely low.

[0037] In some embodiments, the molar ratio of the blocking agent to the remaining NCO groups can be any value or a range between any two of 0.8:1, 0.9:1, 1.0:1, 1.1:1, and 1.2:1. The oxime blocking agent can be methyl ethyl ketone oxime, cyclohexanone oxime, etc., or two of the above blocking agents can be used in combination.

[0038] Through the above steps, a blocked isocyanate crosslinking agent that is stable at room temperature and deblocks at baking temperature can be prepared. The preparation of the multiple modified resin in step S2 will be explained next.

[0039] S2: Bisphenol epoxy resin is subjected to a ring-opening and chain-extending reaction with mixed amines to obtain amination epoxy resin. The amination epoxy resin is then reacted with fatty acids to generate amide bonds and with phenolic resin to introduce multifunctional crosslinking nodes to obtain a multi-modified resin.

[0040] The mixed amine specifically includes polyamines and alkanolamines, wherein the polyamines are selected from one or more of diethylenetriamine and triethylenetetramine, and the alkanolamines are selected from one or more of methylethanolamine and diethanolamine.

[0041] Polyamines (such as diethylenetriamine DETA) contain multiple active hydrogens, enabling them to undergo multi-site ring-opening reactions with epoxy groups. This forms branched structures between molecular chains and introduces a large number of primary amine groups, providing sufficient reactive sites for subsequent fatty acid amidation modification and crosslinking reactions with blocked isocyanates in step S3. Alkanolamines (such as methylethanolamine MEA), on the other hand, contain only one active hydrogen, used to block excessive epoxy end groups and control the amine value of the final resin within a reasonable range. This prevents excessively high amine values ​​from causing brittle paint films or making it difficult to control the pH of the electrophoresis bath.

[0042] If all polyamines are used, the branching of the resin will be too high, the viscosity will rise sharply, and too many primary amine groups may cause uncontrolled side reactions in subsequent processes. If all alkanolamines are used, the number of amine groups introduced will be insufficient, the degree of cationization after neutralization will be low, and the stability of the emulsion and the efficiency of electrophoretic deposition will be affected.

[0043] In some embodiments, the bisphenol-type epoxy resin can be bisphenol A type epoxy resin E-20 or E-51, and its amount can be any value or a range between any two of 8, 20, 30, 40, and 50 parts. The amount of the polyamine can be any value or a range between any two of 15, 18, 20, 22, and 25 parts. Of course, triethylenetetramine can be used instead of diethylenetriamine, or diethanolamine can be used instead of methylethanolamine, as long as the selected amine can achieve epoxy ring opening and introduce the amino groups required for cationization.

[0044] In some embodiments, the dropping temperature of the mixed amine should be controlled to not exceed 80–105°C, for example, it can be any value among 80, 85, 90, 95, 100, and 105°C or a range between any two values.

[0045] In some embodiments, the holding temperature for the amination ring-opening and chain extension reaction can be 50–135°C, for example, any value or any range between two of 50, 60, 65, 70, 80, 85, 90, 120, 130, and 135°C; the holding time can be 1–6 hours, for example, any value or any range between two of 1, 1.5, 2, 3, 4, 4.5, 5.5, and 6 hours.

[0046] The insulation temperature and time need to be determined comprehensively based on the epoxy equivalent of the selected epoxy resin and the reactivity of the mixed amine. E-20 type resin with a larger epoxy equivalent has lower reactivity and requires a higher insulation temperature and a longer reaction time; E-51 type resin has a smaller epoxy equivalent and higher reactivity, so the insulation temperature can be reduced accordingly.

[0047] It is understandable that the amidation reaction of fatty acids with aminated epoxy resin is a key step in imparting the coating film with over-baking flexibility. The carboxyl groups in the fatty acids undergo a condensation reaction with the remaining primary amine groups in the aminated epoxy resin to form amide bonds. These amide bonds have better thermal stability than ordinary ester bonds, with a decomposition temperature above 300℃, and can withstand over-baking conditions of 200℃.

[0048] At the same time, the introduction of C18 long-chain alkyl groups of fatty acids into the molecular network plays an internal plasticizing role, giving the molecular network an appropriate degree of freedom of chain segment movement without reducing the hardness of the coating film. This effectively releases the internal stress accumulation caused by the difference in thermal expansion coefficients during the over-baking and heating process of the high cross-linking density system, and avoids microcracks and surface roughening caused by stress concentration.

[0049] Examples of the fatty acids include long-chain unsaturated fatty acids such as oleic acid and linolenic acid, and two of the above fatty acids may also be used in combination. In some embodiments, the amount of the fatty acid used, by weight, can be any value or a range between any two of 16, 20, 25, 28, or 30 parts.

[0050] In some embodiments, the temperature of the fatty acid amidation reaction can be 40 to 95°C, for example, any value or any two of 40, 50, 55, 60, 65, 70, 80, 85, 90, and 95°C; the amidation reaction time can be 0.5 to 2 hours, for example, any value or any two of 0.5, 1, 1.5, and 2 hours.

[0051] It should be noted that the degree of amidation of fatty acids can be controlled by monitoring changes in the acid value of the system. When the acid value drops to near zero, it indicates that the carboxyl groups of the fatty acids have fully participated in the reaction. If the amidation reaction is incomplete, the residual free fatty acids will compete with organic acids for binding sites on the amino groups in the subsequent S3 neutralization step, reducing the neutralization efficiency and affecting the stability of the emulsion.

[0052] Furthermore, phenolic resin is introduced after the fatty acid modification is completed.

[0053] In this application, the phenolic resin is selected from cashew phenolic resin or tert-butylphenolic resin. The phenolic resin molecule contains abundant phenolic hydroxyl groups and methylene bridging structures. The phenolic hydroxyl groups can react with the remaining epoxy groups and some secondary amine groups in the resin to establish multifunctional rigid cross-linking nodes in the molecular network.

[0054] Furthermore, the reaction temperature of the phenolic resin can be 45–120°C, for example, any value or any two values ​​among 45, 50, 60, 70, 80, 85, 90, 100, 110, and 120°C; the reaction time can be 0.5–2.5 hours, for example, any value or any two values ​​among 0.5, 1, 1.5, 2, and 2.5 hours.

[0055] In some embodiments, the phenolic resin is preferably cashew phenolic resin.

[0056] In some embodiments, the amount of phenolic resin (as a 60% solids solution) may be any value or a range between any two of 18, 22, 28, 30, or 35 parts by weight.

[0057] In some embodiments, step S2 may further include a pre-reaction stage: Before the ring-opening and chain-extending reaction, the phenolic resin and another epoxy resin are pre-reacted under an inert atmosphere to obtain a modified epoxy component, which is then combined with a bisphenol epoxy resin to participate in the subsequent amination reaction.

[0058] It is understandable that the phenolic structure is pre-embedded in the epoxy skeleton to form a heat-resistant rigid matrix, and the phenolic resin introduced in the flexible network in the latter half of S2 forms a bilayer phenolic modified structure—the former focuses on the thermal stability of the matrix, and the latter focuses on crosslinking enhancement.

[0059] In some embodiments, the epoxy resin used in the pre-reaction can be hydrogenated bisphenol A epoxy resin, alicyclic epoxy resin, naphthalene-cyclic epoxy resin, phosphorus-containing epoxy resin, nano-modified epoxy resin, bio-based epoxy resin, etc. The mass ratio of phenolic resin to epoxy resin in the pre-reaction can be any value from 30:15, 35:20, 40:25, 45:30, 50:35, or a range between any two values.

[0060] In some embodiments, the pre-reaction temperature can be 45–180°C, for example, any value or a range between any two of 45, 70, 100, 110, 115, 120, 130, 150, and 180°C; the pre-reaction time can be 0.5–10 hours, for example, any value or a range between any two of 0.5, 1.5, 2, 4, 5, 6, 8, and 10 hours.

[0061] S3: The blocked isocyanate crosslinking agent is blended with the multiple modified resin and then neutralized with an organic acid to a neutralization degree of 40% to 90% to obtain a cationic resin salt; wherein the molar ratio of the blocked NCO group in the blocked isocyanate crosslinking agent to the amine group in the multiple modified resin is 0.01 to 0.1:1; That is, for every 100 reactable amine groups (primary amine + secondary amine), 1 to 10 blocked NCO groups are configured. If this ratio is higher than 0.1:1, the large number of NCO groups participating in crosslinking after unblocking will lead to excessively high crosslinking density in the coating film, causing it to become brittle and lose the flexibility advantage given by the fatty acid chain segments; if this ratio is lower than 0.01:1, the crosslinking density is insufficient, the thermal dimensional stability of the coating film is reduced, and gloss decay will still occur under over-baking conditions.

[0062] In some embodiments, the molar ratio can be any value or a range between any two values ​​from 0.01:1, 0.03:1, 0.04:1, 0.05:1, 0.06:1, 0.08:1, 0.1:1.

[0063] It is understandable that the degree of neutralization determines the water dispersion stability and electrophoretic efficiency of the resin. If the neutralization is too low, the degree of cationization is insufficient, and the resin will have difficulty forming a stable colloidal dispersion system in water, resulting in problems such as demulsification, layering, or even failure to form a film. If the neutralization is too high, the excess acid radicals will increase the ionic impurity content in the coating film, which will have an adverse effect on the corrosion resistance and water resistance of the coating film. At the same time, an excessively high degree of neutralization will also reduce the electrophoretic deposition efficiency.

[0064] In some embodiments, the degree of neutralization can be any value or a range between any two of 40%, 50%, 55%, 60%, 65%, 70%, 80%, and 90%. The organic acid can be lactic acid, acetic acid, etc., or two of the above organic acids can be used in combination.

[0065] It should be noted that the blending process must be carried out below the unblocking temperature of the blocking agent to avoid premature unblocking of the blocked isocyanate, which could trigger an premature reaction between the NCO groups and the amine groups. In some embodiments, the blending temperature can be 30–90°C, for example, any value or any two of 30, 40, 50, 60, 65, 70, 80, and 90°C; the blending time can be 0.5–5 hours, for example, any value or any two of 0.5, 1, 1.5, 2, 2.5, 3, 3.5, and 5 hours.

[0066] The neutralization reaction is an exothermic process, and the organic acid must be added slowly and in batches. In some embodiments, the neutralization reaction temperature should be controlled between 20 and 60°C, for example, any value or any two of 20, 25, 30, 35, 38, 40, 50, and 60°C. After adding the organic acid, continue stirring for 20 to 40 minutes, for example, any value or any two of 20, 25, 30, 35, and 40 minutes, to ensure a thorough and uniform neutralization reaction.

[0067] S4: The cationic resin salt is dispersed in deionized water under shearing action to obtain the cathodic electrophoretic emulsion.

[0068] The neutralized cationic resin salt was slowly dripped into deionized water containing a small amount of co-solvent in a thin stream, forming a stable oil-in-water (O / W) emulsion under high shear. The cationic amine salt groups of the resin molecules faced the aqueous phase, and the hydrophobic framework formed the core, creating emulsion particles with a core-shell structure. The morphology of the emulsion is as follows: Figure 1 As shown.

[0069] In some embodiments, the amount of co-solvent (such as isopropanol) can be any value or a range between any two of 5, 6, 7, or 8 parts. The amount of deionized water can be any value or a range between any two of 80, 100, 120, 150, or 160 parts. The final solids content of the emulsion is adjusted to 10%–50%, and the pH value is controlled at 5.0–7.0.

[0070] In some embodiments, after emulsification, the emulsion is matured in a closed environment with stirring at room temperature to fully stabilize the emulsion particles and make the particle size distribution more uniform. The maturation time can be any value among 10, 20, 30, 40, and 50 hours or any range between two values.

[0071] In some embodiments, the high shear dispersion time can be 10 to 60 minutes, for example, any value or a range between any two of 10, 20, 30, 40, 50, and 60 minutes.

[0072] The present application will be further described below with reference to specific embodiments and comparative examples. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0073] Example 1: A method for preparing a cathodic electrophoretic emulsion with gloss retention after baking, comprising the following steps: S1: Add 15 parts isophorone diisocyanate and 10 parts acetone to a dry reactor. Slowly add 10 parts polycaprolactone diol (molecular weight approximately 1000) dropwise at 30°C, with an NCO / OH molar ratio of 2:1, and react for 2 hours to obtain an NCO-terminated prepolymer. Cool the system to 50°C and slowly add 6 parts butanone oxime (MEKO), with a blocking agent to remaining NCO groups molar ratio of 1:1, and react for 1 hour to obtain a blocked isocyanate crosslinking agent.

[0074] S2: In another reactor, add 40 parts of cashew phenolic resin and 20 parts of hydrogenated bisphenol A epoxy resin, and react at 100°C for 4 hours under a nitrogen atmosphere to obtain a modified epoxy component. Mix the modified epoxy component with 8 parts of bisphenol A type epoxy resin E-20 and 10 parts of propylene glycol methyl ether, and heat to 60°C while stirring to dissolve. Slowly add 20 parts of diethylenetriamine and 5 parts of methylethanolamine, controlling the temperature not to exceed 80°C. After the addition is complete, maintain the temperature at 70°C for 2 hours to obtain an amination epoxy resin. Cool the amination epoxy resin to 70°C and add 30 parts of oleic acid to react for 1 hour. Add 35 parts of cashew phenolic resin solution (60% solid content), and heat to 90°C to react for 1 hour. Add 8 parts of ethylene glycol butyl ether to adjust the solid content to 65%, and cool to below 55°C to obtain a multi-modified resin.

[0075] S3: Under stirring, the blocked isocyanate crosslinking agent prepared in S1 is slowly added to the multi-modified resin in S2, with a molar ratio of blocked NCO groups to amine groups of 0.1:1. Stir at 70°C for 1 hour. Cool down to 40°C, slowly add 10 parts of lactic acid (concentration 85%), control the degree of neutralization at 55%, and stir for 30 minutes to obtain the cationic resin salt.

[0076] S4: Add 80 parts deionized water and 5 parts isopropanol to the emulsification tank. Under vigorous stirring in a high-shear emulsifier, add the cationic resin salt dropwise in a thin stream. After the addition is complete, disperse at high speed under shear for 30 minutes. Adjust the solid content to 30% with deionized water and control the pH value at 6.5.

[0077] The obtained cathodic electrophoretic emulsion, after coating, is as follows: Figure 2 As shown.

[0078] Example 2: A method for preparing a cathodic electrophoretic emulsion with gloss retention after baking, comprising the following steps: S1: Add 25 parts IPDI and 18 parts methyl ethyl ketone (MEK). Add 16 parts polyether diol dropwise at 45°C, with an NCO / OH molar ratio of 2:1, and react for 2.5 hours. Cool to 40°C and add 8 parts MEK oxime for blocking, to obtain a blocked isocyanate crosslinking agent.

[0079] S2: Under a nitrogen atmosphere, add 30 parts of naphthalene-cyclic epoxy resin and 25 parts of alicyclic epoxy resin, and react at 120℃ for 5 hours to obtain a modified epoxy component. Mix the modified epoxy component with 50 parts of bisphenol A type epoxy resin E-51 and 12 parts of propylene glycol methyl ether, and heat to 80℃ to dissolve. Add 15 parts of diethylenetriamine and 5 parts of methylethanolamine dropwise, controlling the temperature not to exceed 100℃, and maintain the reaction at 85℃ for 3 hours. Cool to 65℃ and add 25 parts of linoleic acid, reacting for 1 hour. Add 30 parts of tert-butylphenol resin solution (60% solid content), and react at 80℃ for 1.5 hours. Add 5 parts of ethylene glycol butyl ether to adjust the solid content to 60%.

[0080] S3: Under stirring, slowly add the blocked isocyanate crosslinking agent prepared in S1 to the multiple modified resin in S2, with a molar ratio of blocked NCO groups to amine groups of 0.05:1, and stir at 65°C for 2 hours. Cool down to 35°C, add 10 parts of acetic acid (concentration 85%), control the degree of neutralization at 60%, and stir for 40 minutes.

[0081] S4: In 120 parts deionized water and 8 parts isopropanol, emulsify under high shear for 30 minutes. Adjust the solids content to 25% and control the pH at 5.6.

[0082] The obtained cathodic electrophoretic emulsion, after coating, is as follows: Figure 3 As shown.

[0083] Example 3: S1: Add 24 parts IPDI and 13 parts methyl ethyl ketone (MEK). Add 22 parts polycaprolactone diol dropwise at 45°C, with an NCO / OH molar ratio of 2:1, and react for 3 hours. Cool to 42°C and add 30 parts MEK oxime for blocking.

[0084] S2: Under a nitrogen atmosphere, add 50 parts of naphthalene-cyclic epoxy resin and 35 parts of phosphorus-containing epoxy resin, and react at 110°C for 5 hours. Add 30 parts of bisphenol A type epoxy resin E-20 and 25 parts of propylene glycol methyl ether, and dissolve at 70°C. Add 18 parts of diethylenetriamine and 5 parts of methylethanolamine dropwise, controlling the temperature to not exceed 90°C, and react at 80°C for 3 hours. Cool to 65°C and add 16 parts of oleic acid, reacting for 2 hours. Add 28 parts of cashew phenolic resin solution (60% solid content), and react at 85°C for 1.5 hours. Add 10 parts of propylene glycol methyl ether to adjust the solid content to 55%.

[0085] S3: Under stirring, slowly add the blocked isocyanate crosslinking agent prepared in S1 to the multi-modified resin in S2, with a molar ratio of blocked NCO groups to amine groups of 0.04:1, and stir at 60°C for 2 hours. Cool down to 35°C, add 10 parts of acetic acid (concentration 88%), control the degree of neutralization at 60%, and stir for 35 minutes.

[0086] S4: Emulsify and disperse in 160 parts deionized water and 6 parts isopropanol for 35 minutes. Adjust the solids content to 28% and control the pH at 5.8.

[0087] The obtained cathodic electrophoretic emulsion, after coating, is as follows: Figure 4 As shown.

[0088] Example 4: S1: Add 26 parts IPDI and 20 parts methyl ethyl ketone (MEK). Add 20 parts polycaprolactone diol dropwise at 40°C, with an NCO / OH molar ratio of 2:1, and react for 1.5 hours. Cool to 45°C and add 14 parts MEK oxime for blocking.

[0089] S2: Under a nitrogen atmosphere, add 35 parts cashew phenolic resin and 30 parts bio-based epoxy resin, and react at 115°C for 4 hours. Add 40 parts bisphenol A type epoxy resin E-20 and 30 parts propylene glycol methyl ether, and dissolve at 75°C. Add 25 parts diethylenetriamine and 5 parts methylethanolamine dropwise, controlling the temperature not to exceed 100°C, and react at 80°C for 3 hours. Cool to 60°C and add 28 parts linoleic acid, reacting for 2 hours. Add 18 parts cashew phenolic resin solution (60% solid content), and react at 80°C for 1 hour. Add 12 parts ethylene glycol butyl ether to adjust the solid content to 60%.

[0090] S3: Under stirring, slowly add the blocked isocyanate crosslinking agent prepared in S1 to the multiple modified resin in S2, with a molar ratio of blocked NCO groups to amine groups of 0.06:1, and stir at 60°C for 2.5 hours. Cool down to 38°C, add 10 parts of lactic acid (concentration 85%), control the degree of neutralization at 65%, and stir for 30 minutes.

[0091] S4: Emulsify and disperse in 150 parts deionized water and 7 parts isopropanol for 30 minutes. Adjust the solids content to 25% and control the pH at 6.2.

[0092] The obtained cathodic electrophoretic emulsion, after coating, is as follows: Figure 5 As shown.

[0093] Comparative Example 1: The difference between Comparative Example 1 and Example 1 is that no cashew phenolic resin was added in the pre-reaction stage and the modification stage after amidation in step S2, that is, all phenolic resin was removed, and the remaining reaction conditions and parameters were the same as in Example 1.

[0094] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that in step S1, the amount of diisocyanate was reduced to 8 parts, the amount of hydroxyl-containing compound was reduced to 5 parts, and the amount of blocked isocyanate was reduced to 1.6 parts. The remaining reaction conditions and parameters were the same as in Example 2.

[0095] Comparative Example 3: The difference between Comparative Example 3 and Example 3 is that the degree of neutralization in step S3 is controlled at 20%, while the other reaction conditions and parameters are the same as in Example 3.

[0096] Performance testing and data analysis The emulsions prepared in Examples 1-4 and Comparative Examples 1-3 were added to the electrophoresis bath solution, and the coating was prepared under standard electrophoretic coating conditions using cold-rolled steel sheet as the cathode.

[0097] After baking at 200℃ for 30 minutes, the resulting coating film was subjected to a 60° gloss test and gloss retention rate calculation using a gloss meter, referring to GB / T9754-2007 "Determination of 20°, 60° and 85° Specular Gloss of Paint Films Without Metallic Pigments". Gloss retention rate (%) = (60° gloss after baking / 60° gloss after standard baking) × 100%.

[0098] The impact resistance height of the coating was determined by the free fall method with a heavy hammer, in accordance with GB / T 1732-2020 "Determination of Impact Resistance of Coating Film". The maximum single-sided corrosion width on both sides of the scratch was determined by continuous exposure in a neutral salt spray environment for 500 hours, in accordance with GB / T 10125-2021 "Civilized Atmosphere Corrosion Test - Salt Spray Test".

[0099] The test data for each group are summarized in Table 1.

[0100] Table 1 Note: Comparative Example 3 had extremely poor emulsion stability due to its low neutralization (20%), making it unable to form a film. Therefore, subsequent coating performance tests could not be conducted.

[0101] Based on the test data in Table 1, a comparative analysis was conducted item by item: Compared with Comparative Example 1, Comparative Example 1 shows that due to the complete removal of phenolic resin, the molecular network of Comparative Example 1 lacks the multifunctional rigid crosslinking nodes provided by the phenolic structure, resulting in insufficient crosslinking density and reduced Tg. Under the over-baking condition of 200°C, the coating film has approached or even exceeded Tg and entered the high-elasticity state. The intensified movement of molecular chain segments has roughened the surface, and the gloss retention rate has decreased by 11 percentage points. The absolute gloss has dropped significantly from >70 GU to 45 GU.

[0102] Furthermore, the emulsion in Comparative Example 1 showed stratification after only 2 months of storage, further confirming the irreplaceable role of phenolic resin in imparting internal density to emulsion particles and inhibiting particle aggregation.

[0103] Compared with Comparative Example 2, Comparative Example 2 shows that because the total amount of blocked isocyanate added is only about 1 / 5 of that in Example 2, the number of NCO groups released during the baking process is seriously insufficient, and they cannot form a complete urea bond crosslinking network with the amine groups. The defect of the crosslinking network causes the coating film to produce local relaxation and surface micro-undulations at high temperature, and the gloss retention rate decreases by 17 percentage points, and the absolute gloss decreases from >70 GU to 50 GU.

[0104] Comparing Example 3 with Comparative Example 3, it can be seen that Comparative Example 3 reduced the neutralization degree to 20%, which is far below the lower limit of 40% specified in claim 1. The degree of cationization is seriously insufficient, and the positive charge density on the surface of the resin particles is too low to form sufficient electrostatic repulsion to maintain the stability of the colloid. As a result, demulsification occurs within 1 month, making electrophoretic coating completely impossible, let alone gloss evaluation.

[0105] Furthermore, a comprehensive analysis was conducted by combining the mechanical properties and corrosion resistance data in Table 1.

[0106] Comparing Example 1 with Comparative Example 1, it can be seen that Comparative Example 1, due to the complete removal of phenolic resin, lost the multifunctional rigid crosslinking nodes provided by the phenolic structure in the molecular network. The pencil hardness decreased from 2H to HB, a decrease of three levels, indicating that the crosslinking density and rigidity of the coating film decreased significantly. The adhesion decreased from the highest level 0 to level 2, and the impact resistance decreased from 50cm to 30cm. This shows that the rigid nodes established by the phenolic resin are not only the key to obtaining high hardness, but also play an irreplaceable supporting role in substrate interface adhesion and coating cohesive strength.

[0107] Compared with Comparative Example 2, Example 2 shows that because the total amount of blocked isocyanate crosslinking agent added in Comparative Example 2 is only about 1 / 5 of that in Example 2, the number of NCO groups that are unblocked and participate in the crosslinking reaction is seriously insufficient during the baking process. The pencil hardness drops from 2H to B, a decrease of four levels; the adhesion drops from level 0 to level 1; the impact resistance drops from 50cm to 35cm; and the salt spray corrosion increases from <1.8mm to >2.8mm.

[0108] The simultaneous deterioration of the above indicators indicates that the blocked isocyanate crosslinking agent is not only a key guarantee for gloss retention, but also the foundation for the overall mechanical properties and long-term durability of the coating. The incomplete crosslinking network leads to a significant decrease in the coating in three dimensions: cohesive strength, interfacial bonding force, and corrosion shielding efficiency.

[0109] A comparative analysis of Examples 1-4 reveals that while the four examples employed different types of epoxy resin matrices, different fatty acids, different phenolic resins, and different molar ratios and neutralization degrees of NCO:amine groups, the gloss retention rate remained consistently between 95% and 97%, the gloss at a 60° angle was >70 GU, the pencil hardness reached 2H to 3H, the adhesion was the highest grade 0, the impact resistance was 50 cm, the salt spray corrosion diffusion was <2 mm after 500 hours, and the storage stability was over 6 months.

[0110] In summary, although the present invention has been disclosed above with reference to preferred embodiments, the above preferred embodiments are not intended to limit the present invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the present invention.

Claims

1. A method for preparing a cathodic electrophoretic emulsion with no gloss reduction after baking, characterized in that, Includes the following steps: S1: An aliphatic or alicyclic diisocyanate is reacted with a hydroxyl-containing compound to obtain an NCO-terminated prepolymer, which is then reacted with an oxime blocking agent to obtain a blocked isocyanate crosslinking agent; wherein, the NCO / OH molar ratio of the diisocyanate to the hydroxyl-containing compound is 1.5 to 2.5:

1. S2: Bisphenol epoxy resin is subjected to a ring-opening and chain-extending reaction with mixed amines to obtain amination epoxy resin. The amination epoxy resin is then reacted with fatty acids to generate amide bonds and with phenolic resin to introduce multifunctional crosslinking nodes to obtain a multi-modified resin. S3: The blocked isocyanate crosslinking agent is blended with the multiple modified resin and then neutralized with an organic acid to a neutralization degree of 40% to 90% to obtain a cationic resin salt; wherein the molar ratio of the blocked NCO group in the blocked isocyanate crosslinking agent to the amine group in the multiple modified resin is 0.01 to 0.1:1; S4: The cationic resin salt is dispersed in deionized water under shearing action to obtain the cathodic electrophoretic emulsion.

2. The preparation method according to claim 1, characterized in that, In step S1, the diisocyanate is selected from any one or more of isophorone diisocyanate and hexamethylene diisocyanate; and / or, The hydroxyl-containing compound is selected from any one or more of polycaprolactone diol and polyether diol; and / or, The oxime blocking agent is selected from any one or more of butanone oxime and cyclohexanone oxime, and the molar ratio of the blocking agent to the remaining NCO group is 0.8 to 1.2:

1.

3. The preparation method according to claim 2, characterized in that, In step S1, the amount of diisocyanate used is 15-26 parts by weight, the amount of hydroxyl-containing compound used is 10-22 parts, and the amount of blocking agent used is 6-30 parts.

4. The preparation method according to claim 2, characterized in that, In step S1, the synthesis reaction temperature of the NCO-terminated prepolymer is 30–60°C, and the blocking reaction temperature is 30–60°C.

5. The preparation method according to claim 1, characterized in that, In step S2, the bisphenol type epoxy resin is selected from any one or more of bisphenol A type epoxy resin E-20 and bisphenol A type epoxy resin E-51; and / or, The mixed amine includes a polyamine and an alcoholamine, wherein the polyamine is selected from one or more of diethylenetriamine and triethylenetetramine, and the alcoholamine is selected from one or more of methylethanolamine and diethanolamine.

6. The preparation method according to claim 5, characterized in that, In step S2, the amount of fatty acid used is 16 to 30 parts by weight, and the amount of phenolic resin used is 18 to 35 parts; the fatty acid is selected from any one or more of oleic acid and linolenic acid, and the phenolic resin is selected from any one or more of cashew phenolic resin and tert-butylphenolic resin.

7. The preparation method according to claim 5, characterized in that, Step S2 also includes: Before the ring-opening and chain-extending reaction, phenolic resin and epoxy resin are pre-reacted under an inert atmosphere to obtain a modified epoxy component, which then participates in the subsequent reaction together with the bisphenol type epoxy resin.

8. The preparation method according to claim 7, characterized in that, The epoxy resin used in the pre-reaction is selected from any one or more of hydrogenated bisphenol A epoxy resin, alicyclic epoxy resin, naphthalene-cyclic epoxy resin, and phosphorus-containing epoxy resin.

9. The preparation method according to claim 1, characterized in that, In step S3, the organic acid is selected from any one or more of lactic acid and acetic acid; and / or, In step S4, the cationic resin salt is emulsified by dripping it into deionized water containing a co-solvent in a thin stream. The solid content of the emulsion is adjusted to 10% to 50%, and the pH value is controlled at 5.0 to 7.

0. After emulsification, the emulsion is sealed and stirred for 10 to 50 hours at room temperature.

10. A cathodic electrophoretic emulsion with no gloss degradation after baking, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 9.

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