A Wet-and-Water Resistant Automotive Mid-Coat Based on Interface Self-Reinforcement and Multi-Network Synergy, Its Preparation Method and Application

Abstract

This invention belongs to the field of automotive coating technology, specifically relating to a wet-on-wet waterborne automotive intermediate coating based on interface self-reinforcement and multi-network synergy, its preparation method, and application. It consists of the following components: a waterborne main resin system, amino resin, an aqueous solution of zirconium carbonate ammonium, polyethylene glycol-block-polycaprolactone-urea copolymer, flash-drying wax emulsion, a volatile pH adjuster, solvent, additives, waterborne colorant, and water. The intermediate coating of this invention utilizes the thermosensitive properties of zirconium carbonate to rapidly form an ionic cross-linked network during flash-drying, instantly enhancing wet film strength and fundamentally preventing problems such as undercoating and bleeding. Through the synergistic effect of the chemical cross-linking of the amino resin and the dynamic hydrogen bond network of the thermosensitive hydrogen bond enhancer, the coating film simultaneously possesses high hardness, chemical resistance, and excellent flexibility and stone chip resistance. The flash-drying wax emulsion forms a network on the coating film surface, improving scratch resistance; during baking, it melts and interpenetrates with the upper and lower coatings, forming a reinforced interface and locking in interlayer adhesion.

Description

Technical Field

[0001] This invention belongs to the field of automotive coating technology, specifically relating to a wet-on-wet water-based automotive intermediate coating based on interface self-reinforcing and multi-network synergy, its preparation method, and its application. Background Technology

[0002] Driven by increasingly stringent VOCs emission standards, compact painting processes have become a key option for major automakers building new painting units. Traditional automotive painting lines can stretch for kilometers, resulting in tight production cycles. Any technology that can shorten the process flow and increase production speed is highly attractive. The baking process in the painting workshop is a major energy-consuming unit. Reducing the number of baking cycles or lowering the baking temperature can directly lead to significant operating cost savings. While ensuring quality, reducing equipment investment, factory space, and labor costs is a perpetual pursuit for enterprises.

[0003] Traditional automotive body painting processes typically employ a "three-coat, three-bake" or "three-coat, two-bake" system: electrophoretic primer → (bake) → intermediate coat → (bake) → color coat → clear coat → (bake). In this system, the intermediate coat plays a crucial role: filling and leveling, stone chip resistance, and interlayer adhesion. In traditional processes, the independent baking step after the intermediate coat is the main bottleneck: it consumes a huge amount of energy, requires a large area, and has a long production cycle.

[0004] To overcome the aforementioned bottlenecks, the "wet-on-wet" application technique was introduced into the intermediate coating stage. "Wet-on-wet" refers to the application of the next layer of paint directly onto the wet film of the underlying coating without drying it or only subjecting it to slight flash-drying (solvent evaporation), and then baking all the coatings together.

[0005] After adopting the wet-on-wet process for intermediate coats, the workflow is simplified to: electrophoretic primer → (drying) → intermediate coat → (flash-drying) → color coat → clear coat → (drying). This successfully simplifies the "three coats and three bakes" process to "three coats and two bakes," directly eliminating the baking step of the intermediate coat. Although the wet-on-wet technology has obvious advantages, its successful application is highly dependent on the performance of the intermediate coat itself. Traditional intermediate coats are designed for independent baking, and their direct application to wet-on-wet systems will cause a series of severe challenges: interlayer compatibility and interface stability issues, undercoating and bleeding, interface mixing, appearance quality control difficulties, bubbles and pinholes, performance matching and assurance challenges, stone chip resistance, adhesion, etc. Therefore, developing an intermediate coat specifically adapted to the wet-on-wet process that can systematically solve the above challenges has become the key to the successful application of this technology. Summary of the Invention

[0006] To address the shortcomings of existing technologies and the challenges faced by traditional intermediate coatings when applied to wet-on-wet processes, such as low flash-drying efficiency, poor wet film stability, and difficulty in achieving both toughness and hardness of the final paint film, the present invention aims to provide a wet-on-wet waterborne automotive intermediate coating based on interface self-reinforcing and multi-network synergy.

[0007] The present invention also provides a preparation method that is simple, easy to implement, and suitable for large-scale production.

[0008] The present invention also provides its application: after the intermediate coat is sprayed, it can be flash-dried at 70~90℃ for 6-10 minutes before subsequent paint spraying can be carried out, and finally dried at 140-150℃.

[0009] The wet-on-wet water-based automotive intermediate coating based on interface self-reinforcement and multi-network synergy described in this invention is composed of the following components in parts by weight:

[0010] Waterborne primary resin system: 25-40 parts;

[0011] Amino resin: 6-8 parts;

[0012] Flash-drying sensitized crosslinking agent: 1.0-4.0 parts;

[0013] Temperature-sensitive self-assembly-reconstruction type dynamic hydrogen bond enhancer: 0.5-3.0 parts;

[0014] Flash-drying wax emulsion: 1.0-5.0 parts;

[0015] Volatile pH adjuster: 0.5-1.5 parts;

[0016] Solvent: 0.5-1.5 parts;

[0017] Additives: 0.5-1.5 parts;

[0018] Water-based pigment: 25-35 parts;

[0019] Water: 15-20 parts;

[0020] The thermosensitive self-assembly-reconstruction type dynamic hydrogen bond enhancer is a polyethylene glycol-block-polycaprolactone-urea copolymer (PEG-b-PCL-Urea).

[0021] The flash-drying sensitized crosslinking agent is an aqueous solution of ammonium zirconium carbonate; during the flash-drying stage, it undergoes ionic crosslinking with the carboxyl groups on the resin surface, rapidly improving the early strength and anti-collapse properties of the wet film.

[0022] During the storage and application of the coating, at a pH ≥ 8.5, ammonium zirconium carbonate forms a stable anionic complex in the presence of excess ammonia and carbonate ions, which does not react with the resin, thus ensuring the fluidity and storage stability of the system.

[0023] When the coating undergoes a flash-drying process at 70-90°C, water and volatile ammonia in the system rapidly escape. The departure of the alkali disrupts the chemical equilibrium of ammonium zirconium carbonate, causing it to decompose rapidly and release highly reactive zirconium cations. At this point, the carboxyl groups on the resin segments, which were originally used to provide water solubility, are quickly captured by the zirconium cations and undergo a polydentate coordination reaction, forming a strong cross-linked network of "resin-Zr-resin". This cross-linking reaction can be completed within minutes, thus fundamentally eliminating undercut, streaks, and sagging. The final high-temperature baking then densifies the coating through amino resin.

[0024] The flash-drying wax emulsion is a modified montmorillonite wax emulsion. Montmorillonite wax is a natural mineral wax extracted from lignite, which naturally contains waxy acids and waxy alcohols, exhibiting a certain degree of polarity. After modification through esterification and saponification, it becomes even more polar and has a certain degree of compatibility with the water-based main resin system. Due to its own polar groups, it forms dipole-dipole interactions and hydrogen bonds with the groups on the main resin chain of the intermediate coat, firmly fixing itself to the surface of the intermediate coat rather than simply floating on it, forming a protective layer on the paint film surface, improving scratch resistance and smoothness. During the final baking stage at 140-150℃, the wax network further melts, undergoing a certain degree of interpenetration and fusion with the interface of the main resin and the upper color paint, forming a physically interlocking and chemically interacting interpenetrating network interface, further locking in interlayer adhesion.

[0025] The aqueous main resin system is a mixture of aqueous polyurethane dispersion and aqueous acrylate dispersion in a mass ratio of 1:1-2.

[0026] The preferred waterborne polyurethane dispersions are aliphatic polycarbonate type (Covestro's Bayhydrol® U2757) or polyester type (DAOTAN® VTW 6462 / 36WA from Amazing Resins (China) Co., Ltd.) PUDs.

[0027] Aqueous acrylate dispersions, preferably styrene-acrylic copolymer emulsions, provide hardness and wettability for pigments. Wantipro® 0678 from Wanhua Chemical Group Co., Ltd. is preferred.

[0028] The amino resin is methyl etherified high-imino melamine resin; Cymel® 325 from ZN Resins (China) Co., Ltd.

[0029] The volatile pH adjuster is 25 wt.% ammonia.

[0030] The solvent is an alcohol ether solvent, preferably any one or more of ethylene glycol monobutyl ether, propylene glycol monomethyl ether, dipropylene glycol methyl ether, diethylene glycol monobutyl ether, and dipropylene glycol butyl ether.

[0031] The additives include wetting and leveling agents and defoamers.

[0032] The preferred wetting and leveling agent is BYK-347 or BYK-348 from BYK Chemical GmbH, Germany.

[0033] The preferred defoamer is BYK-024 or BYK-028 from BYK Chemical GmbH, Germany.

[0034] The water-based colorant was purchased from X-Rite Technology Co., Ltd., specifically the general white ASL-9031 and the general super black ASL-9029.

[0035] The preparation method of the polyethylene glycol-block-polycaprolactone-urea (PEG-b-PCL-Urea) copolymer is as follows:

[0036] (1) Synthesis of mPEG-b-PCL-OH block copolymer: Dehydrated polyethylene glycol monomethyl ether (mPEG) was used as a macromolecular initiator and ε-caprolactone was subjected to ring-opening polymerization under the catalysis of stannous octoate. The reaction temperature was 130±3℃ and the reaction time was 8-12 hours. After the reaction was completed, the copolymer was dissolved, precipitated and purified in low temperature n-hexane, and then vacuum dried.

[0037] (2) Synthesis of mPEG-b-PCL-Urea: After dehydration of mPEG-b-PCL-OH obtained in step (1) with solvent via azeotropic reaction, it is reacted with excess hexamethylene diisocyanate (HDI) in the presence of catalyst to generate terminal isocyanate prepolymer; the end-capping reaction is carried out by adding toluene solution of n-butylamine dropwise at cooling. After the reaction is completed, the product is purified and dried to obtain the target product.

[0038] Polyethylene glycol-block-polycaprolactone-urea (PEG-b-PCL-Urea) copolymer is an amphiphilic block copolymer that can self-assemble into core-shell micelles in aqueous media. The PEG segments form the hydrophilic shell, ensuring stable dispersion of the micelles in the aqueous phase, while the PCL segments form the hydrophobic core, with their ends modified with highly self-associative bisurea groups.

[0039] PEG-b-PCL-Urea remains dormant at room temperature. In water-based coatings, this copolymer self-assembles into independent micelles with a "PCL-urea" core and a PEG shell, without forming a network and without affecting moisture evaporation. During the flash-drying step, at baking temperatures (>60°C), the PCL core melts, causing the micelles to disintegrate. The freely moving PCL-urea segments then reconstruct in situ with the surrounding main resin segments PUD and acrylic acid through intermolecular hydrogen bonding, forming a physically reinforcing network that spans the entire film and is both flexible and dynamically reversible.

[0040] The preparation method of wet-on-wet water-based automotive intermediate coating based on interface self-reinforcement and multi-network synergy according to the present invention includes the following steps:

[0041] A. Premixing and Dispersion:

[0042] Add water and additives to the reaction vessel and stir evenly at 300-500 r / min. Add water-based pigment and continue stirring for 10-15 min. Add water-based main resin system and continue stirring at 500-800 r / min for 20-30 min to ensure that the resin and pigment are fully wetted and mixed.

[0043] B. Addition of functional components:

[0044] Reduce the stirring speed to 300-500 r / min, add the solvent, amino resin, thermosensitive self-assembly-reconstruction dynamic hydrogen bond enhancer, and flash-drying wax emulsion. Add each component, stir thoroughly before adding the next component. Add the flash-drying sensitized crosslinking agent and continue stirring for 15-20 min. Use a volatile pH adjuster to control the final pH value of the system within the range of 8.5-9.5. Use the remaining water to adjust the application viscosity of the coating by 20-35 s (25℃, Ford 4# cup). Allow the finished paint to stand and mature at room temperature for 8-24 h. After filtering with a 1-5 μm filter bag, package the product.

[0045] The intermediate coating described in this invention rapidly establishes physical strength during the flash-drying stage, completely eliminating construction defects. The core of this success lies in the unique role of ammonium zirconium carbonate: it is stable under alkaline storage conditions but rapidly decomposes upon flash-drying at temperatures above 60°C, releasing highly active zirconium cations that rapidly cross-link with resin carboxyl groups. This reaction forms a preliminary "resin-Zr-resin" network in the wet film within minutes, significantly improving the early strength, anti-tack, and solvent resistance of the wet film. This fundamentally solves the interfacial interference problems such as "biting," "bleeding," and "sagging" that are prone to occur in wet-on-wet processes.

[0046] Through the synergy of PEG-b-PCL-Urea and amino resin, a multi-layered network of "chemical crosslinking + dynamic physical crosslinking" is constructed during the final baking stage: the amino resin provides a strong and durable chemical crosslinking network, ensuring final hardness and chemical resistance. When the temperature exceeds the melting point of PCL, the micelles of PEG-b-PCL-Urea disintegrate, and the diurea groups at the chain ends extensively form a large number of dynamically reversible hydrogen-bonded physical networks with the main resin. This network effectively dissipates impact energy, endowing the coating film with excellent flexibility, stone impact resistance, and internal stress relaxation capabilities, overcoming the traditional challenge of achieving a balance between hardness and toughness in intermediate coatings.

[0047] Flash-drying wax emulsion-modified montmorillonite wax forms a protective network on the paint film surface after flash drying, improving scratch resistance and application smoothness. During the final high-temperature baking, the wax layer melts and interpenetrates with the main resin and the upper color paint, forming a reinforced interface that combines physical anchoring and chemical action, further locking in interlayer adhesion and preventing performance failure caused by interlayer separation.

[0048] The core inventiveness and outstanding effect of this invention lies in the precise spatiotemporal synergistic effect of its various functional components:

[0049] Ammonium zirconium carbonate acts rapidly during the initial flash-drying stage, providing a working window; PEG-b-PCL-Urea and amino resin interact deeply during the baking period, determining the final properties. The ionic crosslinking of ammonium zirconium carbonate provides immediate strength; the hydrogen bond network of PEG-b-PCL-Urea provides toughening and dynamism; the covalent bond network of the amino resin provides permanent strength and durability; and the interfacial modification of the wax emulsion enhances surface and interlayer properties. Ultimately, a triple crosslinking of "ionic bonds-hydrogen bonds-covalent bonds" is formed, interpenetrating with the wax-modified reinforced interface, constituting an integrated reinforcement system from surface to bottom, from instantaneous to permanent, and from rigid to dynamic. This synergy enables the coating of this invention to not only exhibit stable and reliable performance in wet-on-wet processes, but its overall mechanical properties also surpass those of traditional coatings designed for independent baking.

[0050] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0051] 1) The intermediate coating of the present invention utilizes the properties of ammonium zirconium carbonate that is stable under alkaline conditions, decomposes under neutral or heated conditions, and rapidly forms an ionic cross-linking network during flash drying, instantly improving the strength of the wet film and fundamentally preventing problems such as bottom bleeding and color bleeding.

[0052] 2) This invention enables the coating film to simultaneously possess high hardness, chemical resistance, and excellent flexibility and stone impact resistance through the synergistic effect of the chemical crosslinking of amino resin and the dynamic hydrogen bond network of PEG-b-PCL-Urea.

[0053] 3) The flash-drying wax emulsion of the present invention forms a network on the surface of the paint film, which improves scratch resistance; during baking, it melts and interpenetrates with the upper and lower coatings to form a reinforced interface and lock in the interlayer adhesion.

[0054] 4) The ionic bonds, hydrogen bonds, and covalent crosslinks of each component play a precise role in the flash drying and baking stages, and interpenetrate with the wax-modified interface, forming a synergistic enhancement of the entire system from instantaneous to permanent. Detailed Implementation

[0055] The present invention will be further described below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto.

[0056] Unless otherwise specified, all raw materials used in the examples were commercially available.

[0057] Waterborne polyurethane dispersion, aliphatic polycarbonate type, Covestro AG, Bayhydrol® U 2757;

[0058] Aqueous acrylate dispersions, preferably styrene-acrylic acid copolymer emulsions, Wanhua Chemical Group Co., Ltd. Wantipro® 0678;

[0059] Amino resin, methyl etherified high imino melamine resin, ZX Resins (China) Co., Ltd., Cymel® 325;

[0060] Flash-drying sensitized crosslinking agent, 18-22 wt.% ammonium zirconium carbonate aqueous solution; Wuhan Jiyesheng Chemical Co., Ltd.

[0061] Flash-dried wax emulsion, modified montmorillonite wax emulsion; Longkou Yijiu Environmental Protection Technology Co., Ltd., TL-504 montmorillonite wax emulsion, solid content 25%;

[0062] Wetting and leveling agent, BYK-347, BYK Chemical GmbH, Germany;

[0063] Defoamer, BYK-028, BYK Chemical GmbH, Germany;

[0064] Water-based colorant, X-Rite Technology Co., Ltd., General White ASL-9031, General Super Black ASL-9029.

[0065] Thermosensitive self-assembly-reconstruction type dynamic hydrogen bond enhancer: Preparation of polyethylene glycol-block-polycaprolactone-urea (PEG-b-PCL-Urea) copolymer:

[0066] Step 1: Synthesis of mPEG-b-PCL-OH block copolymer

[0067] Polyethylene glycol monomethyl ether (mPEG, molecular weight 2000) was dehydrated by vacuum heating at 110-120℃, cooled to room temperature, and then ε-caprolactone and toluene were added under nitrogen protection. The molar ratio of mPEG to ε-caprolactone was 1:20, and the mass of toluene added was twice the mass of mPEG-OH. The temperature was raised to 80±3℃ to completely dissolve the mPEG, and then the temperature was raised to 130±3℃. Stannous octoate catalyst (0.15‰ of the total mass of the raw materials) was added and the reaction was continued for 10 hours. After cooling to room temperature, the purification steps were performed: the crude product was dissolved in dichloromethane, and the solution was added dropwise to n-hexane at 0℃ with stirring, resulting in the precipitation of a white solid. The process of dissolving the solid in dichloromethane and adding it dropwise to n-hexane at 0℃ was repeated three times to precipitate the solid. Unreacted monomers and catalysts were removed, and the solid was dried under vacuum at 40℃ to constant weight to obtain mPEG-b-PCL-OH. Add mPEG-b-PCL-OH to an excess pyridine solution of phthalic anhydride, heat under reflux for 2 hours to ensure 100% reaction of the terminal hydroxyl groups, and determine the acid value according to GB / T 6743-2008. The calculated molecular weight is 4060 g / mol, which is close to the theoretical value of 4280 g / mol.

[0068] Step 2: Synthesis of mPEG-b-PCL-Urea

[0069] One part by mass of mPEG-b-PCL-OH and two parts by mass of toluene were heated to remove water, then cooled to 60±3℃. Under nitrogen protection, 0.1% by mass of dibutyltin dilaurate catalyst of mPEG-b-PCL-OH was added, followed by the dropwise addition of HDI (molar ratio of HDI to mPEG-b-PCL-OH: 3:1). The reaction was carried out at 60±3℃ for 3-4 hours. A sample was taken and the -NCO content was determined to be 0.91% by the di-n-butylamine-hydrochloric acid titration method according to HG / T 2409-2023, indicating that the terminal -NCO group had been successfully grafted. The temperature was then lowered to 0-5℃, and a 20wt.% toluene solution of n-butylamine was added dropwise, controlling the reaction temperature at 0-5℃. After the addition was completed, the reaction was stirred for another 3 hours at room temperature. The purification step was carried out in the same manner as in step one, and the product was dried under vacuum at 40℃ to constant weight to obtain mPEG-b-PCL-Urea. The -NCO content was tested again and found to be 0.01%, indicating that the urea end-capping was successful.

[0070] The raw material composition of Examples 1-3 and Comparative Examples 1-4 is shown in Table 1 below (by parts by mass):

[0071] Table 1. Raw material composition of Examples 1-3 and Comparative Examples 1-4

[0072]

[0073] The method for preparing wet-on-wet water-based automotive intermediate coating based on interface self-reinforcement and multi-network synergy includes the following steps:

[0074] A. Premixing and Dispersion:

[0075] Add water and additives to the reaction vessel and stir evenly at 400 r / min. Add water-based pigment and continue stirring for 15 min. Add water-based main resin system and continue stirring at 600 r / min for 30 min to ensure that the resin and pigment are fully wetted and mixed.

[0076] B. Addition of functional components:

[0077] Reduce the stirring speed to 400 r / min, add the solvent, amino resin, thermosensitive self-assembly-reconstruction dynamic hydrogen bond enhancer, and flash-drying wax emulsion. Add each component, stir well before adding the next component. Add the flash-drying sensitized crosslinking agent and continue stirring for 20 min. Use a volatile pH adjuster to control the final pH value of the system within the range of 8.5-9.5. Use the remaining water to adjust the application viscosity of the coating by 30 s (25℃, Ford 4# cup). Let the finished paint stand at room temperature for 24 h to mature. After filtering with a 1μm filter bag, package it.

[0078] The technical specifications of water-based automotive intermediate coatings are shown in Table 2 below:

[0079]

[0080] The test results of the technical indicators of water-based automotive intermediate coatings are shown in Table 3 below:

[0081]

[0082] application:

[0083] Construction conditions:

[0084] 23±5℃, 63±5% humidity.

[0085] The following coating is applied to the surface of the workpiece after pretreatment, electrophoretic coating, and drying:

[0086] The water-based intermediate coat described in the spraying examples and comparative examples was allowed to stand at 80±10℃ for 8 minutes to flash dry, forming a wet film with a thickness of 10-15μm; then, a water-based color paint (Nihon BICS Co., Ltd. DBW-Z) was sprayed on and allowed to stand at 80±10℃ for 4 minutes to flash dry, forming a base coat composite layer with a total film thickness of 25-35μm; finally, a clear topcoat (PPG D800) was sprayed on, with the final film thickness controlled at 40-45μm;

[0087] All wet coatings, including the intermediate coat, color coat, and clear coat, were baked at 145±5℃ for 30 minutes to complete the entire coating process. The key technical indicators and testing standards for the resulting composite coating are shown in Table 4 below:

[0088] Table 4 Key technical indicators and testing standards for composite coatings

[0089]

[0090] The test results are shown in Table 5 below:

[0091] Table 5. Composite membrane test results

[0092]

[0093] Comparative Example 1, lacking all three key functional components, lacks a strong flash-drying network and rapid film-setting ability, resulting in problems during wet-on-wet application: extremely poor adhesion and stone impact resistance, severe orange peel appearance, and overall performance degradation. Comparative Example 2, lacking the rapid ionic crosslinking network of ammonium zirconium carbonate, suffers from insufficient early wet film strength, leading to poor stone impact resistance. Furthermore, slight impact from the paint during flash-drying slightly affects adhesion and surface smoothness. Comparative Example 3, lacking the toughening effect of a dynamic hydrogen-bonded network, has sufficient rigidity but insufficient toughness in the paint film, manifested as a significant decrease in impact resistance and flexibility, and a slight increase in hardness. Comparative Example 4, lacking a flash-drying wax emulsion, will lead to certain degradation of the coating in three dimensions: surface appearance, interlayer adhesion, and overall mechanical properties affected by interfaces.

Claims

1. A wet-on-wet water-based automotive intermediate coating based on interface self-reinforcement and multi-network synergy, characterized in that, It consists of the following components in parts by mass: Waterborne primary resin system: 25-40 parts; Amino resin: 6-8 parts; Flash-drying sensitized crosslinking agent: 1.0-4.0 parts; Temperature-sensitive self-assembly-reconstruction type dynamic hydrogen bond enhancer: 0.5-3.0 parts; Flash-drying wax emulsion: 1.0-5.0 parts; Volatile pH adjuster: 0.5-1.5 parts; Solvent: 0.5-1.5 parts; Additives: 0.5-1.5 parts; Water-based pigment: 25-35 parts; Water: 15-20 parts; The thermosensitive self-assembly-reconstruction type dynamic hydrogen bond enhancer is a polyethylene glycol-block-polycaprolactone-urea copolymer; The flash-drying sensitizing crosslinking agent is an aqueous solution of ammonium zirconium carbonate; The flash-drying wax emulsion is a modified montana wax emulsion; The aqueous main resin system is a mixture of aqueous polyurethane dispersion and aqueous acrylate dispersion in a mass ratio of 1:1-2; The volatile pH adjuster is ammonia. The solvent is an alcohol ether solvent; The preparation method of the polyethylene glycol-block-polycaprolactone-urea copolymer is as follows: (1) Synthesis of mPEG-b-PCL-OH block copolymer: Dehydrated polyethylene glycol monomethyl ether was used as a macromolecular initiator and ε-caprolactone was subjected to ring-opening polymerization under the catalysis of stannous octoate. The reaction temperature was 130±3℃ and the reaction time was 8-12 hours. After the reaction was completed, the product was dissolved, precipitated and purified, and then vacuum dried. (2) Synthesis of mPEG-b-PCL-Urea: After dehydration of mPEG-b-PCL-OH obtained in step (1) with solvent via azeotropic reaction, it is reacted with excess hexamethylene diisocyanate (HDI) in the presence of catalyst to generate terminal isocyanate prepolymer; the end-capping reaction is carried out by adding toluene solution of n-butylamine dropwise at cooling. After the reaction is completed, the product is purified and dried to obtain the target product.

2. The wet-on-wet water-based automotive intermediate coating based on interface self-reinforcement and multi-network synergy as described in claim 1, characterized in that, The amino resin is a methyl etherified high-imino melamine resin.

3. The wet-on-wet water-based automotive intermediate coating based on interface self-reinforcement and multi-network synergy as described in claim 1, characterized in that, The additives include wetting and leveling agents and defoamers.

4. A method for preparing a wet-on-wet water-based automotive mid-coat paint based on interface self-reinforcement and multi-network synergy as described in any one of claims 1-3, characterized in that, Includes the following steps: A. Premixing and Dispersion: Add water and additives to the reaction vessel, stir well, add water-based color paste, continue stirring, add water-based main resin system, and continue stirring. B. Addition of functional components: Add solvent, amino resin, thermosensitive self-assembly-reconstruction dynamic hydrogen bond enhancer, and flash-drying wax emulsion. After each component is added, stir until homogeneous before adding the next component. Add flash-drying sensitized crosslinking agent and continue stirring. Use a volatile pH adjuster to control the final pH value of the system within the range of 8.5-9.

5. Use the remaining water to adjust the coating to the application viscosity. Allow the finished paint to stand and mature at room temperature, filter it using a filter bag, and then package it.

5. An application of the wet-on-wet water-based automotive intermediate coating based on interface self-reinforcement and multi-network synergy as described in any one of claims 1-3, characterized in that, After the intermediate coat is applied, it should be flash-dried at 70-90℃ before subsequent paints are applied, and finally dried at 140-150℃.

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