A method for preparing pigment paste for electrophoretic coatings

Abstract

This invention belongs to the field of electrophoretic coating technology, specifically relating to a method for preparing color pastes for electrophoretic coatings. The modified waterborne dispersion resin of this invention forms a strong chemical anchoring effect between functional monomers and the pigment surface. Combined with the inertness of the blocked crosslinking agent during storage, this endows the color paste with exceptional dispersion and storage stability, fundamentally ensuring product quality consistency and the reliability of electrophoretic application. Secondly, based on a highly crosslinked dense network structure, the reinforcing effect of nano-barium sulfate, and the synergistic effect of a highly efficient composite catalyst, the final cured paint film exhibits excellent comprehensive performance, possessing superior water resistance, impact resistance, and neutral salt spray resistance. Furthermore, this technology is based on a water-based system and uses an environmentally friendly composite catalyst to replace the toxic organotin catalyst, improving product performance while meeting increasingly stringent environmental regulations, demonstrating significant technological advancement and market application value.

Description

Technical Field

[0001] This invention belongs to the field of electrophoretic coating technology, and specifically relates to a method for preparing color paste for electrophoretic coating. Background Technology

[0002] Cathodic electrophoretic coatings are widely used in electronic components, building hardware, and bathroom products due to their excellent corrosion resistance, adaptability to automated coating, and uniformity. With the increasing demand for decorative properties in high-end fields, colored electrophoretic pigments have become a research hotspot, but existing technologies still have the following key problems: (1) Stability problem: Pigment particles have a high specific gravity and are prone to sedimentation and clumping during storage, resulting in color difference, clogging of spray guns, or instability of electrophoretic bath liquid, affecting production efficiency and product quality. (2) Compatibility problem with resin: Conventional dispersion resins have insufficient bonding force with the pigment surface, resulting in low dispersion efficiency, difficulty in achieving the required grinding fineness, and affecting the density and adhesion of the paint film. (3) Contradiction between environmental protection and performance: Water-based is an environmentally friendly trend, but simple water-based systems are often inferior to solvent-based systems in key performance aspects such as corrosion resistance and impact resistance. At the same time, traditional catalysts (such as organotin) pose environmental and health risks. (4) Single function: Traditional pigments mainly provide color and have limited contribution to enhancing the overall mechanical properties and corrosion resistance of the paint film.

[0003] Therefore, developing a composite electrophoretic coating pigment that combines excellent stability, superior film performance, and high environmental friendliness has become a pressing technical problem to be solved in this field. Summary of the Invention

[0004] The purpose of this invention is to address the existing problems by providing a method for preparing color paste for electrophoretic coatings.

[0005] This invention is achieved through the following technical solution:

[0006] A method for preparing a color paste for electrophoretic coatings includes the following steps:

[0007] S1. Add the water-based dispersion resin, dispersant EFKA-4560 and deionized water to the paint mixing tank and stir until uniform;

[0008] S2. Add pigment, nano-barium sulfate, crosslinking agent, and composite catalyst in sequence, and stir for 30-40 minutes.

[0009] S3. Then transfer to a nano-grind mill and grind for 60-90 minutes. If the fineness is ≤10μm as measured by a laser particle size analyzer, stop grinding. Filter with a 100-mesh filter to remove zirconium beads, and add deionized water to adjust the viscosity of the pigment to 20±2.

[0010] Further, the weight parts of each raw material in step S1 are: 25-30 parts of water-based dispersion resin, 1.5-2.5 parts of dispersant EFKA-4560, and 5-8 parts of deionized water.

[0011] Furthermore, the stirring speed is controlled at 400~500 rpm during the stirring process described in step S1.

[0012] Furthermore, the preparation of the aqueous dispersion resin described in step S1 includes the following steps:

[0013] S101. Prepare a four-necked flask, set the stirring speed to 300 rpm, purge the air in the system with nitrogen three times, add 20-27 parts of deionized water and 0.5-0.8 parts of emulsifier OP-10 to the flask, stir to dissolve, heat to 79-83℃, keep warm for 10-14 min, add the monomer mixture through dropping funnel A, stir evenly, add APS aqueous solution through dropping funnel B, add simultaneously through both funnels, control the dropping rate to complete the dropping within 3-3.5 h, and maintain the reaction system temperature at 79-83℃.

[0014] S102. After the addition is complete, heat to 85~88℃ and keep warm for 1~1.5h. Add the remaining APS aqueous solution and continue the reaction for 2~2.5h. Then cool down to 40~45℃ and add 10~14 parts of N,N-dimethylaminopropylamine dropwise over 25~35min. After the addition is complete, keep warm for 1~2h. Adjust the pH of the system to 7.0~7.5 with lactic acid and stir for 30~35min to obtain the water-based dispersion resin for later use.

[0015] Further, the monomer mixture described in step S101 is composed of the following components in parts by weight: 15-18 parts of dibutyl itaconic acid (DBI), 15-18 parts of glycidyl methacrylate (GMA), 14-16 parts of hydroxyethyl acrylate (HEA), and 20-25 parts of methyl methacrylate (MMA).

[0016] Further, the weight parts of each raw material in step S2 are: 20-25 parts of pigment, 8-10 parts of nano barium sulfate, and 0.03-0.05 parts of composite catalyst;

[0017] The amount of crosslinking agent is measured according to a molar ratio of resin hydroxyl groups to crosslinking agent-NCO of 1:1.1.

[0018] Furthermore, the pigment can be any one of Pigment Red 254, Pigment Red 177, Pigment Red 122, Pigment Yellow 151, Pigment Violet 23, Pigment Blue 15:3, Pigment Green 7, and Carbon Black.

[0019] Furthermore, in step S2, the stirring speed is controlled to be 1000~2000 rpm.

[0020] Further, the preparation method of the crosslinking agent in step S2 is as follows: Prepare a three-necked flask, set the stirring speed to 200 rpm, add 18-22 parts of IPDI, heat to 60°C, start stirring, and dropwise add 8-10 parts of pre-melted ε-caprolactam to the IPDI over 1-1.5 hours, controlling the reaction temperature at 78-82°C. Take samples every 30 minutes and monitor the reaction at 2270 cm⁻¹ using Fourier transform infrared spectroscopy. -1 The reaction is stopped when the -NCO characteristic peak is completely disappeared, and the mixture is cooled to room temperature to obtain the crosslinking agent for later use.

[0021] Furthermore, the preparation of the composite catalyst described in step S2 includes the following steps:

[0022] S201. Add cashew nut shellac to deionized water at a solid-liquid ratio of 1:1, and heat to 50-55°C while stirring. Under constant temperature conditions, add 40% dimethylamine aqueous solution (molar ratio of dimethylamine to cashew nut shellac is 1.2:1) and 37% formaldehyde aqueous solution (molar ratio of formaldehyde to cashew nut shellac is 1.1:1) dropwise over 30-40 minutes. Heat to 78-82°C and maintain the temperature for 2.5-3.5 hours. After the reaction is complete, allow the mixture to stand and separate into layers. Take the upper organic phase, wash it three times with deionized water, and remove the residual water by vacuum distillation to obtain cashew nut shellac amine.

[0023] S202. Add octanoic acid to deionized water at a solid-liquid ratio of 1:5.5, stir to dissolve, adjust the pH to 5.0-5.5 with 10% sodium hydroxide solution, add zinc oxide (molar ratio to octanoic acid is 1:2), heat to 60℃, react at 400-600 rpm for 2-2.5 h, after the reaction is completed, filter while hot (100 mesh filter), cool the filtrate to room temperature, precipitate crystals, and vacuum dry at 50-55℃ for 4-5 h to obtain zinc octanoate powder;

[0024] S203. Add β-cyclodextrin to deionized water at a solid-liquid ratio of 1:3, stir to dissolve, then add 40% sodium hydroxide solution (mass ratio of β-cyclodextrin to 1:10), and add propylene oxide (molar ratio of β-cyclodextrin hydroxyl group to 3:1) dropwise over 1-1.5 hours, controlling the temperature at 40-43°C. After the addition is complete, raise the temperature to 60°C and maintain the reaction for 3.5-4.5 hours, adjusting the pH of the system to 10-11 with 10% dilute hydrochloric acid during the reaction. After the reaction is complete, neutralize to neutral with hydrochloric acid, concentrate under reduced pressure to 1 / 3 of the original volume, add ethanol to precipitate the product, filter, and vacuum dry at 60-65°C for 6-7 hours to obtain hydroxypropyl-β-cyclodextrin.

[0025] S204. Add hydroxypropyl-β-cyclodextrin to deionized water at a solid-liquid ratio of 1:92. Heat to 50-55℃ while stirring at 700-900 rpm and dissolve for 10-15 minutes. Then, add cashew phenolic acid amine (mass ratio of 5:1 to hydroxypropyl-β-cyclodextrin) and zinc octanoate (mass ratio of 2:1 to hydroxypropyl-β-cyclodextrin) in sequence. Continue stirring for 25-35 minutes and then transfer to a high-pressure homogenizer. Process twice at 30 MPa to refine the emulsion particles to a particle size ≤500 nm.

[0026] Furthermore, in step S3, the grinding temperature is set to 28~34℃ and the grinding speed is 3000~4000rpm.

[0027] The present invention has the following advantages over the prior art:

[0028] To address the problems of existing technologies, this invention provides a method for preparing pigment paste for electrophoretic coatings, which significantly improves the physical and mechanical properties and protective performance of the final electrophoretic film while ensuring the stability of the pigment paste. First, the modified waterborne dispersion resin of this invention forms a strong chemical anchoring effect between the functional monomers and the pigment surface. Combined with the inertness of the blocked crosslinking agent during storage, this endows the pigment paste with exceptional dispersion and storage stability. Its centrifugal stability is higher than 99% in absorbance, and it exhibits no precipitation or floating color during long-term storage at 40°C, fundamentally ensuring the consistency of product quality and the reliability of electrophoretic application. Second, based on the highly crosslinked dense network structure, the reinforcing effect of nano-barium sulfate, and the synergistic effect of the highly efficient composite catalyst, the final cured film exhibits excellent comprehensive performance. Its resistance to neutral salt spray reaches 420 hours, its impact resistance reaches 65 cm, and its water resistance shows no abnormalities after 240 hours, significantly superior to traditional formulations. Furthermore, this technology is based on an aqueous system and uses an environmentally friendly composite catalyst to replace the toxic organotin catalyst. While improving product performance, it also meets increasingly stringent environmental regulations, demonstrating significant technological advancement and market application value. Detailed Implementation

[0029] To further explain the present invention, the following specific embodiments are described.

[0030] Unless otherwise specified, the raw materials used in this invention are all from commercially available conventional products. Example 1

[0031] A method for preparing a color paste for electrophoretic coatings includes the following steps:

[0032] S1. Synthesis and modification of waterborne dispersion resins:

[0033] S101. Prepare a four-necked flask (equipped with a mechanical stirrer, thermometer, condenser, double dropping funnel, and nitrogen protection device). Set the stirring speed to 300 rpm, purge the air from the system with nitrogen three times, add 2 parts of deionized water (2 / 3 of the total volume) and 0.5 parts of emulsifier OP-10 to the flask, stir to dissolve, and then heat to 79°C and keep warm for 10 min. Add the monomer mixture (15 parts DBI + 15 parts GMA + 14 parts HEA + 20 parts MMA) through dropping funnel A, stir evenly, and add the APS aqueous solution (0.7 parts APS dissolved in 5 parts deionized water) through dropping funnel B. Add the solution simultaneously through both funnels, controlling the dropping rate to complete the addition within 3 hours, and maintain the reaction system temperature at 79°C.

[0034] S102. After the addition is complete, heat to 85℃ and keep warm for 1 hour. Add the remaining APS aqueous solution (0.3 parts APS dissolved in 3 parts deionized water), continue the reaction for 2 hours, then cool to 40℃ and add 10 parts N,N-dimethylaminopropylamine dropwise over 25 minutes. After the addition is complete, keep warm for 1 hour, adjust the pH of the system to 7.0 with lactic acid, and stir for 30 minutes to obtain an aqueous dispersion resin for later use.

[0035] S2, Preparation of crosslinking agent:

[0036] Prepare a three-necked flask (equipped with a stirrer, thermometer, and reflux condenser). Set the stirring speed to 200 rpm, add 18 parts of IPDI, heat to 60°C, and start stirring. Add 8 parts of pre-melted ε-caprolactam dropwise to the IPDI over 1 hour, maintaining the reaction temperature at 78°C. Take samples every 30 minutes and monitor the reaction at 2270 cm⁻¹ using Fourier transform infrared spectroscopy (FT-IR). -1 The reaction was stopped when the -NCO characteristic peak completely disappeared, and the mixture was cooled to room temperature to obtain the crosslinking agent for later use.

[0037] S3. Preparation of color paste:

[0038] S301. Add 25 parts of the water-based dispersion resin obtained in step S1, 1.5 parts of the dispersant EFKA-4560 and 5 parts of deionized water to the paint mixing tank and stir at 400 rpm until uniform.

[0039] S302, add 20 parts of Pigment Red 254 (or multiple pigments), 8 parts of nano barium sulfate, crosslinking agent (measured according to the molar ratio of resin hydroxyl to crosslinking agent-NCO 1:1.1), and 0.03 parts of composite catalyst in sequence, and stir at 1000 rpm for 30 min;

[0040] S303, then transfer to a nano-sand mill (zirconia beads diameter 0.1mm, filling rate 80%), set the grinding temperature to 28℃, the grinding speed to 3000rpm, and circulate grinding for 60min. If the fineness is ≤10μm as detected by a laser particle size analyzer, stop grinding, filter with a 100-mesh filter to remove zirconia beads, and add deionized water to adjust the viscosity of the color paste to 20±2.

[0041] The preparation of the composite catalyst includes the following steps:

[0042] (1) Cashew phenol was added to deionized water at a solid-liquid ratio of 1:1 (g:mL), and the temperature was raised to 50°C while stirring. Under constant temperature conditions, 40% dimethylamine aqueous solution (molar ratio of cashew phenol to cashew phenol was 1.2:1) and 37% formaldehyde aqueous solution (molar ratio of cashew phenol to cashew phenol was 1.1:1) were added dropwise over a period of 30 min. The temperature was raised to 78°C and the reaction was kept at this temperature for 2.5 h. After the reaction was completed, the mixture was allowed to stand and separate into layers. The upper organic phase was taken, washed three times with deionized water, and the residual water was removed by vacuum distillation to obtain cashew phenol amine.

[0043] (2) Add octanoic acid to deionized water at a solid-liquid ratio of 1:5.5 (g:mL), stir to dissolve, adjust the pH to 5.0 with 10% sodium hydroxide solution, add zinc oxide (molar ratio of 1:2 with octanoic acid), heat to 60℃, react at 400rpm for 2h, after the reaction is completed, filter while hot (100 mesh filter), cool the filtrate to room temperature, precipitate crystals, and vacuum dry at 50℃ for 4h to obtain zinc octanoate powder;

[0044] (3) Add β-cyclodextrin to deionized water at a solid-liquid ratio of 1:3 (g:mL), stir to dissolve, add 40% sodium hydroxide solution (mass ratio of β-cyclodextrin to 1:10), add propylene oxide (molar ratio of β-cyclodextrin hydroxyl group to 3:1) dropwise, add for 1 hour, control the temperature at 40℃, after the addition is complete, raise the temperature to 60℃, keep the reaction at 3.5 hours, adjust the pH of the system to 10 with 10% dilute hydrochloric acid during the reaction, after the reaction is complete, neutralize to neutral with hydrochloric acid, concentrate under reduced pressure to 1 / 3 of the original volume, add ethanol to precipitate the product, filter, and vacuum dry at 60℃ for 6 hours to obtain hydroxypropyl-β-cyclodextrin;

[0045] (4) Add hydroxypropyl-β-cyclodextrin to deionized water at a solid-liquid ratio of 1:92, heat to 50°C under stirring at 700 rpm, dissolve for 10 min, then add cashew phenolic amine (mass ratio of 5:1 to hydroxypropyl-β-cyclodextrin) and zinc octanoate (mass ratio of 2:1 to hydroxypropyl-β-cyclodextrin) in sequence, continue stirring for 25 min, then transfer to a high-pressure homogenizer and process twice at 30 MPa to refine the emulsion particles to a particle size ≤500 nm. Example 2

[0046] A method for preparing a color paste for electrophoretic coatings includes the following steps:

[0047] S1. Synthesis and modification of waterborne dispersion resins:

[0048] S101. Prepare a four-necked flask, set the stirring speed to 300 rpm, purge the air from the system with nitrogen three times, add deionized water (2 / 3 of the total volume, 23 parts) and emulsifier OP-10 0.65 parts to the flask, stir to dissolve, heat to 80℃, keep warm for 12 min, add monomer mixture (DBI 16 parts + GMA 16 parts + HEA 15 parts + MMA 22 parts) through dropping funnel A, stir evenly, add APS aqueous solution (APS 0.85 parts dissolved in 5 parts deionized water) through dropping funnel B, add simultaneously through both funnels, control the dropping rate to complete the dropping within 3.3 h, and maintain the reaction system temperature at 81℃;

[0049] S102. After the addition is complete, the temperature is raised to 87℃ and kept at this temperature for 1.2h. The remaining APS aqueous solution (0.5 parts of APS dissolved in 3 parts of deionized water) is added, and the reaction continues for 2.2h. Then the temperature is lowered to 42℃, and 12 parts of N,N-dimethylaminopropylamine are added dropwise over a period of 30min. After the addition is complete, the temperature is kept at this temperature for 1.5h. The pH of the system is adjusted to 7.2 with lactic acid, and the mixture is stirred for 32min to obtain an aqueous dispersion resin for later use.

[0050] S2, Preparation of crosslinking agent:

[0051] Prepare a three-necked flask, set the stirring speed to 200 rpm, add 20 parts of IPDI, heat to 60℃, start stirring, and dropwise add 9 parts of pre-melted ε-caprolactam to the IPDI over 1.2 hours, controlling the reaction temperature at 80℃. Take samples every 30 minutes and monitor the reaction at 2270 cm⁻¹ using Fourier transform infrared spectroscopy (FT-IR). -1 The reaction was stopped when the -NCO characteristic peak completely disappeared, and the mixture was cooled to room temperature to obtain the crosslinking agent for later use.

[0052] S3. Preparation of color paste:

[0053] S301. Add 27 parts of the water-based dispersion resin obtained in step S1, 2 parts of the dispersant EFKA-4560 and 6.5 parts of deionized water to the paint mixing tank and stir at 450 rpm until uniform.

[0054] S302, add 22 parts of Pigment Red 254 (or multiple pigments), 9 parts of nano barium sulfate, crosslinking agent (measured according to the molar ratio of resin hydroxyl to crosslinking agent-NCO 1:1.1), and 0.04 parts of composite catalyst in sequence, and stir at 1500 rpm for 35 min;

[0055] S303, then transfer to a nano-grind mill (zirconia beads diameter 0.2mm, filling rate 80%), set the grinding temperature to 31℃, the grinding speed to 3500rpm, and circulate grinding for 75min. If the fineness is ≤10μm as detected by a laser particle size analyzer, stop grinding, filter with a 100-mesh filter to remove the zirconia beads, and add deionized water to adjust the viscosity of the color paste to 20±2.

[0056] The preparation of the composite catalyst includes the following steps:

[0057] (1) Add cashew phenol to deionized water at a solid-liquid ratio of 1:1, and heat to 52°C while stirring. Under constant temperature conditions, add 40% dimethylamine aqueous solution (molar ratio of cashew phenol to cashew phenol is 1.2:1) and 37% formaldehyde aqueous solution (molar ratio of cashew phenol to cashew phenol is 1.1:1) dropwise over a period of 35 min. Heat to 80°C and keep the reaction at this temperature for 3 h. After the reaction is complete, allow the mixture to stand and separate into layers. Take the upper organic phase, wash it three times with deionized water, and remove the residual water by vacuum distillation to obtain cashew phenol amine.

[0058] (2) Add octanoic acid to deionized water at a solid-liquid ratio of 1:5.5, stir to dissolve, adjust the pH to 5.2 with 10% sodium hydroxide solution, add zinc oxide (molar ratio of 1:2 with octanoic acid), heat to 60℃, react at 500rpm for 2.2h, after the reaction is completed, filter while hot (100 mesh filter), cool the filtrate to room temperature, precipitate crystals, and vacuum dry at 52℃ for 4.5h to obtain zinc octanoate powder;

[0059] (3) Add β-cyclodextrin to deionized water at a solid-liquid ratio of 1:3, stir to dissolve, add 40% sodium hydroxide solution (mass ratio of β-cyclodextrin to 1:10), add propylene oxide (molar ratio of β-cyclodextrin hydroxyl group to 3:1) dropwise, add for 1.2 h, control the temperature at 41℃, after the addition is complete, raise the temperature to 60℃, keep the reaction at 4 h, adjust the pH of the system with 10% dilute hydrochloric acid to maintain at 10.5 during the reaction, neutralize to neutral with hydrochloric acid, concentrate under reduced pressure to 1 / 3 of the original volume, add ethanol to precipitate the product, filter, and vacuum dry at 62℃ for 6.5 h to obtain hydroxypropyl-β-cyclodextrin;

[0060] (4) Add hydroxypropyl-β-cyclodextrin to deionized water at a solid-liquid ratio of 1:92, heat to 52°C under stirring at 800 rpm, dissolve for 12 min, then add cashew phenolic amine (mass ratio of 5:1 to hydroxypropyl-β-cyclodextrin) and zinc octanoate (mass ratio of 2:1 to hydroxypropyl-β-cyclodextrin) in sequence, continue stirring for 30 min, then transfer to a high-pressure homogenizer and process twice at 30 MPa to refine the emulsion particles to a particle size ≤500 nm. Example 3

[0061] A method for preparing a color paste for electrophoretic coatings includes the following steps:

[0062] S1. Synthesis and modification of waterborne dispersion resins:

[0063] S101. Prepare a four-necked flask, set the stirring speed to 300 rpm, purge the air from the system with nitrogen three times, add deionized water (2 / 3 of the total volume, 27 parts) and emulsifier OP-10 0.8 parts to the flask, stir to dissolve, heat to 83℃, keep warm for 14 min, add monomer mixture (18 parts DBI + 18 parts GMA + 16 parts HEA + 25 parts MMA) through dropping funnel A, stir evenly, add APS aqueous solution (1.05 parts APS dissolved in 5 parts deionized water) through dropping funnel B, add simultaneously through both funnels, control the dropping rate to complete the dropping within 3.5 h, and maintain the reaction system temperature at 83℃;

[0064] S102. After the addition is complete, heat to 88℃ and keep warm for 1.5h. Add the remaining APS aqueous solution (0.45 parts APS dissolved in 3 parts deionized water), continue the reaction for 2.5h, then cool to 45℃ and add 14 parts N,N-dimethylaminopropylamine dropwise over 35min. After the addition is complete, keep warm for 2h, adjust the pH of the system to 7.5 with lactic acid, and stir for 35min to obtain an aqueous dispersion resin for later use.

[0065] S2, Preparation of crosslinking agent:

[0066] Prepare a three-necked flask, set the stirring speed to 200 rpm, add 22 parts of IPDI, heat to 60℃, start stirring, and dropwise add 10 parts of pre-melted ε-caprolactam to the IPDI over 1.5 hours, controlling the reaction temperature at 82℃. Take samples every 30 minutes and monitor the reaction at 2270 cm⁻¹ using Fourier transform infrared spectroscopy (FT-IR). -1 The reaction was stopped when the -NCO characteristic peak completely disappeared, and the mixture was cooled to room temperature to obtain the crosslinking agent for later use.

[0067] S3. Preparation of color paste:

[0068] S301. Add 30 parts of the water-based dispersion resin obtained in step S1, 2.5 parts of the dispersant EFKA-4560 and 8 parts of deionized water to the paint mixing tank and stir at 500 rpm until uniform.

[0069] S302, add 25 parts of Pigment Red 254 (or multiple pigments), 10 parts of nano barium sulfate, crosslinking agent (measured according to the molar ratio of resin hydroxyl to crosslinking agent-NCO 1:1.1), and 0.05 parts of composite catalyst in sequence, and stir at 2000 rpm for 40 min;

[0070] S303, then transfer to a nano-sand mill (zirconia beads diameter 0.3mm, filling rate 80%), set the grinding temperature to 34℃, the grinding speed to 4000rpm, and circulate grinding for 90min. If the fineness is ≤10μm as detected by a laser particle size analyzer, stop grinding, filter with a 100-mesh filter to remove zirconia beads, and add deionized water to adjust the viscosity of the color paste to 20±2.

[0071] The preparation of the composite catalyst includes the following steps:

[0072] (1) Add cashew phenol to deionized water at a solid-liquid ratio of 1:1, and heat to 55°C while stirring. Under constant temperature conditions, add 40% dimethylamine aqueous solution (molar ratio of cashew phenol to cashew phenol is 1.2:1) and 37% formaldehyde aqueous solution (molar ratio of cashew phenol to cashew phenol is 1.1:1) dropwise over a period of 40 min. Heat to 82°C and keep the reaction at this temperature for 3.5 h. After the reaction is complete, allow the mixture to stand and separate into layers. Take the upper organic phase, wash it three times with deionized water, and remove the residual water by vacuum distillation to obtain cashew phenol amine.

[0073] (2) Add octanoic acid to deionized water at a solid-liquid ratio of 1:5.5, stir to dissolve, adjust the pH to 5.5 with 10% sodium hydroxide solution, add zinc oxide (molar ratio of 1:2 with octanoic acid), heat to 60℃, react at 600rpm for 2.5h, after the reaction is completed, filter while hot (100 mesh filter), cool the filtrate to room temperature, precipitate crystals, and vacuum dry at 55℃ for 5h to obtain zinc octanoate powder;

[0074] (3) Add β-cyclodextrin to deionized water at a solid-liquid ratio of 1:3, stir to dissolve, add 40% sodium hydroxide solution (mass ratio of β-cyclodextrin to 1:10), add propylene oxide (molar ratio of β-cyclodextrin hydroxyl group to 3:1) dropwise, add for 1.5 h, control the temperature at 43 °C, after the addition is complete, raise the temperature to 60 °C, keep the reaction at 4.5 h, adjust the pH of the system to 11 with 10% dilute hydrochloric acid during the reaction, after the reaction is complete, neutralize to neutral with hydrochloric acid, concentrate under reduced pressure to 1 / 3 of the original volume, add ethanol to precipitate the product, filter, and vacuum dry at 65 °C for 7 h to obtain hydroxypropyl-β-cyclodextrin;

[0075] (4) Add hydroxypropyl-β-cyclodextrin to deionized water at a solid-liquid ratio of 1:92, heat to 55°C under stirring at 900 rpm, dissolve for 15 min, then add cashew phenolic amine (mass ratio of 5:1 to hydroxypropyl-β-cyclodextrin) and zinc octanoate (mass ratio of 2:1 to hydroxypropyl-β-cyclodextrin) in sequence, continue stirring for 35 min, then transfer to a high-pressure homogenizer and process twice at 30 MPa to refine the emulsion particles to a particle size ≤500 nm.

[0076] Comparative Example 1

[0077] Compared with Example 2, Comparative Example 1 omits step S1 and the entire process of synthesizing and modifying the aqueous dispersion resin. The aqueous dispersion resin in step S3 is replaced with ordinary aqueous acrylic resin. The other steps are the same as in Example 2.

[0078] Comparative Example 2

[0079] Compared with Example 2, Comparative Example 2 changes the amount of crosslinking agent in step S302, replacing "resin hydroxyl group to crosslinking agent-NCO molar ratio 1:1.1" with "resin hydroxyl group to crosslinking agent-NCO molar ratio 1:0.9", while the other steps are the same as in Example 2.

[0080] Comparative Example 3

[0081] Compared with Example 2, Comparative Example 3 omits the addition of nano-barium sulfate in step S302, while the other steps are the same as in Example 2.

[0082] Comparative Example 4

[0083] Compared with Example 2, Comparative Example 4 replaces the composite catalyst with an equal amount of conventional organotin catalyst (dibutyltin dilaurate) and omits the preparation of the composite catalyst. The other steps are the same as in Example 2.

[0084] 1. Pigment performance testing

[0085] Color pastes were prepared using the methods described in Examples 1-3 and Comparative Examples 1-4, respectively, and then centrifugal stability and storage stability tests were performed.

[0086] (1) Centrifugal stability test:

[0087] Take a small amount of color paste, dilute it 4000 times, and measure the absorbance A0 at a wavelength of 510 nm using a UV spectrophotometer. Take 10 g of color paste and place it in a 30 mL centrifuge tube. Centrifuge at 3000 rpm for 30 min. Insert a pipette below the liquid surface in the centrifuge tube, take 1 mL of color paste, dilute it 4000 times, and measure its absorbance A at 510 nm. Compare this absorbance with the absorbance A0 of the system before centrifugation. Calculate the specific absorbance r according to the following formula.

[0088]

[0089] The closer the absorbance is to 100%, the better the stability of the system.

[0090] (2) Storage stability test:

[0091] After sealing and storing the pigment at 40°C for 30 days, observe the sedimentation at the bottom of the pigment.

[0092] The test results are shown in Table 1 below.

[0093] Table 1. Results of Color Paste Performance Tests

[0094]

[0095] As shown in Table 1 above, the specific absorbance of the pigment pastes in Examples 1-3 is close to 100%. After centrifugation at 3000 rpm for 30 minutes, the pigment particles showed almost no sedimentation, indicating a stable dispersion system. Comparative Example 1, however, had a specific absorbance of only 68.2%, and significant precipitation occurred after 30 days of storage at 40°C. This may be because the ordinary water-based acrylic resin lacks the epoxy groups of GMA and the ester groups of DBI, failing to form a strong bond with the pigment surface. During centrifugation, the pigment precipitates rapidly, resulting in absorbance loss. Furthermore, the ordinary resin has poor thermal stability, leading to a decrease in its encapsulation force on the pigment during storage, causing the pigment particles to settle. In Comparative Example 2, reducing the amount of crosslinking agent had little impact on the comparative absorbance and storage stability. This may be because the crosslinking agent of this invention is in a closed state during storage and does not participate in the pigment dispersion process, thus not affecting storage stability. The difference between Comparative Example 3 (omitting nano-barium sulfate) and Example 2 is minimal because nano-barium sulfate is primarily used to improve the density and scratch resistance of the paint film and does not participate in the pigment dispersion system. Furthermore, the particle size of nano-barium sulfate differs significantly from that of the pigment, thus it does not compete with the pigment for adsorption dispersant and does not induce aggregation. Therefore, it has almost no impact on centrifugal and storage stability. Comparative Example 4 has a specific absorbance of 94.8% and exhibits a small amount of precipitation when stored at 40°C, showing weaker stability than the examples. This indicates that the use of the composite catalyst has a certain promoting effect on the centrifugal and storage stability of the color paste.

[0096] 2. Coating film performance testing

[0097] The color pastes prepared according to the methods of Examples 2 and Comparative Examples 1-4 were mixed with acrylic cathodic electrophoretic coating and deionized water at a mass ratio of 1:1:5 to prepare working solutions. The solutions were allowed to mature at room temperature for 24 hours. Electrophoretic coating was applied to the sample surface at an electrophoretic voltage of 60-80V and an electrophoretic time of 60-90s, and cured at 160℃ for 30 minutes to obtain an electrophoretic paint film. The water resistance of the paint film was then tested according to GB / T 1733-2020, the impact resistance according to GB / T 1732-2020, and the neutral salt spray resistance according to GB / T 1771-2007.

[0098] The test results are shown in Table 2 below.

[0099] Table 2 Results of Coating Film Performance Test

[0100]

[0101] As shown in Table 2 above, compared with Example 2, Comparative Example 1, using ordinary acrylic resin, exhibited poorer water resistance, an impact resistance of only 50 cm, and a salt spray resistance of 280 h. The modified dispersion resin is fundamental to the high mechanical properties of the paint film. Comparative Example 2, lacking sufficient crosslinking agent, saw its water resistance drop from level 0 to level 1, and its salt spray resistance decrease from 420 h to 300 h. This may be because insufficient crosslinking agent allows -NCO to react with only 90% of the resin hydroxyl groups, leaving uncrosslinked hydroxyl groups in the paint film. Water molecules easily penetrate and cause bubbling, and the unreacted hydroxyl groups also reduce the corrosion resistance of the paint film. Comparative Example 3, without nano-barium sulfate, saw its impact resistance decrease from 65 cm to 55 cm, and its salt spray resistance decrease from 420 h to 320 h. This indicates that the nano-reinforcing effect of nano-barium sulfate helps improve the hardness of the paint film, making it less prone to deformation under impact and providing protection to the substrate. Comparative Example 4 used an organotin catalyst, and the salt spray resistance of the paint film decreased from 420h to 380h, indicating that the catalytic activity of organotin was lower than that of the composite catalyst of the present invention, resulting in incomplete crosslinking reaction, a small amount of unreacted groups remaining in the paint film, and a decrease in salt spray resistance.

[0102] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a color paste for electrophoretic coatings, characterized in that, Includes the following steps: S1. Add the water-based dispersion resin, dispersant EFKA-4560 and deionized water to the paint mixing tank and stir until uniform; S2. Add pigment, nano-barium sulfate, blocked crosslinking agent, and composite catalyst in sequence, and stir until uniform; S3. Then transfer to a nano-grind mill for circulating grinding. When the fineness is ≤10μm as detected by a laser particle size analyzer, stop grinding, filter out zirconium beads, and add deionized water to adjust the viscosity of the color paste to 20±2. The preparation of the aqueous dispersion resin described in step S1 includes the following steps: S101. Prepare a four-necked flask, set the stirring speed to 300 rpm, purge the air in the system with nitrogen three times, add 20-27 parts of deionized water and 0.5-0.8 parts of emulsifier OP-10 to the flask, stir to dissolve, heat to 79-83℃, keep warm for 10-14 min, add the monomer mixture through dropping funnel A, stir evenly, add APS aqueous solution through dropping funnel B, add simultaneously through both funnels, control the dropping rate to complete the dropping within 3-3.5 h, and maintain the reaction system temperature at 79-83℃. S102. After the addition is complete, heat to 85~88℃ and keep warm for 1~1.5h. Add the remaining APS aqueous solution and continue the reaction for 2~2.5h. Then cool down to 40~45℃ and add 10~14 parts of N,N-dimethylaminopropylamine dropwise over 25~35min. After the addition is complete, keep warm for 1~2h. Adjust the pH of the system to 7.0~7.5 with lactic acid and stir for 30~35min to obtain an aqueous dispersion resin for later use. The monomer mixture described in step S101 consists of the following components in parts by weight: 15-18 parts of dibutyl itaconic acid, 15-18 parts of glycidyl methacrylate, 14-16 parts of hydroxyethyl acrylate, and 20-25 parts of methyl methacrylate. The weight parts of each raw material in step S1 are: 25-30 parts of water-based dispersion resin, 1.5-2.5 parts of dispersant EFKA-4560, and 5-8 parts of deionized water; The weight parts of each raw material in step S2 are: 20-25 parts of pigment, 8-10 parts of nano barium sulfate, and 0.03-0.05 parts of composite catalyst; The amount of crosslinking agent used is measured according to a molar ratio of resin hydroxyl groups to crosslinking agent-NCO of 1:1.1; The preparation of the composite catalyst described in step S2 includes the following steps: S201. Add cashew phenol to deionized water at a solid-liquid ratio of 1:1, and heat to 50-55°C while stirring. Under constant temperature conditions, add 40% dimethylamine aqueous solution and 37% formaldehyde aqueous solution dropwise over 30-40 minutes. Heat to 78-82°C and maintain the temperature for 2.5-3.5 hours. After the reaction is complete, allow the mixture to stand and separate into layers. Take the upper organic phase, wash it three times with deionized water, and remove the residual water by vacuum distillation to obtain cashew phenol amine. S202. Add octanoic acid to deionized water at a solid-liquid ratio of 1:5.5, stir to dissolve, adjust the pH to 5.0-5.5 with 10% sodium hydroxide solution, add zinc oxide, heat to 60℃, and react at 400-600 rpm for 2-2.5 h. After the reaction is complete, filter while hot, cool the filtrate to room temperature to precipitate crystals, and vacuum dry at 50-55℃ for 4-5 h to obtain zinc octanoate powder. S203. Add β-cyclodextrin to deionized water at a solid-liquid ratio of 1:3, stir to dissolve, add 40% sodium hydroxide solution, add propylene oxide dropwise over 1-1.5 hours, and control the temperature at 40-43°C. After the addition is complete, raise the temperature to 60°C and keep the reaction at this temperature for 3.5-4.5 hours. During this period, adjust the pH of the system with 10% dilute hydrochloric acid to maintain it at 10-11. After the reaction is complete, neutralize to neutral with hydrochloric acid, concentrate under reduced pressure to 1 / 3 of the original volume, add ethanol to precipitate the product, filter, and vacuum dry at 60-65°C for 6-7 hours to obtain hydroxypropyl-β-cyclodextrin. S204. Add hydroxypropyl-β-cyclodextrin to deionized water at a solid-liquid ratio of 1:

92. Heat the mixture to 50-55°C while stirring at 700-900 rpm. After dissolving for 10-15 minutes, add cashew phenolic acid and zinc octanoate in sequence. Continue stirring for 25-35 minutes and then transfer the mixture to a high-pressure homogenizer. Process the mixture twice at 30 MPa to refine the emulsion particles to a particle size ≤500 nm.

2. The method for preparing a color paste for electrophoretic coating according to claim 1, characterized in that, In step S1, the stirring speed is controlled at 400~500 rpm.

3. The method for preparing a color paste for electrophoretic coating according to claim 1, characterized in that, In step S2, the stirring speed is controlled at 1000~2000 rpm and the stirring time is 30~40 min.

4. The method for preparing a color paste for electrophoretic coating according to claim 1, characterized in that, The preparation method of the crosslinking agent in step S2 is as follows: Prepare a three-necked flask, set the stirring speed to 200 rpm, add 18-22 parts of IPDI, heat to 60℃, start stirring, and dropwise add 8-10 parts of pre-melted ε-caprolactam to the IPDI over a period of 1-1.5 hours. Control the reaction temperature at 78-82℃, take samples every 30 minutes, and monitor the reaction at 2270 cm⁻¹ using Fourier transform infrared spectroscopy. -1 The reaction is stopped when the -NCO characteristic peak is completely disappeared, and the mixture is cooled to room temperature to obtain the crosslinking agent for later use.

5. The method for preparing a color paste for electrophoretic coating according to claim 1, characterized in that, In step S3, the grinding temperature is set to 28~34℃, the grinding speed is 3000~4000rpm, and the grinding time is 60~90min.

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