Low temperature cure high edge coverage cathodic electrophoretic coating and method of making same

Abstract

This invention belongs to the technical field of water-based cathodic electrophoretic coatings, specifically relating to a low-temperature curing cathodic electrophoretic coating with high edge coverage and its preparation method. The preparation method includes: firstly, synthesizing a cationic epoxy resin with allyl double bonds in the side chain as the main resin; then pre-incorporating a phenolic resin oligomer and a quaternary phosphine salt latent base into the resin, followed by neutralization and emulsification to obtain a mother liquor; simultaneously, preparing thermally triggered microcapsules with melamine-formaldehyde as the shell, encapsulating a multifunctional thiol compound and a thermal initiator; finally, incorporating the microcapsules into the resin mother liquor to obtain the final product. This invention successfully solves the technical challenge of simultaneously achieving low-temperature curing and high edge coverage in cathodic electrophoretic coatings. The coating can be completely cured at a low temperature of 148-152℃, with an edge film thickness ratio exceeding 82%, and also exhibits excellent salt spray corrosion resistance exceeding 790 hours, significantly improving the edge protection performance and coating efficiency of the workpiece.

Description

Technical Field

[0001] This invention belongs to the technical field of water-based cathodic electrophoretic coatings, specifically relating to a low-temperature curing cathodic electrophoretic coating with high edge coverage and its preparation method. Background Technology

[0002] Cathodic electrophoretic coatings, as a type of environmentally friendly coating that uses water as a dispersion medium, have been widely used in the metal surface coating of automobile bodies, hardware components, household appliance housings, and various industrial equipment due to their excellent anti-corrosion performance, high penetration, good edge coverage, and high degree of automation in coating.

[0003] However, in actual production, for workpieces with complex geometries, especially at edges or abrupt geometric changes such as flanges, corners, bends, welds, and openings, electrophoretic coatings often exhibit significant thinning of film thickness, decreased gloss, and even localized exposure of the substrate after baking and curing. This "edge effect" or "sharp point effect" leads to a significant deterioration in the coating's corrosion resistance, insulation, and mechanical protection performance in that area, becoming a long-standing technical pain point affecting the overall protective quality of the workpiece. The root cause of this problem lies in the low-viscosity melt-leveling stage that the electrophoretic coating film undergoes in the early stages of thermal curing. During this stage, the edge and corner areas, due to their large specific surface area and small heat capacity, heat up faster than the planar areas, resulting in a decrease in surface tension and the formation of a surface tension gradient between them. This gradient triggers Marangoni convection, driving the liquid coating material to migrate from the edges to the planar areas. If this low-viscosity window lasts too long or the system viscosity is too low, the coating material at the edges will be lost in large quantities before the resin fully cross-links and cures, ultimately forming defects.

[0004] On the other hand, with increasingly stringent global requirements for energy conservation, emission reduction, and carbon reduction, low-temperature curing of electrophoretic coatings has become a clear technological development trend. However, lowering the curing temperature often leads to a decrease in the crosslinking reaction rate, which can easily result in incomplete curing, insufficient hardness, and reduced chemical resistance of the coating film. In addition, the commonly used blocked isocyanate curing agents typically have high unblocking temperatures (generally above 170°C), which not only limits further reduction in curing temperature but also prolongs the time the coating film remains in a low-viscosity state, thus exacerbating the migration and loss of coating material at the edges.

[0005] To improve the overall performance of coatings, existing technologies have made various attempts. For example, patent CN103319976A discloses a thick-film high-corrosion-resistant cathodic electrophoretic coating, which uses a high-crosslinking-density microgel and a fully enclosed isocyanate compound to achieve a 35-40μm thick film coating and improve the edge shrinkage phenomenon to a certain extent. However, its curing temperature still needs to be above 170℃, resulting in high energy consumption, and the problem of paint loss at sharp edges has not been fundamentally solved. Patent CN106922151A improves edge corrosion resistance by adding silica-based inorganic fillers, but the organotin catalyst used has bioaccumulative toxicity, which does not meet the increasingly stringent environmental regulations. In addition, the charge matching between the inorganic filler and the resin matrix is ​​poor, and the long-term storage of the bath solution is prone to stability problems such as stratification and precipitation. Patent CN114231077A discloses a low-temperature curing, matte cathodic electrophoretic coating that uses a dual-sealing agent modified isocyanate curing agent to reduce the curing temperature to around 140℃. However, this technology lacks specific design for edge coverage, resulting in poor film thickness uniformity at sharp edges and weld seams, with no significant improvement in the edge film thickness ratio. Furthermore, it still relies on organotin catalysts, limiting its environmental friendliness. Patent CN110437665A improves the anti-corrosion performance of sharp edges by combining a sharp edge shrinkage inhibitor with an anti-corrosion functional group additive. However, this method relies solely on additive optimization and does not address the root cause of the resin crosslinking structure. The additive addition amount is as high as 10%-30%, which may adversely affect the long-term stability of the bath and the mechanical properties of the coating film.

[0006] In summary, existing cathodic electrophoretic coating technologies generally suffer from the following common problems that are difficult to reconcile: a low edge film thickness ratio, resulting in insufficient salt spray corrosion resistance at sharp edges and welds; a high curing temperature, which contradicts the trend of energy conservation and consumption reduction; and the difficulty in simultaneously ensuring bath stability and coating mechanical properties. Therefore, developing a novel cathodic electrophoretic coating that can achieve low-temperature curing, ensure dense coating protection for complex structural edges, and simultaneously possess environmental friendliness and excellent bath stability has significant practical implications and broad application prospects. Summary of the Invention

[0007] The purpose of this invention is to provide a low-temperature curing cathodic electrophoretic coating with high edge coverage and its preparation method, so as to solve the technical problems of existing cathodic electrophoretic coatings, such as thinning of film thickness and reduction of protective performance at edges and corners due to paint migration, as well as high energy consumption and difficulty in achieving coating performance due to the need for high-temperature curing.

[0008] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of this invention provides a method for preparing a low-temperature curing, high-edge-coverage cathodic electrophoretic coating, comprising the following steps: (1) Using bisphenol A type epoxy resin with an epoxy equivalent of 180-330 g / eq as the matrix, chain extension is carried out by ring-opening addition reaction with hydroxyl compounds to obtain chain extension intermediate with an epoxy equivalent of 800-1300 g / eq; allyl glycidyl ether (AGE) is etherified with the chain extension intermediate to obtain epoxy resin modified with double bonds; the epoxy resin modified is reacted with amine compounds to introduce tertiary amine or quaternary ammonium groups, and neutralized with acid to obtain cationic epoxy main resin; (2) Add the phenolic resin oligomer and the quaternary phosphine salt latent base to the cationic epoxy main resin obtained in step (1), mix them, and obtain mixture A; neutralize the mixture A with acid, add deionized water to emulsify, and obtain resin emulsion mother liquor; (3) Using melamine-formaldehyde prepolymer as the shell, a mixture of droplets containing multifunctional thiol compounds and thermally decomposable free radical initiators is encapsulated as the core to prepare a microcapsule dispersion; (4) Add the microcapsule dispersion to the resin emulsion mother liquor obtained in step (2). The solid mass of the added microcapsule is 0.5 to 5.0 parts by weight, based on 100 parts by weight of the solid mass of the cationic epoxy main resin obtained in step (1). Mix to obtain the cathodic electrophoretic coating.

[0009] Further, in step (1), the hydroxyl compound is at least one of bisphenol A, propylene glycol, or diethylene glycol; the amount of the hydroxyl compound added is, based on the equivalent of the hydroxyl groups it contains, in a ratio of 0.20 to 0.35:1 to the equivalent of the epoxy groups contained in the bisphenol A type epoxy resin matrix; the ring-opening addition reaction is carried out in the presence of a catalyst, the catalyst being triphenylphosphine, and its amount is 0.05 to 0.20 wt% of the mass of the bisphenol A type epoxy resin matrix.

[0010] Further, in step (1), the molar equivalent ratio of the allyl glycidyl ether to the hydroxyl group contained in the chain extension intermediate is 0.10 to 0.20:1; the etherification reaction is carried out in the presence of a catalyst and a free radical polymerization inhibitor, wherein the catalyst is triphenylphosphine or imidazole, and the amount of catalyst used is 0.05 to 0.20 wt% of the mass of the chain extension intermediate; the free radical polymerization inhibitor is hydroquinone or p-hydroxyanisole.

[0011] Further, in step (1), the amine compound is diethylamine, diisopropylamine or a quaternized monomer; the amount of the amine compound added is such that the amine value of the cationic epoxy resin is 40 to 120 mg KOH / g.

[0012] Step (1) increases the molecular weight and epoxy equivalent of the epoxy resin through chain extension, constructing a more rigid aromatic ring-ether bond skeleton, which helps to improve the glass transition temperature of the resin. The introduced allyl double bond provides active sites for the subsequent low-temperature mercapto-ene click reaction, which can quickly form crosslinking points in the early stage of curing. Amination and neutralization treatment endow the resin with the cationic properties required for water dispersion, enabling it to be stably dispersed in aqueous systems and laying the foundation for electrophoretic deposition.

[0013] Further, in step (2), the hydroxyl value of the phenolic resin oligomer is 200-300 mg KOH / g; based on 100 parts by weight of the solid mass of the cationic epoxy main resin obtained in step (1), the amount of phenolic resin oligomer added is 6-10 parts by weight, and the amount of quaternary phosphine salt latent base added is 0.10-0.25 parts by weight.

[0014] Further, in step (2), the quaternary phosphine salt latent base is tetraphenylphosphine bromide (TPPBr) or diphenyl(3-nitrobenzyl)phosphine bromide (TBPP).

[0015] Step (2) involves pre-incorporating phenolic resin oligomers and quaternary phosphine salt latent bases into the resin system. Phenolic resin, as a polyhydroxy crosslinking agent, can undergo a condensation reaction with residual epoxy groups in the resin during the high-temperature curing stage, forming a dense ether bond network that enhances the coating's chemical resistance and mechanical strength. Quaternary phosphine salt, as a latent catalyst, is activated at high temperatures, promoting the epoxy-phenolic hydroxyl reaction, thereby achieving medium-temperature curing without relying on high-temperature unsealing curing agents, which helps reduce energy consumption.

[0016] Further, in step (3), the multifunctional thiol compound is a mixture of pentaerythritol tetra(3-mercaptopropionate) (PETMP) and trimethylolpropane tri(3-mercaptopropionate) (TMPTMP) in a mass ratio of 7:(2-4); the thermally decomposable free radical initiator is di-tert-butyl benzoate peroxide.

[0017] Further, in step (3), the preparation process of the microcapsule dispersion includes: (a) Pentaerythritol tetra(3-mercaptopropionate) and trimethylolpropane tri(3-mercaptopropionate) were mixed at a mass ratio of 7:(2-4) to obtain a basic core mixture. Di-tert-butyl benzoate peroxide and p-hydroxyanisole were added to the basic core mixture and stirred to obtain a core premix. The amount of di-tert-butyl benzoate peroxide added was 1.5 to 2.5 wt% of the mass of the basic core mixture; the amount of p-hydroxyanisole added was 200 to 300 ppm of the mass of the basic core mixture. (b) Prepare an aqueous solution of melamine-formaldehyde prepolymer with a solid content of 8-12 wt%, adjust the pH to 8.5-9.0, and prepolymerize at 40-50°C for 20-30 minutes to obtain the outer shell aqueous phase; (c) At 40–45°C and under shearing conditions, the core premix is ​​added dropwise to the outer shell aqueous phase and sheared emulsified for 8–12 minutes to form a core drop emulsion with a particle size of 0.8–1.5 μm; (d) Adjust the pH of the nuclear drop emulsion to 4.0-4.5, react at 55-60°C for 1-3 hours to mature, and form a microcapsule dispersion.

[0018] Further, in step (3), after the microcapsule dispersion is formed, it is surface treated with a cationic polymer electrolyte. Specifically, the microcapsule dispersion is cooled to 25-30°C and the pH is adjusted to 5.0-5.5. Then, a cationic polymer electrolyte is added and stirred for 20-30 minutes. The cationic polymer electrolyte is polydiallyldimethylammonium chloride.

[0019] The microcapsules prepared in step (3) have shells formed after the condensation and curing of melamine-formaldehyde prepolymer, which can rupture within the temperature range of 95-110℃. After the release of the polyfunctional thiol compounds and thermal initiators in the core, they can rapidly undergo a thiol-ene click reaction with the allyl double bonds in the resin. This reaction can quickly form a local cross-linked network in the early stages of curing, effectively increasing the viscosity of the coating in the edge areas, inhibiting paint loss due to the Marangoni effect, and thus improving edge coverage.

[0020] A second aspect of the present invention provides a low-temperature curing high edge coverage cathodic electrophoretic coating prepared by the above method.

[0021] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: 1. Outstanding edge coverage and superior protective performance: By designing clickable allyl double bonds in the main resin and combining it with thermally triggered microencapsulation technology, a rapid thiol-ene click reaction can be initiated at the low temperature stage of the initial baking process. This allows the coating in the edge area to gel rapidly and increase viscosity, effectively inhibiting paint migration from the edge to the plane caused by the Marangoni effect. The measured edge film thickness ratio can reach 82-85%, significantly better than traditional technologies. This ensures that easily corroded and vulnerable areas such as workpiece edges and welds receive sufficient and dense coating protection, and the overall neutral salt spray resistance exceeds 790 hours.

[0022] 2. Achieves low-temperature curing, resulting in significant energy savings: A novel crosslinking system composed of phenolic resin oligomers and quaternary phosphine salt latent base replaces the traditional blocked isocyanate curing agent that requires temperatures above 170℃ to deblock the reaction. This system can be efficiently activated in the mid-temperature range of 140-165℃, promoting the condensation reaction between residual epoxy groups and phenolic hydroxyl groups in the resin to form a dense crosslinked network. The minimum temperature for complete curing of the coating of this invention can be reduced to 148-152℃, which is about 20℃ lower than the traditional process, significantly reducing baking energy consumption.

[0023] 3. Excellent overall performance: This invention ensures low-temperature activity while imparting a high cross-linking density to the coating. The coating achieves excellent edge coverage while simultaneously possessing high hardness, strong adhesion, and outstanding wear resistance, thus achieving a balance and improvement in multiple key performance characteristics. Attached Figure Description

[0024] Figure 1 The Fourier transform infrared spectra of the cured coatings of Example 1 of the present invention are compared with those of Comparative Examples 1, 3, and 5. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] Bisphenol A type epoxy resin, CAS: 25085-99-8, purchased from Wuhan Kemic Biomedical Technology Co., Ltd. The phenolic resin oligomers were purchased from Shandong Dingyi New Material Co., Ltd., with a hydroxyl value of 200-300 mg KOH / g. Tetraphenylphosphine bromide, CAS: 2751-90-8, purchased from Azeroth Bio; Pentaerythritol tetra(3-mercaptopropionate), CAS: 7575-23-7, purchased from Aladdin Reagents; Trimethylolpropane tris(3-mercaptopropionate), CAS: 33007-83-9, purchased from Hubei Xingyan New Material Technology Co., Ltd. Di-tert-butyl benzoate peroxide, CAS: 614-45-9, purchased from Jiangsu Peixing Chemical Co., Ltd. p-Hydroxyanisole, CAS: 150-76-5, purchased from Jingzhou Yinjie Chemical Co., Ltd. Melamine-formaldehyde prepolymer, CAS: 9003-08-1, purchased from Shijiazhuang Hanwoz Trading Co., Ltd.

[0027] Example 1 This embodiment provides a low-temperature curing high edge coverage cathodic electrophoretic coating, the preparation method of which includes the following steps: (1) In a reactor equipped with a stirrer, thermometer, nitrogen inlet, and condenser, 1000 parts by mass of bisphenol A type epoxy resin (matrix) with an epoxy equivalent of 220 g / eq were added. The temperature was raised to 125 °C, and 1.5 parts by mass of triphenylphosphine (≥98%) as a catalyst were added. Bisphenol A and diethylene glycol (mass ratio 2.1:1, with a total hydroxyl equivalent to epoxy group equivalent ratio of 0.28:1) that were pre-molten and mixed were slowly added dropwise to the reaction system. The reaction was stirred at 125 °C for 5 hours, and the epoxy equivalent was monitored to increase to about 1000 g / eq to obtain the chain extension intermediate.

[0028] The system was cooled to 95°C, and allyl glycidyl ether (AGE, with a molar ratio of epoxy group to chain extension intermediate hydroxyl group of 0.15:1), 1.0 part by mass of imidazole catalyst, and 200 ppm of free radical polymerization inhibitor p-hydroxyanisole (MEHQ) were added sequentially. The reaction was continued at this temperature for 2 hours to obtain an epoxy resin modified with double bonds.

[0029] The system was cooled to 70°C, and diethylamine was slowly added dropwise (the amount added was such that the final resin amine value was approximately 80 mg KOH / g), and the reaction was allowed to proceed for 60 minutes. Subsequently, neutralization was performed using a mixed acid-water solution of lactic acid and acetic acid (equivalent ratio 1:1), controlling the degree of neutralization to approximately 40%. A cationic epoxy resin with a solid content of approximately 75% was obtained.

[0030] (2) Take 100 parts by weight of the cationic epoxy resin obtained in step (1) and place it in a container, then heat it to 70°C. Add 8 parts by weight of phenolic resin oligomer (Novolac) with a hydroxyl value of 250 mg KOH / g and 0.15 parts by weight of tetraphenylphosphine bromide (TPPBr), a quaternary phosphine salt latent base. Stir at 400 rpm for 45 minutes until the system becomes a clear and homogeneous mixture A. The viscosity of the system is about 2.8 Pa·s, and no diluent is added. Then, remove the solvent and microbubbles under a vacuum of -0.09 MPa for 15 minutes and cool to 55°C.

[0031] At 55°C, mixture A was neutralized in two stages using a lactic acid / acetic acid (1:1 equivalent) mixture: the first stage neutralized to approximately 40% neutralization and allowed to stand for 15 minutes; the second stage continued neutralization to approximately 45% final neutralization, with a system pH of approximately 5.7. Subsequently, a three-stage emulsification process was performed: the first stage added 25% deionized water and emulsified at high shear (approximately 10 m / s) for 15 minutes; the second stage added 35% deionized water and emulsified at medium shear (approximately 10 m / s) for 12 minutes; the third stage added the remaining deionized water at low shear (400 rpm) to adjust the mother liquor solid content to 35 wt%. After aging for 20 minutes, the mixture was filtered through a 50 μm filter bag to obtain the resin emulsion mother liquor.

[0032] (3)(a) Core premix: Under nitrogen protection, pentaerythritol tetra(3-mercaptopropionate) (PETMP) and trimethylolpropane tri(3-mercaptopropionate) (TMPTMP) were mixed at a mass ratio of 7:3 to obtain a basic core mixture. 2.0 parts by mass of di-tert-butylparaben (TBPB) and 250 ppm (by mass) of p-hydroxyanisole (MEHQ) were added to 100 parts by mass of the basic core mixture, and the mixture was stirred at 25°C for 25 minutes until completely homogeneous to obtain the core premix solution, which was stored protected from light.

[0033] (b) Preparation of the outer shell aqueous phase: The commercially available melamine-formaldehyde prepolymer was prepared into an aqueous solution with a solid content of 10 wt%. The pH was adjusted to 8.8 with sodium carbonate solution and prepolymerized at 45°C for 25 minutes to obtain the outer shell aqueous phase.

[0034] (c) Emulsification and nucleation: The aqueous phase of the shell was maintained at 42°C, and under high-speed shear at 6000 rpm, the emulsion was nucleated at a rate of approximately 0.8 mL·s. -1 ·kg -1 All the core premixed solution was added dropwise at a rate of [missing information]. After the addition was complete, high-shear emulsification was continued at 8000 rpm for 10 minutes to form a stable core drop emulsion with a particle size of approximately 1.2 μm.

[0035] (d) Condensation to form a shell: The pH of the system was slowly adjusted to 4.2 with a 10% citric acid aqueous solution, and the reaction was carried out at a constant temperature of 58°C for 2 hours. Then the temperature was raised to 70°C and the mixture was allowed to mature for another hour to obtain a milky white microcapsule dispersion.

[0036] (e) Cool the microcapsule dispersion to 28°C and adjust the pH to 5.2 with dilute formic acid. Slowly add 20 wt% of a polydiallyldimethylammonium chloride (PDADMAC) aqueous solution, which is 0.3 wt% of the total mass of the dispersion (based on PDADMAC solids), and stir for 25 minutes. Finally, centrifuge and wash, redisperse in deionized water, and adjust the solid content to 30 wt% to obtain the microcapsule dispersion.

[0037] (4) Take 100 parts by weight (solids) of the resin emulsion mother liquor obtained in step (2) and slowly add it in batches to the microcapsule dispersion obtained in step (3) (equivalent to 2.0 parts by weight of microcapsule solids) under gentle stirring at 25°C and 300 rpm. The addition interval is 3 minutes. After all the materials are added, continue stirring for 15 minutes to mix evenly and obtain the cathodic electrophoretic coating of this embodiment.

[0038] The working solution was prepared by mixing cathodic electrophoretic coating, carbon black paste, and deionized water in a mass ratio of 1:4:7. The solid content of the solution was approximately 20 wt%, and the pH was adjusted to 5.8. The solution was allowed to mature for 24 hours before use.

[0039] Example 2 This embodiment provides a low-temperature curing high edge coverage cathodic electrophoretic coating, the preparation method of which includes the following steps: (1) In a reactor equipped with a stirrer, thermometer, nitrogen inlet, and condenser, 1000 parts by mass of bisphenol A type epoxy resin (matrix) with an epoxy equivalent of 260 g / eq were added. The temperature was raised to 128 °C, and 1.8 parts by mass of triphenylphosphine as a catalyst were added. Bisphenol A (with a hydroxyl equivalent to epoxy group equivalent ratio of 0.25:1) was slowly added dropwise to the reaction system. The reaction was stirred at 128 °C for 4.5 hours, and the epoxy equivalent was monitored to increase to about 900 g / eq to obtain the chain extension intermediate.

[0040] The system was cooled to 100°C, and allyl glycidyl ether (AGE, with a molar ratio of epoxy group to chain extension intermediate hydroxyl group of 0.12:1), 1.2 parts by mass of imidazole catalyst, and 180 ppm of hydroquinone (HQ), a free radical polymerization inhibitor, were added sequentially. The reaction was continued at this temperature for 2.5 hours to obtain an epoxy resin modified with double bonds.

[0041] The system was cooled to 75°C, and diethylamine was slowly added dropwise (the amount added was such that the final resin amine value was approximately 65 mg KOH / g), and the reaction was allowed to proceed for 75 minutes. Subsequently, neutralization was performed using a mixed acid-water solution of lactic acid and acetic acid (equivalent ratio 1:1), controlling the degree of neutralization to approximately 45%. A cationic epoxy resin with a solid content of approximately 77% was obtained.

[0042] (2) Take 100 parts by weight of the cationic epoxy resin obtained in step (1) and place it in a container, then heat it to 75°C. Add 7.5 parts by weight of phenolic resin oligomer (Novolac) with a hydroxyl value of 230 mg KOH / g and 0.18 parts by weight of quaternary phosphine salt latent base diphenyl (3-nitrobenzyl)phosphine bromide (TBPP). Stir at 450 rpm for 40 minutes to obtain mixture A. After stirring, remove the solvent and microbubbles under a vacuum of -0.092 MPa for 18 minutes, then cool to 58°C.

[0043] At 58°C, mixture A was neutralized in two stages using a lactic acid / acetic acid (1:1 equivalent) mixture: the first stage neutralized to approximately 42% and allowed to stand for 18 minutes; the second stage continued neutralization to approximately 48% final neutralization, with a system pH of approximately 5.8. Subsequently, a three-stage emulsification process was performed: the first stage added 28% deionized water and emulsified at high speed for 18 minutes; the second stage added 32% deionized water and emulsified at medium speed for 14 minutes; the third stage added the remaining deionized water at low shear at 350 rpm, adjusting the mother liquor solids content to 32 wt.%. After aging for 25 minutes, the mixture was filtered through a 50 μm filter bag to obtain the resin emulsion mother liquor.

[0044] (3) (a) Core premix: Under nitrogen protection, pentaerythritol tetra(3-mercaptopropionate) (PETMP) and trimethylolpropane tri(3-mercaptopropionate) (TMPTMP) were mixed at a mass ratio of 7:3 to obtain a basic core mixture. 1.8 parts by mass of di-tert-butylparaben peroxide (TBPB) and 280 ppm (by mass) of p-hydroxyanisole (MEHQ) were added to 100 parts by mass of the basic core mixture, and the mixture was stirred at 25°C for 28 minutes until completely homogeneous to obtain the core premix solution, which was stored in the dark.

[0045] (b) Preparation of the outer shell aqueous phase: The commercially available melamine-formaldehyde prepolymer was prepared into an aqueous solution with a solid content of 9 wt%. The pH was adjusted to 8.7 with sodium carbonate solution and prepolymerized at 48°C for 28 minutes to obtain the outer shell aqueous phase.

[0046] (c) Emulsification and nucleation: The aqueous phase of the shell was maintained at 43°C, and under high-speed shear at 7000 rpm, the emulsion was nucleated at a rate of approximately 0.7 mL·s. -1 ·kg -1 All the core premixed solution was added dropwise at a rate of [missing information]. After the addition was complete, high-shear emulsification was continued at 8500 rpm for 11 minutes to form a stable core drop emulsion with a particle size of approximately 1.0 μm.

[0047] (d) Condensation to form a shell: The pH of the system was slowly adjusted to 4.3 with a 10% citric acid aqueous solution, and the reaction was carried out at 57°C for 2 hours. Then the temperature was raised to 70°C and the mixture was allowed to mature for another hour to obtain a milky white microcapsule dispersion.

[0048] (e) Surface cationization: The dispersion was cooled to 27°C, and the pH was adjusted to 5.3 with dilute formic acid. A 20 wt% aqueous solution of polydiallyl dimethyl ammonium chloride (PDADMAC) was slowly added, amounting to 0.35 wt% of the total mass of the dispersion (based on PDADMAC solids). The mixture was stirred for 22 minutes. Finally, the mixture was centrifuged, washed, and redispersed in deionized water. The solid content was adjusted to 29 wt% to obtain the microcapsule dispersion.

[0049] (4) Take 100 parts by weight (solids) of the resin emulsion mother liquor obtained in step (2) and slowly add it in batches to the microcapsule dispersion obtained in step (3) (equivalent to 1.5 parts by weight of microcapsule solids) under gentle stirring at 28°C and 350 rpm. The addition interval is 2.5 minutes. After all the materials are added, continue stirring for 20 minutes to mix evenly and obtain the cathodic electrophoretic coating of this embodiment.

[0050] The working solution was prepared by mixing cathodic electrophoretic coating, carbon black paste, and deionized water in a mass ratio of 1:3.5:6. The solid content of the solution was approximately 18 wt.%, and the pH was adjusted to 5.5. The solution was allowed to mature for 24 hours before use.

[0051] Comparative Example 1 The difference between this comparative example and Example 1 is that in step (3), no microcapsule dispersion is prepared; instead, the same mass of unencapsulated pentaerythritol tetra(3-mercaptopropionate) (PETMP), trimethylolpropane tri(3-mercaptopropionate) (TMPTMP), and di-tert-butylparaben peroxide (TBPB) are added directly to the resin emulsion mother liquor as in Example 1. The remaining steps are the same as in Example 1.

[0052] Comparative Example 2 The difference between this comparative example and Example 1 is that in step (2), the quaternary phosphine latent base (TPPBr) is replaced with an equal mass of the conventional tertiary amine catalyst N,N-dimethylbenzylamine. The remaining steps are the same as in Example 1.

[0053] Comparative Example 3 The difference between this comparative example and Example 1 is that no phenolic resin oligomer (Novolac) is added in step (2). The remaining steps and parameters are exactly the same as in Example 1.

[0054] Comparative Example 4 The difference between this comparative example and Example 1 is that in step (2), the phenolic resin oligomer (Novolac) and the quaternary phosphine latent base (TPPBr) are replaced with an equal mass of caprolactam-blocked toluene diisocyanate (TDI) trimer. The remaining steps and parameters are exactly the same as in Example 1.

[0055] Comparative Example 5 The difference between this comparative example and Example 1 is that in step (1) of preparing the cationic epoxy main resin, the reaction that introduces allyl double bonds is not carried out, that is, allyl glycidyl ether (AGE) and related catalysts and inhibitors are not added. The remaining steps are the same as in Example 1.

[0056] Performance testing To evaluate the overall performance of the cathodic electrophoretic coating provided by the present invention, particularly its corrosion resistance, edge protection capability, and abrasion resistance, the coatings prepared in Examples 1 and 2 and Comparative Examples 1-5 were coated and tested. The test substrates were cold-rolled steel sheets that had undergone standard phosphating treatment.

[0057] 1. Neutral salt spray resistance: According to GB / T 10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test". After the test plate is scratched, it is continuously sprayed in the salt spray chamber, and the number of hours when the corrosion width on one side of the scratch reaches 2 mm is recorded.

[0058] 2. Edge film thickness ratio: The dry film thickness was measured using a magnetic thickness gauge in both the standard flat area (average of 5 points within a Φ50mm circle) and a pre-set sharp edge (R≈0.3mm, average of 5 points). The ratio (%) of the edge film thickness to the flat film thickness was calculated to quantify edge coverage capability.

[0059] 3. Abrasion resistance: Tested according to GB / T 23988-2009 Determination of abrasion resistance of coatings by falling sand method. Record the mass (g) of standard sand consumed when the coating is worn through and the substrate is exposed.

[0060] 4. Adhesion: Tested according to GB / T 9286-2021 Paints and Varnishes Cross-cut Test, with an evaluation level of 0-5 (level 0 being the best).

[0061] 5. Minimum complete curing condition: determined by measuring the gel fraction of the cured coating at different temperatures / times using the acetone wiping method to be ≥95%. Record the minimum baking temperature (time fixed at 25 minutes) to achieve complete curing.

[0062] The test results are shown in Table 1.

[0063] Table 1 Performance Test Results As can be seen from the above performance test results, Examples 1 and 2 exhibit excellent comprehensive performance. The coating has excellent corrosion resistance, good edge protection capability, and high mechanical strength. At the same time, it achieves the goal of low-temperature curing.

[0064] The low edge film thickness ratio in Comparative Example 1 may be due to the partial consumption or migration of unencapsulated thiol compounds and initiators in the electrophoresis bath or early baking stage, preventing them from concentrating and effectively initiating the click reaction within the designed low-temperature window, thus weakening the effects of rapid setting and edge protection. Comparative Example 2 uses a tertiary amine with catalytic activity at room temperature. While this has little impact on adhesion, it may lead to partial pre-crosslinking or changes in the reaction pathway, resulting in a decrease in coating density. Simultaneously, the temperature required for complete curing increases, indicating that the latent properties of quaternary phosphine salts are crucial for achieving low-temperature curing. Comparative Example 3, lacking phenolic resin oligomers, cannot form a dense ether bond crosslinking network with epoxy groups at high temperatures, leading to a significant decrease in coating crosslinking density, a substantial reduction in salt spray resistance and abrasion resistance, and a significantly higher complete curing temperature. The edge film thickness ratio is only 54%, fully demonstrating that phenolic resin oligomers are the key component for achieving low-temperature dense curing and high edge coverage. Comparative Example 4, using a conventional high-temperature unsealing isocyanate curing agent, exhibited the highest minimum complete curing temperature, indicating that the phenolic / quaternary phosphine salt system of this invention possesses superior mid-temperature reactivity. Comparative Example 5 showed a significant decrease in all properties, particularly the lowest edge film thickness ratio. The presumed reason may be that the resin lacks sites for low-temperature mercapto-ene click reactions, preventing the rapid establishment of a cross-linking network to inhibit paint flow during the initial baking stage, resulting in severely insufficient edge coverage and consequently affecting overall corrosion resistance and abrasion resistance.

[0065] Figure 1 The figures show the Fourier transform infrared spectra of the cured coatings prepared in Example 1 and Comparative Examples 1, 3, and 5 of this invention. As can be seen from the figures, Example 1 exhibits the highest Fourier transform infrared (FTIR) spectra at 2570 cm⁻¹. -1 The characteristic peak of thiol group completely disappeared at 1640 cm⁻¹ -1 The allyl double bond peak at 910 cm⁻¹ was significantly weakened. -1 The characteristic peak of the epoxy group disappears, and a peak at 1550 cm⁻¹ appears. -1 melamine The characteristic peak of formaldehyde shell is at 1510 cm⁻¹ -1 Phenolic benzene ring characteristic peaks; Comparative Examples 1, 3, and 5 showed obvious thiol residues, epoxy residues, or missing characteristic peaks due to the lack of microcapsules, phenolic crosslinking agents, and alkenylated main resins, respectively, confirming the effectiveness of the two-stage curing reaction and structural design of this invention.

[0066] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a low-temperature curing, high-edge-coverage cathodic electrophoretic coating, characterized in that, Includes the following steps: (1) Using bisphenol A type epoxy resin with an epoxy equivalent of 180-330 g / eq as the matrix, chain extension is carried out by ring-opening addition reaction with hydroxyl compound to obtain chain extension intermediate with an epoxy equivalent of 800-1300 g / eq; allyl glycidyl ether is etherified with chain extension intermediate to obtain epoxy resin modified body containing double bond; epoxy resin modified body is reacted with amine compound to introduce tertiary amine or quaternary ammonium group, and neutralized with acid to obtain cationic epoxy main resin; (2) Add phenolic resin oligomer and quaternary phosphine salt latent base to cationic epoxy main resin, mix, and obtain mixture A; neutralize mixture A with acid, add deionized water for emulsification, and obtain resin emulsion mother liquor; (3) Using melamine-formaldehyde prepolymer as the shell, a mixture of droplets containing multifunctional thiol compounds and thermally decomposable free radical initiators is encapsulated as the core to prepare a microcapsule dispersion, and the microcapsule dispersion is subjected to surface cationization treatment. (4) Add the surface-cationized microcapsule dispersion to the resin emulsion mother liquor. The solid mass of the added microcapsules is 0.5 to 5.0 parts by weight, based on 100 parts by weight of the solid mass of the cationic epoxy main resin. Mix to obtain the cathodic electrophoretic coating.

2. The preparation method according to claim 1, characterized in that, In step (1), the hydroxyl compound is at least one of bisphenol A, propylene glycol, or diethylene glycol; the amount of the hydroxyl compound added is, based on the equivalent of the hydroxyl groups it contains, in a ratio of 0.20 to 0.35:1 to the equivalent of the epoxy groups contained in the bisphenol A type epoxy resin matrix; the ring-opening addition reaction is carried out in the presence of a catalyst, the catalyst being triphenylphosphine, and its amount is 0.05 to 0.20 wt% of the mass of the bisphenol A type epoxy resin matrix.

3. The preparation method according to claim 1, characterized in that, In step (1), the molar equivalent ratio of the allyl glycidyl ether to the hydroxyl group contained in the chain extension intermediate is 0.10 to 0.20:1; the etherification reaction is carried out in the presence of a catalyst and a free radical polymerization inhibitor, wherein the catalyst is triphenylphosphine or imidazole, and the amount of catalyst used is 0.05 to 0.20 wt% of the mass of the chain extension intermediate; the free radical polymerization inhibitor is hydroquinone or p-hydroxyanisole.

4. The preparation method according to claim 1, characterized in that, In step (1), the amine compound is diethylamine, diisopropylamine or a quaternized monomer; the amount of the amine compound added is such that the amine value of the cationic epoxy resin is 40 to 120 mg KOH / g.

5. The preparation method according to claim 1, characterized in that, In step (2), the hydroxyl value of the phenolic resin oligomer is 200-300 mg KOH / g; based on 100 parts by weight of the solid mass of the cationic epoxy main resin obtained in step (1), the amount of phenolic resin oligomer added is 6-10 parts by weight, and the amount of quaternary phosphine salt latent base added is 0.10-0.25 parts by weight.

6. The preparation method according to claim 1, characterized in that, In step (2), the quaternary phosphine salt latent base is tetraphenylphosphine bromide or diphenyl(3-nitrobenzyl)phosphine bromide.

7. The preparation method according to claim 1, characterized in that, In step (3), the multifunctional thiol compound is a mixture of pentaerythritol tetra(3-mercaptopropionate) and trimethylolpropane tri(3-mercaptopropionate) in a mass ratio of 7:(2-4); the thermally decomposable free radical initiator is di-tert-butyl benzoate peroxide.

8. The preparation method according to claim 1, characterized in that, In step (3), the preparation process of the microcapsule dispersion includes: (a) Pentaerythritol tetra(3-mercaptopropionate) and trimethylolpropane tri(3-mercaptopropionate) were mixed to obtain a basic core mixture. Di-tert-butyl benzoate peroxide and p-hydroxyanisole were added to the basic core mixture and stirred to obtain a core premix. (b) Prepare an aqueous solution of melamine-formaldehyde prepolymer with a solid content of 8-12 wt%, adjust the pH to 8.5-9.0, and prepolymerize at 40-50°C for 20-30 minutes to obtain the outer shell aqueous phase; (c) The core premix is ​​added dropwise to the aqueous phase of the outer shell, and shear emulsified to form a core drop emulsion; (d) Adjust the pH of the nuclear drop emulsion to 4.0-4.5, react at 55-60°C for 1-3 hours to mature, and form a microcapsule dispersion.

9. The preparation method according to claim 8, characterized in that, In step (a), the mass ratio of pentaerythritol tetra(3-mercaptopropionate) to trimethylolpropane tri(3-mercaptopropionate) is 7:(2-4); the amount of di-tert-butylbenzoate peroxide added is 1.5 to 2.5 wt% of the mass of the basic core mixture; and the amount of p-hydroxyanisole added is 200 to 300 ppm of the mass of the basic core mixture.

10. A low-temperature curing high edge coverage cathodic electrophoretic coating prepared by the preparation method according to any one of claims 1-9.

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