Novel water-based self-stratifying coating and preparation method thereof

Abstract

The invention belongs to the technical field of anti-corrosion coatings, and particularly relates to a novel water-based self-stratifying coating and a preparation method of the novel water-based self-stratifying coating, and the pH value of the novel water-based self-stratifying coating is 7.5-8. Comprising the following components in percentage by weight: 42.5 to 42.87 percent of maleimide modified hydroxyl-terminated hyperbranched polyester, 17 to 17.15 percent of modified polydimethylsiloxane of which the chain end is modified by sulfydryl, 25.5 to 25.72 percent of bisphenol A type epoxy resin, 0.3 to 0.6 percent of dispersing agent, 0.1 to 0.3 percent of defoaming agent, 0.05 to 0.2 percent of flatting agent, 0.1 to 0.3 percent of wetting agent and 13.6 to 13.72 percent of deionized water. The water-based self-stratifying coating disclosed by the invention takes water as a dispersion medium, is extremely low in VOC (Volatile Organic Compounds) content and meets environmental protection requirements, and meanwhile, a traditional'primer and finishing coat 'double-layer system can be replaced by single-channel construction, so that the construction process is remarkably simplified, the construction efficiency is improved, the energy consumption is reduced without repeated baking, and the water-based self-stratifying coating has extremely high industrial application value.

Description

Technical Field

[0001] This invention belongs to the field of anti-corrosion coating technology, specifically relating to a novel water-based self-stratifying coating and its preparation method. Background Technology

[0002] Water-based coatings, due to their low volatile organic compound content and safety / environmental friendliness, have become an important development direction in the coatings industry. Among them, multifunctional coatings possessing multiple excellent properties, such as coatings that simultaneously exhibit excellent adhesion, mechanical strength, and superior hydrophobicity and abrasion resistance, are widely in demand in areas such as metal protection, marine antifouling, and industrial flooring. Traditional technical approaches mainly fall into two categories: 1. Multi-layer coating system: This involves first applying a primer (such as epoxy resin) to provide adhesion and mechanical strength, followed by a topcoat (such as silicone or fluorocarbon) to provide surface properties. While this method is reliable, it has inherent drawbacks such as complex production processes, high energy consumption, long production cycles, and high costs.

[0003] 2. Single-layer self-separating coatings: These methods involve mixing resins with different properties (such as epoxy resin and silicone resin), utilizing their surface energy differences to spontaneously separate and form a gradient functional coating during film formation. This method simplifies the process and is currently a research hotspot, but several key technical challenges remain unresolved: First, there is the contradiction between compatibility and stability. Epoxy resin and polydimethylsiloxane (PDMS) have vastly different polarities, resulting in extremely poor compatibility when directly blended. This leads to a high likelihood of phase separation, causing poor storage stability of the coating system and even deterioration before application. While adding compatibilizers is a common practice, traditional compatibilizers often only improve compatibility during mixing but inhibit self-delamination during film formation. This results in the final coating failing to form a clear gradient structure, significantly reducing surface properties (such as hydrophobicity).

[0004] Secondly, the interfacial adhesion is weak. Even if preliminary self-delamination is achieved through physical blending, the low surface energy PDMS, after being enriched on the surface, only exhibits weak physical adsorption with the underlying epoxy resin. This weak interfacial adhesion makes the coating prone to interlayer delamination when subjected to external forces or thermal stress, greatly impairing the coating's mechanical properties and durability.

[0005] Finally, there is the challenge of balancing functionality and crosslinking. Existing technologies struggle to construct a robust and uniform crosslinking network within the coating. If epoxy resin and PDMS crosslink independently, interlayer interface issues persist; if a common crosslinking agent is used, uneven crosslinking density may result due to reaction rate and compatibility problems, preventing the optimal performance of both resins from being achieved simultaneously.

[0006] Therefore, there is an urgent need in this field for a new technical solution that can effectively solve the compatibility problem between epoxy resin and PDMS and ensure the stability of the coating system. At the same time, it can not only promote sufficient self-delamination during film formation, but also firmly bond the two layers together through strong chemical bonds after delamination, forming a dense network structure with both excellent surface and bottom layer properties and strong interlayer bonding. Summary of the Invention

[0007] The purpose of this invention is to provide a novel water-based self-stratifying coating to solve the above-mentioned technical problems.

[0008] To achieve the above-mentioned technical objectives, the technical solution of the present invention is as follows: a novel water-based self-stratifying coating, wherein the pH of the novel water-based self-stratifying coating is 7.5-8, and by weight comprises the following components: 42.5-42.87% maleimide-modified hydroxyl-terminated hyperbranched polyester, 17-17.15% modified polydimethylsiloxane with thiol-terminated ends, 25.5-25.72% bisphenol A type epoxy resin, 0.3-0.6% dispersant, 0.1-0.3% defoamer, 0.05-0.2% leveling agent, 0.1-0.3% wetting agent, and 13.6-13.72% deionized water.

[0009] As a further improvement, the preparation method of the maleimide-modified hydroxyl-terminated hyperbranched polyester is as follows: S11. Take trimethylolpropane and the catalyst p-toluenesulfonic acid, add 2,2-dimethylolpropionic acid in three portions, purge with nitrogen, react at 120~140℃ for 7~8h, then continue to react under vacuum for 3~4h, concentrate and dry the product to obtain hydroxyl-terminated hyperbranched polyester. S12. Dissolve the dried hydroxyl-terminated hyperbranched polyester in anhydrous xylene, add the catalyst dibutyltin disilicate, and under nitrogen protection, add isophorone diisocyanate dropwise in an ice-water bath at 0-5°C. After the addition is complete, raise the temperature to 60-80°C and react for 5-6 hours to obtain a solution containing the prepolymer. Cool the solution containing the prepolymer to room temperature and add N-(2-hydroxyethyl)maleimide solution dropwise. React at 30-40°C for 6-8 hours. After vacuum distillation, precipitation, washing, and vacuum drying, obtain the maleimide-modified hydroxyl-terminated hyperbranched polyester.

[0010] As a further improvement, in step S11, the specific process of adding 2,2-dimethylolpropionic acid in three stages is as follows: The molar ratio of 2,2-dimethylolpropionic acid to trimethylolpropane added for the first time was 3:1, and the reaction time was 2 hours. The second addition of 2,2-dimethylolpropionic acid and trimethylolpropane had a molar ratio of 6:1 and a reaction time of 2 hours. The third addition of 2,2-dimethylolpropionic acid and trimethylolpropane was carried out at a molar ratio of 12:1, and the reaction time was 3-4 hours.

[0011] As a further improvement, in step S12, the amount of dibutyltin disilicate added is 0.1~0.5% of the mass of the hydroxyl-terminated hyperbranched polyester, the molar ratio of the hydroxyl-terminated hyperbranched polyester to isophorone diisocyanate is 1:6, the concentration of the N-(2-hydroxyethyl)maleimide solution is 1g / mL, and the solvent is anhydrous tetrahydrofuran.

[0012] As a further improvement, the method for preparing the modified polydimethylsiloxane with thiol-modified chain ends is as follows: S21. Dissolve hexamethylcyclotrisiloxane in anhydrous tetrahydrofuran, add brominated ester as an initiator, and couple the reaction at -15 to -10℃ for 1 to 10 hours. Add dimethylchlorosilane to terminate the coupling reaction. Wash with saturated sodium bicarbonate solution until neutral, separate the liquid and keep the organic phase. Dry the organic phase with anhydrous magnesium sulfate, then distill under reduced pressure and precipitate and purify repeatedly with methanol to obtain terminal functionalized polydimethylsiloxane. S22. Dissolve terminal-functionalized polydimethylsiloxane in anhydrous dimethylformamide, add 4-dimethylaminopyridine, mercaptoacetic acid and N,N'-diisopropylcarbodiimide, under nitrogen protection, and react at room temperature for 6-12 h. After the reaction is complete, filter, wash the filtrate with deionized water, evaporate and dry under reduced pressure to obtain modified polydimethylsiloxane with mercapto-modified chain ends.

[0013] As a further improvement, in step S21, the ratio of hexamethylcyclotrisiloxane to anhydrous tetrahydrofuran is 4g:1mL, the amount of brominated ester added is 6.5% of the mass of hexamethylcyclotrisiloxane, and the amount of dimethylchlorosilane added is 3.5% of the mass of hexamethylcyclotrisiloxane.

[0014] As a further improvement, in step S22, the ratio of terminal-functionalized polydimethylsiloxane to anhydrous dimethylformamide is 1.5 g: 1 mL, the amount of 4-dimethylaminopyridine added is 1-5% of the mass of terminal-functionalized polydimethylsiloxane, and the molar ratio of terminal-functionalized polydimethylsiloxane, mercaptoacetic acid, and N,N'-diisopropylcarbodiimide is 1:1:1.

[0015] As a further improvement, the dispersant is sodium polycarboxylate, the defoamer is an organic polyether ester, the leveling agent is a polyether-modified silicone, and the wetting agent is a polyether-modified silicone.

[0016] This invention also provides a method for preparing a novel waterborne self-stratifying coating. The method involves preheating bisphenol A type epoxy resin at 60-80°C for 20 minutes; mixing a maleimide-terminated hydroxyl-terminated hyperbranched polyester, a mercapto-terminated modified polydimethylsiloxane, the preheated bisphenol A type epoxy resin, and deionized water to form a mixed system; stirring the mixture; adding ammonium bicarbonate aqueous solution to adjust the pH of the mixed system to 7.5-8; then adding a dispersant, defoamer, leveling agent, and wetting agent to the mixed system; shearing at 1000-3000 rpm for 10-30 minutes to form a uniform and stable dispersion; and allowing it to stand for 1-2 hours to obtain the final product.

[0017] Due to the adoption of the above technical solution, the beneficial effects of the present invention are as follows: the surface layer exhibits excellent hydrophobicity, low coefficient of friction, anti-adhesion and stain resistance brought by modified polydimethylsiloxane; the bottom layer exhibits strong adhesion to a variety of substrates, high hardness and excellent corrosion resistance brought by bisphenol A type epoxy resin; and the overall coating has excellent impermeability, wear resistance and mechanical strength due to the dense click chemical cross-linking structure.

[0018] This water-based self-stratifying coating uses water as the dispersion medium and has extremely low VOC content, meeting environmental protection requirements. At the same time, a single coat can replace the traditional "primer + topcoat" two-layer system, significantly simplifying the construction process, improving construction efficiency, and reducing energy consumption by eliminating the need for multiple baking processes. It has extremely high industrial application value. Attached Figure Description

[0019] Figure 1 This is the 1H NMR spectrum of a terminal hydroxyl hyperbranched polyester modified with maleimide at the chain ends; Figure 2 This is the 1H NMR spectrum of a modified polydimethylsiloxane with thiol groups at the chain ends. Detailed Implementation

[0020] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. 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. Where specific conditions are not specified in the embodiments, conventional conditions are followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0021] This invention provides a novel water-based self-stratifying coating, the specific operation steps of which are as follows: S1. Preparation of hydroxyl-terminated hyperbranched polyester: Weigh out trimethylolpropane and add it to the reactor. Then add 2,2-dimethylolpropionic acid in a molar amount three times that of trimethylolpropane. Add 0.3-1% of p-toluenesulfonic acid as a catalyst. React for 2 hours. Then add 2,2-dimethylolpropionic acid in a molar amount six times that of trimethylolpropane. React for 2 hours. Then add 2,2-dimethylolpropionic acid in a molar amount twelve times that of trimethylolpropane. React for 3-4 hours. Then start evacuating the reactor to a vacuum level ≤ -0.09 MPa. Continue the reaction for 3-4 hours. Nitrogen gas is purged throughout the reaction. The temperature is always controlled at 120-140℃.

[0022] After the reaction was completed, the product was concentrated and dried to obtain hydroxyl-terminated hyperbranched polyester (HBP-OH), as shown in the following reaction formula.

[0023]

[0024]

[0025]

[0026] S2. Maleimide-terminated maleic acid group of hydroxyl-terminated hyperbranched polyester: Dry hydroxyl-terminated hyperbranched polyester was dissolved in anhydrous xylene and placed in a reactor. 0.1-0.5% of dibutyltin disilicate by mass of the hydroxyl-terminated hyperbranched polyester was added. The temperature was controlled at 0-5°C in an ice-water bath. Diisocyanate with a molar amount six times that of the hydroxyl-terminated hyperbranched polyester was added dropwise. After the addition was complete, the temperature was raised to 60-80°C and the reaction was continued for 5-6 hours to obtain a solution containing HO-HBP-NCO prepolymer, as shown in the following reaction formula. Nitrogen gas was purged throughout the reaction process for protection.

[0027]

[0028] Cool the above solution containing HO-HBP-NCO prepolymer to room temperature, weigh maleimide with a molar amount equal to that of diisocyanate, and dissolve it in anhydrous tetrahydrofuran at a material-to-liquid ratio of 1 g / mL to prepare a maleimide solution. Stir the above solution containing HO-HBP-NCO prepolymer and add the maleimide solution dropwise. After the addition is complete, control the temperature at 30~40℃ and react for 6~8 h.

[0029] After the reaction was completed, the solvent was removed by vacuum distillation. The product was precipitated with cold methanol, washed, and dried under vacuum to obtain a terminal maleimide-modified hyperbranched polyester (HO-HBP-MI), as shown in the following reaction formula. The proton NMR spectrum is as follows. Figure 1 As shown.

[0030]

[0031] S3. Preparation of terminal-functionalized polydimethylsiloxane (PDMS): Anhydrous tetrahydrofuran was added to the reaction vessel, followed by hexamethylcyclotrisiloxane at a feed-to-liquid ratio of 4 g / mL. 6.5% of the mass of hexamethylcyclotrisiloxane was then injected as an initiator bromoester. The reaction was carried out for 1-10 h at a temperature controlled at -15 to -10 °C. Finally, 3.5% of the mass of hexamethylcyclotrisiloxane was added as dimethylchlorosilane to terminate the coupling reaction.

[0032] The reaction solution was washed with saturated sodium bicarbonate solution until neutral, separated, and the organic phase was retained. The organic phase was dried with anhydrous magnesium sulfate, then distilled under reduced pressure, and then repeatedly precipitated and purified with methanol to obtain terminal functionalized polydimethylsiloxane.

[0033] S4. Preparation of modified polydimethylsiloxane (PDMS-SH) with thiol-terminated chains: The terminal-functionalized polydimethylsiloxane was added to the reactor, and anhydrous dimethylformamide was added as solvent at a feed-to-liquid ratio of 1.5 g / mL. The mixture was stirred to dissolve the terminal-functionalized polydimethylsiloxane. 1-5% of the mass of the terminal-functionalized polydimethylsiloxane was added as a catalyst, and thioglycolic acid (the molar amount of the terminal-functionalized polydimethylsiloxane) was slowly added. A condensing agent was also added, and the mixture was stirred. The reaction was carried out for 6-12 hours under nitrogen protection, and the temperature was kept at room temperature throughout the reaction.

[0034] After the reaction was complete, the precipitate was filtered, and the filtrate was collected. The filtrate was washed with deionized water and the solvent was removed under reduced pressure to obtain modified polydimethylsiloxane (PDMS-SH) with thiol-terminated chains. The 1H NMR spectrum is shown below. Figure 2 As shown.

[0035]

[0036] S5. Preparation of water-based self-stratifying coatings: Bisphenol A type epoxy resin was preheated at 60-80℃ for 20 min. HO-HBP-MI (prepared in S2), PDMS-SH (prepared in S4), bisphenol A type epoxy resin, and deionized water were mixed in a ratio of 25:10:15:8 to form a mixture. The mixture was stirred, and ammonium bicarbonate aqueous solution was added to adjust the pH to 7.5-8, ensuring that HO-HBP-MI and PDMS-SH were miscible but did not react. Sodium polycarboxylate dispersant diluted with water was then added... Defoamer organic polyether ester, leveling agent polyether-modified silicone, and wetting agent polyether-modified silicone are added to a mixing system. The amount of sodium polycarboxylate added is 0.3~0.6% of the total mass, the amount of organic polyether ester added is 0.1~0.3% of the total mass, the amount of polyether-modified silicone added is 0.05~0.2% of the total mass, and the amount of polyether-modified silicone added is 0.1~0.3% of the total mass. The mixing system is sheared at 1000-3000 rpm for 10-30 min using a high-speed disperser to form a uniform and stable dispersion. After standing for 1~2 h, a water-based self-stratifying coating is prepared.

[0037] Depending on the variations in the formulation, the content ranges of each component are as follows: maleimide-modified hydroxyl-terminated hyperbranched polyester (HO-HBP-MI) 42.5~42.87%, modified polydimethylsiloxane (PDMS-SH) with thiol-terminated ends 17~17.15%, bisphenol A type epoxy resin 25.5~25.72%, dispersant 0.3~0.6%, defoamer 0.1~0.3%, leveling agent 0.05~0.2%, wetting agent 0.1~0.3%, and deionized water 13.6~13.72%.

[0038] The water-based self-stratifying coating provided by this invention has excellent compatibility and storage stability: The HO-HBP-MI prepared by S1 and S2 is characterized by the introduction of maleimide groups into approximately half of its hydroxyl groups, resulting in a near-equal balance between the amounts of hydroxyl and maleimide groups in the molecule. The hydroxyl groups, being hydrophilic, effectively promote the miscibility of HO-HBP-MI with deionized water. By employing terminally hydroxyl hyperbranched polyester as the core compatibilizer and reaction platform, its highly branched three-dimensional spherical structure and abundant terminal functional groups enable it to achieve good miscibility with both polar bisphenol A epoxy resin and non-polar polydimethylsiloxane through physical entanglement and polar interactions.

[0039] This invention solves the problems of thermodynamic incompatibility and easy phase separation when bisphenol A epoxy resin is directly blended with modified polydimethylsiloxane, ensuring that the coating emulsion has a uniform and stable single-phase system before storage and application, thus avoiding stratification and deterioration.

[0040] The water-based self-stratifying coating provided by this invention has a highly efficient and controllable self-stratification effect: Hydroxyl-terminated hyperbranched polyesters are amphiphilic. The numerous terminal hydroxyl groups exhibit good interaction and compatibility with bisphenol A type epoxy resin, water, and the substrate surface. Its internal aliphatic chain structure has good affinity with nonpolar modified polydimethylsiloxane. Hydroxyl-terminated hyperbranched polyester molecules spontaneously accumulate at the phase interface between bisphenol A type epoxy resin and PDMS-SH. Its polar ends penetrate into the epoxy phase, while the nonpolar parts extend into the PDMS phase, thereby significantly reducing the interfacial tension between the two phases and greatly lowering the energy barrier required for phase separation, making the phase separation process easier and faster.

[0041] In the emulsion state, the hydroxyl-terminated hyperbranched polyester, through its amphiphilicity, "encapsulates" or "bridges" the bisphenol A type epoxy resin and PDMS-SH to form stable emulsion particles, preventing macroscopic phase separation during storage.

[0042] In the coated state, moisture begins to evaporate. As moisture decreases, the system concentration increases, polymer chain mobility increases, and the phase separation driving force begins to dominate. The low surface energy PDMS-SH has a strong tendency to migrate towards the air interface. Due to the good compatibility between HO-HBP-MI and PDMS-SH, HO-HBP-MI will migrate to the surface along with the PDMS-SH chains. In this process, HO-HBP-MI will guide and promote the smoother passage of PDMS-SH through the bisphenol A epoxy resin. As moisture continues to evaporate, HO-HBP-MI will not completely migrate to the surface, but will form a gradient transition layer with continuously changing composition between the bisphenol A epoxy resin and PDMS-SH. The concentration of HO-HBP-MI is the highest in this transition layer. HO-HBP-MI combines with the bisphenol A epoxy resin on one side through hydroxyl groups and with PDMS-SH on the other side through chain segment entanglement.

[0043] The water-based self-stratifying coating provided by this invention has strong interlayer bonding and a dense cross-linked network: The PDMS-SH molecules prepared by S3 and S4 are characterized by the introduction of thiol groups (-SH) at one end of most molecules. Thiol groups not only provide some hydrophilicity, but more importantly, they will undergo click chemistry with maleimide groups when the ambient pH is near neutral. The pH of the coating is adjusted to 7.5-8 with ammonium bicarbonate aqueous solution. At this point, the thiol groups and maleimide groups hardly react. After coating, as the ammonium bicarbonate decomposes and water is lost, the pH gradually approaches neutral, and the thiol groups and maleimide groups begin to undergo click chemistry and form high-density covalent bonds. When the phase separation is basically completed, the final role of HO-HBP-MI changes from promoting phase separation to strengthening the interface. The HO-HBP-MI molecule becomes a "chemical bridge" connecting the surface PDMS-SH and the underlying bisphenol A epoxy resin, firmly fixing the physically shared structure through chemical bonds, forming a dense network that is both functionally layered and holistically unified.

[0044] The novel water-based self-stratifying coating and its properties provided by the present invention are described in detail below through examples and comparative examples: Example 1 A novel water-based self-stratifying coating, with the following component addition amounts: 215.52g HO-HBP-MI, 86.21g PDMS-SH, 129.31g bisphenol A type epoxy resin, 68.97g deionized water, 1.5g dispersant sodium polycarboxylate, 0.5g defoamer organic polyether ester, 0.25g leveling agent polyether modified organosilicon, and 0.5g wetting agent polyether modified organosilicon.

[0045] The preparation method of the above-mentioned novel water-based self-stratifying coating includes the following steps: S1. Weigh 11.21g of trimethylolpropane and add it to the reactor, then add 33.5g of 2,2-dimethylolpropionic acid and 1.23g of p-toluenesulfonic acid. Heat to 140℃ and react for 2 hours. Then add 67g of 2,2-dimethylolpropionic acid and react for 2 hours. Then add 134g of 2,2-dimethylolpropionic acid and react for 3 hours. Then start vacuuming, with a vacuum degree ≤-0.09MPa, and continue the reaction for 3 hours. Nitrogen gas is always purged during the reaction, and the temperature is always controlled at 140℃.

[0046] After the reaction was completed, the product was concentrated and dried to obtain hydroxyl-terminated hyperbranched polyester (HBP-OH).

[0047] S2. Dissolve 20g of dried hydroxyl-terminated hyperbranched polyester in 20mL of anhydrous xylene, place it in a reactor, add 0.1g of dibutyltin disilicate (based on the mass of the hydroxyl-terminated hyperbranched polyester), maintain the temperature at 0~5℃ in an ice-water bath, add 5.33g of isophorone diisocyanate dropwise, and after the addition is complete, raise the temperature to 80℃ and continue the reaction for 5h to obtain a solution containing HO-HBP-NCO prepolymer. Nitrogen gas is purged throughout the reaction process for protection.

[0048] The solution containing the HO-HBP-NCO prepolymer was cooled to room temperature. 3.39 g of N-(2-hydroxyethyl)maleimide was weighed and dissolved in 3.4 mL of anhydrous tetrahydrofuran to prepare an N-(2-hydroxyethyl)maleimide solution. The solution containing the HO-HBP-NCO prepolymer was stirred and the N-(2-hydroxyethyl)maleimide solution was added dropwise. After the addition was completed, the temperature was controlled at 40 °C and the reaction was carried out for 6 h.

[0049] After the reaction was completed, the solvent was removed by vacuum distillation of the reaction solution, and the product was precipitated with cold methanol, washed, and dried under vacuum to obtain a terminal maleimide-modified hydroxyl-terminated hyperbranched polyester (HO-HBP-MI).

[0050] S3. Add 300 mL of anhydrous tetrahydrofuran to the reaction vessel, then add 300 g of hexamethylcyclotrisiloxane, inject 19.5 g of initiator bromoester, react for 1 h, and control the temperature at -10 °C. Then add 10.5 g of dimethylchlorosilane to terminate the coupling reaction.

[0051] The reaction solution was washed with saturated sodium bicarbonate solution until neutral, separated, and the organic phase was retained. The organic phase was dried with anhydrous magnesium sulfate, then distilled under reduced pressure, and then repeatedly precipitated and purified with methanol to obtain terminal functionalized polydimethylsiloxane.

[0052] S4. Add 100g of terminal-functionalized polydimethylsiloxane to the reactor, add 67mL of anhydrous dimethylformamide, stir to dissolve the terminal-functionalized polydimethylsiloxane, add 1g of 4-dimethylaminopyridine as a catalyst, slowly add 6.14g of mercaptoacetic acid, add a condensing agent, stir, and react for 6~12h. Nitrogen gas is always purged during the reaction, and the temperature is always controlled at room temperature.

[0053] After the reaction was completed, the precipitate was filtered, the filtrate was collected, the filtrate was washed with deionized water and the solvent was removed under reduced pressure to obtain modified polydimethylsiloxane (PDMS-SH) with thiol-modified chain ends.

[0054] S5. Preheat bisphenol A epoxy resin at 80℃ for 20 min. Mix 215.52 g of HO-HBP-MI, 86.21 g of PDMS-SH, 129.31 g of bisphenol A epoxy resin and 68.97 g of deionized water to prepare a mixture system. Stir and add ammonium bicarbonate aqueous solution to adjust the pH of the system to 7.5~8. Add 1.5 g of dispersant sodium polycarboxylate, 0.5 g of defoamer organic polyether ester, 0.25 g of leveling agent polyether modified organosilicon and 0.5 g of wetting agent polyether modified organosilicon to the mixture system. Use a high-speed disperser to shear the mixture system at 1000 rpm for 10-30 min to form a uniform and stable dispersion. Let it stand for 2 h to obtain the final product.

[0055] Example 2 A novel water-based self-stratifying coating, with the following component addition amounts: 215.52g HO-HBP-MI, 86.21g PDMS-SH, 129.31g bisphenol A type epoxy resin, 68.97g deionized water, 2.25g dispersant sodium polycarboxylate, 1g defoamer organic polyether ester, 0.63g leveling agent polyether modified organosilicon, and 1g wetting agent polyether modified organosilicon.

[0056] The preparation method of the above-mentioned novel water-based self-stratifying coating includes the following steps: S1. Weigh 11.21g of trimethylolpropane and add it to the reactor, then add 33.5g of 2,2-dimethylolpropionic acid and 1.23g of p-toluenesulfonic acid. Heat to 130℃ and react for 2 hours. Then add 67g of 2,2-dimethylolpropionic acid and react for 2 hours. Then add 134g of 2,2-dimethylolpropionic acid and react for 4 hours. Then start vacuuming, with a vacuum degree ≤-0.09MPa, and continue the reaction for 4 hours. Nitrogen gas is always purged during the reaction, and the temperature is always controlled at 130℃.

[0057] After the reaction was completed, the product was concentrated and dried to obtain a hydroxyl-terminated hyperbranched polyester (HBP-OH). S2. Dissolve 20g of dried hydroxyl-terminated hyperbranched polyester in 20mL of anhydrous xylene, place it in a reactor, add 0.1g of dibutyltin disilicate (based on the mass of the hydroxyl-terminated hyperbranched polyester), and maintain the temperature at 0~5℃ in an ice-water bath. Add 5.33g of isophorone diisocyanate dropwise. After the addition is complete, raise the temperature to 70℃ and continue the reaction for 6h to obtain a solution containing HO-HBP-NCO prepolymer. Nitrogen gas is purged throughout the reaction process.

[0058] The solution containing the HO-HBP-NCO prepolymer was cooled to room temperature. 3.39 g of N-(2-hydroxyethyl)maleimide was weighed and dissolved in 3.4 mL of anhydrous tetrahydrofuran to prepare an N-(2-hydroxyethyl)maleimide solution. The solution containing the HO-HBP-NCO prepolymer was stirred and the N-(2-hydroxyethyl)maleimide solution was added dropwise. After the addition was completed, the temperature was controlled at 35 °C and the reaction was carried out for 7 h.

[0059] After the reaction was completed, the solvent was removed by vacuum distillation of the reaction solution, and the product was precipitated with cold methanol, washed, and dried under vacuum to obtain a terminal maleimide-modified hydroxyl-terminated hyperbranched polyester (HO-HBP-MI).

[0060] S3. Add 300 mL of anhydrous tetrahydrofuran to the reaction vessel, then add 300 g of hexamethylcyclotrisiloxane, inject 19.5 g of initiator bromo ester, react for 6 h, and control the temperature at -12 °C. Then add 10.5 g of dimethylchlorosilane to terminate the coupling reaction.

[0061] The reaction solution was washed with saturated sodium bicarbonate solution until neutral, separated, and the organic phase was retained. The organic phase was dried with anhydrous magnesium sulfate, then distilled under reduced pressure, and then repeatedly precipitated and purified with methanol to obtain terminal functionalized polydimethylsiloxane.

[0062] S4. Add 100g of terminal-functionalized polydimethylsiloxane to the reactor, add 67mL of anhydrous dimethylformamide, stir to dissolve the terminal-functionalized polydimethylsiloxane, add 1g of 4-dimethylaminopyridine as a catalyst, slowly add 6.14g of mercaptoacetic acid, add a condensing agent, stir, and react for 9h. Nitrogen gas is purged throughout the reaction, and the temperature is always controlled at room temperature.

[0063] After the reaction was completed, the precipitate was filtered, the filtrate was collected, the filtrate was washed with deionized water and the solvent was removed under reduced pressure to obtain 105g of modified polydimethylsiloxane (PDMS-SH) with thiol-modified chain ends.

[0064] S5. Preheat bisphenol A epoxy resin at 70℃ for 20 min. Mix 215.52 g of HO-HBP-MI, 86.21 g of PDMS-SH, 129.31 g of bisphenol A epoxy resin and 68.97 g of deionized water to prepare a mixture system. Stir and add ammonium bicarbonate aqueous solution to adjust the pH of the system to 7.5~8. Add 2.25 g of dispersant sodium polycarboxylate, 1 g of defoamer organic polyether ester, 0.63 g of leveling agent polyether modified organosilicon and 1 g of wetting agent polyether modified organosilicon to the mixture system. Shear the mixture system at 2000 rpm for 20 min using a high-speed disperser to form a uniform and stable dispersion. Let it stand for 1.5 h to obtain the final product.

[0065] Example 3 A novel water-based self-stratifying coating, with the following component addition amounts: 215.52g HO-HBP-MI, 86.21g PDMS-SH, 129.31g bisphenol A type epoxy resin, 68.97g deionized water, 3g dispersant sodium polycarboxylate, 1.5g defoamer organic polyether ester, 1g leveling agent polyether modified organosilicon, and 1.5g wetting agent polyether modified organosilicon.

[0066] The preparation method of the above-mentioned novel water-based self-stratifying coating includes the following steps: S1. Weigh 11.21g of trimethylolpropane and add it to the reactor, then add 33.5g of 2,2-dimethylolpropionic acid and 1.23g of p-toluenesulfonic acid. Heat to 120℃ and react for 2 hours. Then add 67g of 2,2-dimethylolpropionic acid and react for 2 hours. Then add 134g of 2,2-dimethylolpropionic acid and react for 4 hours. Then start evacuating the vacuum to a vacuum degree ≤-0.09MPa and continue the reaction for 4 hours. Nitrogen gas is always purged during the reaction, and the temperature is always controlled at 120℃.

[0067] After the reaction was completed, the product was concentrated and dried to obtain hydroxyl-terminated hyperbranched polyester (HBP-OH).

[0068] S2. Dissolve 20g of dried hydroxyl-terminated hyperbranched polyester in 20mL of anhydrous xylene, place it in a reactor, add 0.1g of dibutyltin disilicate (based on the mass of the hydroxyl-terminated hyperbranched polyester), maintain the temperature at 0~5℃ in an ice-water bath, add 5.33g of isophorone diisocyanate dropwise, and after the addition is complete, raise the temperature to 60℃ and continue the reaction for 6h to obtain a solution containing HO-HBP-NCO prepolymer. Nitrogen gas is purged throughout the reaction process for protection.

[0069] The solution containing the HO-HBP-NCO prepolymer was cooled to room temperature. 3.39 g of N-(2-hydroxyethyl)maleimide was weighed and dissolved in 3.4 mL of anhydrous tetrahydrofuran to prepare an N-(2-hydroxyethyl)maleimide solution. The solution containing the HO-HBP-NCO prepolymer was stirred and the N-(2-hydroxyethyl)maleimide solution was added dropwise. After the addition was completed, the temperature was controlled at 30 °C and the reaction was carried out for 8 h.

[0070] After the reaction was completed, the solvent was removed by vacuum distillation of the reaction solution, and the product was precipitated with cold methanol, washed, and dried under vacuum to obtain a terminal maleimide-modified hydroxyl-terminated hyperbranched polyester (HO-HBP-MI).

[0071] S3. Add 300 mL of anhydrous tetrahydrofuran to the reaction vessel, then add 300 g of hexamethylcyclotrisiloxane, inject 19.5 g of initiator bromoester, react for 10 h, and control the temperature at -15 °C. Then add 10.5 g of dimethylchlorosilane to terminate the coupling reaction.

[0072] The reaction solution was washed with saturated sodium bicarbonate solution until neutral, separated, and the organic phase was retained. The organic phase was dried with anhydrous magnesium sulfate, then distilled under reduced pressure, and then repeatedly precipitated and purified with methanol to obtain terminal functionalized polydimethylsiloxane.

[0073] S4. Add 100g of terminal-functionalized polydimethylsiloxane to the reactor, add 67mL of anhydrous dimethylformamide, stir to dissolve the terminal-functionalized polydimethylsiloxane, add 1g of 4-dimethylaminopyridine as a catalyst, slowly add 6.14g of mercaptoacetic acid, add a condensing agent, stir, and react for 12h. Nitrogen gas is purged throughout the reaction, and the temperature is always controlled at room temperature.

[0074] After the reaction was completed, the precipitate was filtered, the filtrate was collected, the filtrate was washed with deionized water and the solvent was removed under reduced pressure to obtain 105g of modified polydimethylsiloxane (PDMS-SH) with thiol-modified chain ends.

[0075] S5. Preheat bisphenol A epoxy resin at 60℃ for 20 min. Mix 215.52 g of HO-HBP-MI, 86.21 g of PDMS-SH, 129.31 g of bisphenol A epoxy resin and 68.97 g of deionized water to prepare a mixture system. Stir and add ammonium bicarbonate aqueous solution to adjust the pH of the system to 7.5~8. Add 3 g of dispersant sodium polycarboxylate, 1.5 g of defoamer organic polyether ester, 1 g of leveling agent polyether modified organosilicon and 1.5 g of wetting agent polyether modified organosilicon to the mixture system. Shear the mixture system at 3000 rpm for 30 min using a high-speed disperser to form a uniform and stable dispersion. Let it stand for 2 h to obtain the final product.

[0076] Example 4 A novel water-based self-stratifying coating, with the following component addition amounts: 4310g HO-HBP-MI, 1724g PDMS-SH, 2586g bisphenol A type epoxy resin, 1379g deionized water, 45g dispersant sodium polycarboxylate, 20g defoamer organic polyether ester, 12.5g leveling agent polyether modified organosilicon, and 20g wetting agent polyether modified organosilicon.

[0077] The preparation method of the above-mentioned novel water-based self-stratifying coating includes the following steps: Steps S1 to S4 are scaled up 20 times according to the scale of Example 2 to obtain HO-HBP-MI and PDMS-SH.

[0078] S5. In the mixing tank, preheat 2586g of bisphenol A epoxy resin at 70℃ for 20min, add 4310g of HO-HBP-MI and 1724g of PDMS-SH, stir at 100rpm to mix evenly, add 1379g of deionized water to form a mixed system, stir at 500rpm, add ammonium bicarbonate aqueous solution, adjust the pH of the system to 7.5~8, transfer to a high-speed dispersion tank, add 45g of dispersant sodium polycarboxylate, 20g of defoamer organic polyether ester, 12.5g of leveling agent polyether modified organosilicon and 20g of wetting agent polyether modified organosilicon to the mixed system, shear at 2500rpm for 25min to form a uniform and stable dispersion, let stand for 1.5h to obtain the final product.

[0079] Example 5 A novel water-based self-stratifying coating, with the following component addition amounts: 215.52 kg HO-HBP-MI, 86.21 kg PDMS-SH, 129.31 kg bisphenol A type epoxy resin, 68.97 kg deionized water, 2.25 kg dispersant sodium polycarboxylate, 1 kg defoamer organic polyether ester, 0.63 kg leveling agent polyether modified organosilicon, and 1 kg wetting agent polyether modified organosilicon.

[0080] The preparation method of the above-mentioned novel water-based self-stratifying coating includes the following steps: Steps S1 to S4 are scaled up 1000 times according to the scale of Example 2 to obtain HO-HBP-MI and PDMS-SH.

[0081] S5. In a premixing tank, bisphenol A epoxy resin, HO-HBP-MI, and PDMS-SH are first mixed at 150 rpm for 30 minutes to obtain a homogeneous oil phase. Deionized water (pH adjusted to 7.5-8.0 with ammonia) is slowly added to the premixing tank via a metering pump while the stirring speed is increased to 600 rpm for phase inversion emulsification. The emulsion is then pumped into a high-speed dispersion tank, where 2.25 kg of dispersant sodium polycarboxylate, 1 kg of defoamer organic polyether ester, 0.63 kg of leveling agent polyether-modified silicone, and 1 kg of wetting agent polyether-modified silicone are added under stirring at 500 rpm. The high-speed dispersion disc is turned on, and the speed is gradually increased to 2800-3000 rpm for 30 minutes to perform strong shear emulsification, ensuring a fine and uniform emulsion particle size. The emulsion is then transferred to a settling and maturation tank and stirred at 100 rpm for 1.5 hours to eliminate bubbles and stabilize the system. Finally, the emulsion is filtered through a 200-mesh vibrating screen to obtain the final product.

[0082] Comparative Example 1: A coating was prepared by following the operating steps of Example 2, omitting S1 and S2, and directly omitting the addition of HO-HBP-MI in the S5 preparation procedure. The remaining steps, conditions, and addition amounts were the same as in Example 2.

[0083] The purpose of this comparative example is to verify the core role of hyperbranched polyester in compatibility, delamination promotion, and enhanced interlayer bonding.

[0084] Comparative Example 2: A coating, following the operating steps of Example 2, except that S2 is modified as follows: after preparing the terminal hydroxyl hyperbranched polyester (HBP-OH), maleimide treatment is not performed; S4 is omitted, and the product of S3 is used for the preparation operation of S5. The remaining steps, conditions and amounts are the same as in Example 2, and the coating is prepared.

[0085] The purpose of this comparative example is to verify the key role of mercapto-maleimide click chemical crosslinking in enhancing interlayer bonding and overall performance.

[0086] Comparative Example 3: A coating, following the operating steps of Example 2, with S1 and S2 modified as follows: S1. Add 100g of ε-caprolactone, 1.80g of 1,4-butanediol and 0.1g of catalyst stannous octoate to the reactor, introduce nitrogen gas, heat to 130℃, and stir at 1000rpm for 8h at this temperature until the reaction is complete.

[0087] After the reaction was completed, the obtained viscous polymer was dissolved in 100 mL of anhydrous tetrahydrofuran. Under stirring at 1000 rpm, the polymer solution was slowly poured into excess cold methanol for precipitation. The precipitate was filtered, collected, washed three times with cold methanol, and dried at 40 °C for 24 h to obtain purified linear polycaprolactone diol (HO-PCL-OH).

[0088] S2. Place 20g of HO-PCL-OH in a reactor, add 20mL of anhydrous xylene, and heat and stir until completely dissolved. Add 0.1g of dibutyltin disilicate, maintain the temperature at 0~5℃ in an ice-water bath, and slowly add 10.67g of isophorone diisocyanate. After the addition is complete, raise the temperature to 70℃ and continue the reaction for 6h to obtain a solution containing OCN-PCL-NCO prepolymer. Nitrogen gas is purged throughout the reaction process.

[0089] The solution containing the above OCN-PCL-NCO prepolymer was cooled to room temperature. 6.78 g of maleimide was weighed and dissolved in 6.78 mL of anhydrous tetrahydrofuran to prepare a maleimide solution. The solution containing the above HO-HBP-NCO prepolymer was stirred and the maleimide solution was added dropwise. After the addition was completed, the temperature was controlled at 35 °C and the reaction was carried out for 7 h.

[0090] After the reaction was completed, the solvent was removed by vacuum distillation of the reaction solution, and the product was precipitated with cold methanol, washed, and dried under vacuum to obtain linear polycaprolactone (MI-PCL-MI) modified with maleimide at the chain end.

[0091] The modified S2 product was used to perform the S5 configuration operation, with the remaining steps, conditions, and amounts being the same as in Example 2, to prepare a coating.

[0092] The purpose of this comparative example is to verify the advantages of the unique three-dimensional spherical structure of hyperbranched polymers in terms of interfacial enrichment and efficient promotion of stratification.

[0093] Comparative Example 4: A coating, following the operating steps of Example 2, with S2 modified as follows: S2. Dissolve 20g of dried hydroxyl-terminated hyperbranched polyester in 20mL of anhydrous xylene, place it in a reactor, add 0.1g of dibutyltin disilicate (based on the mass of the hydroxyl-terminated hyperbranched polyester), maintain the temperature at 0~5℃ in an ice-water bath, add 10.7g of diisocyanate dropwise, and after the addition is complete, raise the temperature to 70℃ and continue the reaction for 6h to obtain a solution containing OCN-HBP-NCO prepolymer. Nitrogen gas is purged throughout the reaction process.

[0094] The solution containing the OCN-HBP-NCO prepolymer was cooled to room temperature. 6.8 g of maleimide was weighed and dissolved in 6.8 mL of anhydrous tetrahydrofuran to prepare a maleimide solution. The solution containing the OCN-HBP-NCO prepolymer was stirred and the maleimide solution was added dropwise. After the addition was completed, the temperature was controlled at 35 °C and the reaction was carried out for 7 h.

[0095] After the reaction was completed, the solvent was removed by vacuum distillation of the reaction solution, and the product was precipitated with cold methanol, washed, and dried under vacuum to obtain a hyperbranched polyester (HBP-MI) modified with maleimide at the chain end.

[0096] The modified S2 product was used to perform the S5 configuration operation, with the remaining steps, conditions, and amounts being the same as in Example 2, to prepare a coating.

[0097] The purpose of this comparative example is to verify the indispensable role of the hydroxyl groups (-OH) on hyperbranched polyester molecules in their amphiphilicity and in dynamically guiding the layering process during the water evaporation stage, and to explore the consequences of retaining only the crosslinking function while losing the hydrophilicity and interfacial guiding function.

[0098] The products of each embodiment and comparative example were tested, and the test data were analyzed.

[0099] Emulsion stability test: Method: The prepared coating was placed in a graduated, sealed, transparent glass bottle and stored at room temperature for 30 days. Observe and record whether phenomena such as layering, sedimentation, or flocculation occur.

[0100] Evaluation criteria: The status is recorded every 10 days and ultimately described as "stable", "slight sedimentation / stratification", "severe stratification", etc.

[0101] The test results are shown in Table 1.

[0102] Coating performance test: Method: All coating samples were uniformly coated using a wire bar coater on sanded, cleaned, and dried tinplate. The wet film thickness was controlled at 100 μm. After leveling for 15 min under standard conditions (23±2℃, 50±5% RH), the samples were cured in an 80℃ forced-air oven for 2 hours. The dry film thickness was approximately 40±5 μm.

[0103] Surface hydrophobicity: Using a contact angle meter, 5 μL of deionized water was dropped onto the cured coating surface, and the static water contact angle (WCA) was measured. Five points were measured for each sample, and the average value was taken.

[0104] Adhesion: Using the cross-cut test (ASTM D3359), with a blade spacing of 1mm, cut 6×6 squares, apply 3M tape and quickly peel it off, and observe the coating peeling. Grade 0 (best) to 5 (worst).

[0105] Pencil Hardness (ASTM D3363): Using a pencil hardness tester, scratch the coating from softest to hardest, and take the highest pencil hardness grade that does not scratch the coating as the result.

[0106] Salt water resistance: Immerse the coated sample in a 3.5 wt% NaCl solution and observe the surface for 72 hours to see if blistering, rust, peeling, or other phenomena occur.

[0107] Abrasion resistance (ASTM D4060): The mass loss (mg) of the coating after 100 abrasion cycles was recorded using a Taber abrasion tester with a CS-10 abrasion wheel and a load of 500g.

[0108] The test results are shown in Table 1.

[0109] Table 1. Comparison of detection data between the examples and comparative examples.

[0110] As shown in Table 1, the data of Comparative Example 1 completely failed, proving that hyperbranched polyester is the basis for system compatibility and successful film formation. Without hyperbranched polyester, bisphenol A type epoxy resin and PDMS-SH cannot coexist, resulting in immediate phase separation and failure to form an effective coating.

[0111] The coating performance of Comparative Example 2 was significantly reduced, with adhesion and hardness being significantly lower than those of Example 2. It also exhibited poor abrasion resistance and blistering in salt water resistance. This demonstrates that the key to forming a high-strength, highly cross-linked, and permeability-resistant network through the click chemical reaction of mercapto-maleimide is the lack of covalent bonds, resulting in weak interlayer bonding and a substantial deterioration in performance.

[0112] Although Comparative Example 3 could form a film, all its properties were inferior to those of Example 2. Its water contact angle was much lower than that of Example 2, demonstrating that the hyperbranched structure has unparalleled advantages in guiding PDMS-SH to migrate to the surface and achieving efficient self-stratification. Its poor hardness and abrasion resistance indicate that the linear PCL structure cannot form a dense cross-linked network like the hyperbranched structure.

[0113] The performance of Comparative Example 4 is between that of Comparative Example 2 and Comparative Example 3. Its water contact angle is better than that of Comparative Example 3 but much worse than that of Comparative Example 2, which proves that the completely hydrophobic HBP-MI has lost its ability to guide phase separation. However, its adhesion and hardness are still better than those of Comparative Example 2 and Comparative Example 3, indicating that click chemical crosslinking is still playing a role. However, the lack of hydroxyl interface guiding function means that the overall performance of the coating cannot reach the optimal level. This verifies that the amphiphilicity of HBP is the core of its ability to guide phase separation.

[0114] In summary, this invention provides a waterborne self-stratifying coating that successfully prepares a waterborne self-stratifying coating with excellent comprehensive performance through the amphiphilic guiding effect of hyperbranched polyester and the strong anchoring effect of click chemistry. All comparative data verify, from the opposite perspective, the indispensability and synergy of these two core design points. After optimization, the process exhibits high reliability and scale-up, fully meeting the application requirements of high-performance coatings.

[0115] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A novel water-based self-stratifying coating, characterized in that, The novel water-based self-stratifying coating has a pH of 7.5-8 and, by weight, comprises the following components: 42.5-42.87% maleimide-modified hydroxyl-terminated hyperbranched polyester, 17-17.15% modified polydimethylsiloxane with thiol-terminated ends, 25.5-25.72% bisphenol A type epoxy resin, 0.3-0.6% dispersant, 0.1-0.3% defoamer, 0.05-0.2% leveling agent, 0.1-0.3% wetting agent, and 13.6-13.72% deionized water.

2. The novel water-based self-stratifying coating according to claim 1, characterized in that, The preparation method of the maleimide-modified hydroxyl-terminated hyperbranched polyester is as follows: S11. Take trimethylolpropane and the catalyst p-toluenesulfonic acid, add 2,2-dimethylolpropionic acid in three portions, purge with nitrogen, react at 120~140℃ for 7~8h, then continue to react under vacuum for 3~4h, concentrate and dry the product to obtain hydroxyl-terminated hyperbranched polyester. S12. Dissolve the dried hydroxyl-terminated hyperbranched polyester in anhydrous xylene, add the catalyst dibutyltin disilicate, and under nitrogen protection, add isophorone diisocyanate dropwise in an ice-water bath at 0-5°C. After the addition is complete, raise the temperature to 60-80°C and react for 5-6 hours to obtain a solution containing the prepolymer. Cool the solution containing the prepolymer to room temperature and add N-(2-hydroxyethyl)maleimide solution dropwise. React at 30-40°C for 6-8 hours. After vacuum distillation, precipitation, washing, and vacuum drying, obtain the maleimide-modified hydroxyl-terminated hyperbranched polyester.

3. The novel water-based self-stratifying coating according to claim 2, characterized in that, In step S11, the specific process of adding 2,2-dimethylolpropionic acid in three separate steps is as follows: The molar ratio of 2,2-dimethylolpropionic acid to trimethylolpropane added for the first time was 3:1, and the reaction time was 2 hours. The second addition of 2,2-dimethylolpropionic acid and trimethylolpropane had a molar ratio of 6:1 and a reaction time of 2 hours. The third addition of 2,2-dimethylolpropionic acid and trimethylolpropane was carried out at a molar ratio of 12:1, and the reaction time was 3-4 hours.

4. The novel water-based self-stratifying coating according to claim 3, characterized in that, In step S12, the amount of dibutyltin disilicate added is 0.1~0.5% of the mass of the hydroxyl-terminated hyperbranched polyester, the molar ratio of the hydroxyl-terminated hyperbranched polyester to isophorone diisocyanate is 1:6, the concentration of the N-(2-hydroxyethyl)maleimide solution is 1g / mL, and the solvent is anhydrous tetrahydrofuran.

5. A novel water-based self-stratifying coating according to claim 1, characterized in that, The method for preparing the modified polydimethylsiloxane with thiol-modified chain ends is as follows: S21. Dissolve hexamethylcyclotrisiloxane in anhydrous tetrahydrofuran, add brominated ester as an initiator, and couple the reaction at -15 to -10℃ for 1 to 10 hours. Add dimethylchlorosilane to terminate the coupling reaction. Wash with saturated sodium bicarbonate solution until neutral, separate the liquid and keep the organic phase. Dry the organic phase with anhydrous magnesium sulfate, then distill under reduced pressure and precipitate and purify repeatedly with methanol to obtain terminal functionalized polydimethylsiloxane. S22. Dissolve terminal-functionalized polydimethylsiloxane in anhydrous dimethylformamide, add 4-dimethylaminopyridine, mercaptoacetic acid and N,N'-diisopropylcarbodiimide, under nitrogen protection, and react at room temperature for 6-12 h. After the reaction is complete, filter, wash the filtrate with deionized water, evaporate and dry under reduced pressure to obtain modified polydimethylsiloxane with mercapto-modified chain ends.

6. A novel water-based self-stratifying coating according to claim 5, characterized in that, In step S21, the ratio of hexamethylcyclotrisiloxane to anhydrous tetrahydrofuran is 4g:1mL, the amount of brominated ester added is 6.5% of the mass of hexamethylcyclotrisiloxane, and the amount of dimethylchlorosilane added is 3.5% of the mass of hexamethylcyclotrisiloxane.

7. A novel water-based self-stratifying coating according to claim 6, characterized in that, In step S22, the ratio of terminal-functionalized polydimethylsiloxane to anhydrous dimethylformamide is 1.5 g: 1 mL, the amount of 4-dimethylaminopyridine added is 1-5% of the mass of terminal-functionalized polydimethylsiloxane, and the molar ratio of terminal-functionalized polydimethylsiloxane, mercaptoacetic acid, and N,N'-diisopropylcarbodiimide is 1:1:

1.

8. A novel water-based self-stratifying coating according to claim 1, characterized in that, The dispersant is sodium polycarboxylate, the defoamer is organic polyether ester, the leveling agent is polyether-modified silicone, and the wetting agent is polyether-modified silicone.

9. The method for preparing a novel water-based self-stratifying coating according to claim 1, characterized in that, Bisphenol A type epoxy resin was preheated at 60~80℃ for 20 min; a mixed system was prepared by mixing maleimide-terminated hydroxyl-terminated hyperbranched polyester, thiol-terminated modified polydimethylsiloxane, preheated bisphenol A type epoxy resin, and deionized water, and stirred. An ammonium bicarbonate aqueous solution was added to adjust the pH of the mixed system to 7.5~8; then a dispersant, defoamer, leveling agent, and wetting agent were added to the mixed system, and sheared at 1000~3000 rpm for 10~30 min to form a uniform and stable dispersion. The mixture was allowed to stand for 1~2 h to obtain the final product.

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