Aqueous two-component polyurethane topcoat and use thereof

Abstract

The present application relates to the technical field of vehicle coating, and discloses a water-based two-component polyurethane finish and application thereof. The water-based two-component polyurethane finish is prepared by compounding a hydroxyl-containing water-based dispersion and a water-based polycarbonate aliphatic anion polyurethane dispersion with a high glass transition temperature as a film-forming resin, and selectively adding a water-based aliphatic anion polyurethane dispersion with a low glass transition temperature. Through specific proportioning and synergistic effect of each component, the characteristics of different types of water-based resins are brought to the best state, so that the water-based two-component polyurethane finish has excellent leveling property, thixotropy, elasticity and interlayer adhesion, does not fly ash, has excellent mechanical properties and weather resistance, can be applied in a large area under high temperature, low temperature or high humidity environment, is well matched with a non-sanded intermediate coating, and the finish after coating does not need to be coated with a varnish, has excellent appearance gloss and vividness, and has high practical value and popularization value.

Description

A water-based two-component polyurethane topcoat and its application Technical Field

[0001] This invention relates to the field of automotive coatings technology, and more particularly to a water-based two-component polyurethane topcoat and its application. Background Technology

[0002] Rail transit vehicles are a major mode of public transportation. Their manufacturing largely utilizes lightweight aluminum alloys instead of steel, leading to changes in the types and quality of coatings. Since rail transit vehicles operate in environments ranging from C2 to C5, their coatings must balance corrosion protection with aesthetic appeal.

[0003] Topcoat is a coating applied over primer or intermediate coat, primarily serving to color, cover, and decorate. Traditional rail vehicle topcoat systems consist of a solvent-based 1K base coat and a clear topcoat. Due to their low solids content, the clear topcoat and base coat account for over 60% of the total VOC (volatile organic compound) emissions, wasting resources, polluting the environment, and posing a fire hazard. Furthermore, the base coat + clear topcoat system uses a two-coat, one-bake (2C1B) application method. While this method offers excellent gloss and reflectivity, it results in high raw material and labor costs and relatively low work efficiency. Currently, there is no water-based topcoat that does not require a clear topcoat.

[0004] Water-based topcoats for rail transit vehicles have stringent requirements for the application environment. The painting area of ​​rail transit vehicles is large; for example, a typical high-speed train body is between 24.5m and 27m long and approximately 3.3m high. This leads to problems such as spraying runs, fly ash, and difficulties in connecting spray guns when the ambient temperature is too high, too low, or the humidity is too high. To address the demanding application conditions of water-based topcoats, there is an urgent need to find a water-based topcoat to replace the "base coat and clear coat" coating system. This water-based topcoat should be applicable in high-temperature, low-temperature, or high-humidity environments, while also possessing excellent anti-sag, anti-prickly heat, anti-orange peel, and anti-bubble properties. When spraying large areas, the paint film should have smooth transitions at the joints, a smooth surface, and excellent adhesion to the previous primer layer, thus meeting the practical needs of water-based topcoats in rail transit vehicle painting. Summary of the Invention

[0005] To address the aforementioned issues, this invention provides a water-based two-component polyurethane topcoat and its application. Through the synergistic effect of the various raw material components, it can be applied over a large area in high-temperature, low-temperature, or high-humidity environments. When used in conjunction with a no-sanding primer, the water-based two-component polyurethane topcoat exhibits excellent interlayer adhesion and eliminates the need for a topcoat clear coat. It boasts excellent gloss and reflectivity, comparable to the film appearance of a solvent-based 1K base coat + topcoat clear coat system, and possesses superior overall performance.

[0006] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows:

[0007] In a first aspect, the present invention provides a water-based two-component polyurethane topcoat, comprising component A and component B in a mass ratio of 2:1 to 6.5:1; wherein,

[0008] Component A comprises the following raw material components by mass percentage: 40%–45% hydroxyl-containing aqueous dispersion, 5%–17% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40–50°C, 0–17% aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C, 0.2%–1% first surfactant, 0.2%–1% first defoamer, 0.2%–1% substrate wetting agent, 0.1%–1% leveling agent, 0.1%–1% aqueous ultraviolet absorber, 15%–45% aqueous color paste, 3%–6% first cosolvent, 1%–3% neutralizer, 0.2%–0.5% thickener, and 5%–10% water;

[0009] The hydroxyl-containing aqueous dispersion has a hydroxyl content of 3.5% to 5% and a solid content of 35% to 50%; the aqueous color paste includes water, a dispersant, a second defoamer, a second surfactant, a pigment, a color paste stabilizer, and a neutralizer;

[0010] Component B comprises the following raw material components by mass percentage: 70%–80% curing agent and 20%–30% second cosolvent; the curing agent is at least one of hexamethylene diisocyanate or hydrophilic aliphatic polyisocyanate.

[0011] Compared to existing technologies, the waterborne two-component polyurethane topcoat provided by this invention has component A as the main paint component and component B as the curing component. Component A is a film-forming resin composed of a waterborne dispersion containing hydroxyl groups and a waterborne polycarbonate-containing aliphatic anionic polyurethane dispersion with a high glass transition temperature. A waterborne aliphatic anionic polyurethane dispersion with a low glass transition temperature is selectively added. The film-forming resins have good compatibility. Through specific proportions and the synergistic effect of each component, the characteristics of different types of waterborne resins are brought to their optimal state. This results in a waterborne two-component polyurethane topcoat with a high solids content. The coating film has excellent leveling properties, stone chip resistance, chemical resistance, and weather resistance. It also has high toughness, good elasticity, and gloss (60°) and DOI (dots of clarity) of over 95. It has excellent hiding power and fullness, no haze, and the finished coating film does not require a topcoat, achieving the gloss and DOI of a clear varnish. Furthermore, when used in conjunction with the underlying coating (epoxy or polyurethane), the coating requires no sanding and exhibits excellent interlayer adhesion, achieving a grade of 0. Even during large-area application in high-temperature, low-temperature, or high-humidity conditions, the water-based two-component polyurethane topcoat provided by this invention exhibits smooth transitions at the paint film joints, with no dripping or "flying dust" during spraying, no overlapping marks, and a smooth overall paint film surface. It also demonstrates excellent interlayer adhesion with the intermediate coating, increasing the thixotropic index from the existing 2.5 to over 4.8. Simultaneously, it balances the flexibility and hardness of the cured film, raising its hardness from the conventional HB to over H, fully meeting the needs of large-area spraying on the entire body of rail transit vehicles.

[0012] The coating film prepared by waterborne polyurethane dispersion is soft, with good thixotropic index and anti-sagging properties, and moderate drying speed. It also exhibits excellent transparency, minimal impact of different spraying techniques on the color effect of the two-component polyurethane topcoat, high coating efficiency, and strong adhesion to the intermediate coating layer, ensuring good interlayer adhesion. The flexibility of the molecular chain is the most important factor determining the glass transition temperature (Tg) of polymers; the better the main chain flexibility, the lower the glass transition temperature. Through extensive experiments, this invention has found that waterborne polyurethane dispersions with low glass transition temperatures maintain good flexibility at low temperatures, improving film formation under these conditions. Waterborne polyurethane dispersions with high glass transition temperatures improve the coating's adaptability to high-temperature environments and enhance its high-temperature resistance. Blending waterborne polyurethane dispersions with different glass transition temperatures can adapt to and improve the workability of coating films under various complex conditions, including high and low temperatures, and can form a smooth and glossy coating.

[0013] Furthermore, hydroxyl-containing aqueous dispersions are secondary dispersions, characterized by high solids content (35%–50%), high hydroxyl content (3%–5%), and low volatile organic compound (VOC) content. When used in combination with polyisocyanate curing agents in two-component polyurethane topcoats, they exhibit excellent reactivity and film-forming properties. This results in better adhesion, durability, water resistance, solvent resistance, and thermal stability of the two-component polyurethane topcoat, improving the flexibility and hardness of the coating (pencil hardness H or higher), and giving the coating high gloss (95% gloss at 60°), high fullness, and high DOI (DOI above 95). Compared to low-hydroxyl acrylic resins, hydroxyl-containing aqueous dispersions have higher hydroxyl values, leading to a corresponding increase in the number of hydroxyl groups in the film-forming resin. This enhances the resin's reactivity, significantly increasing the crosslinking speed and degree during curing. Due to the drawbacks of hydroxyl-containing aqueous dispersions, such as rapid drying, rapid film volume shrinkage, numerous pits, and noticeable particles, waterborne two-component polyurethane topcoats formulated with hydroxyl-containing aqueous dispersions often exhibit a bluish or white sheen on the sides and are difficult to color-match. Therefore, using hydroxyl-containing aqueous dispersions alone as film-forming resins cannot meet the requirements for rapid color matching in large-area spraying and subsequent repair paint application on rail transit vehicles. Blending a specific proportion of hydroxyl-containing aqueous dispersions into non-hydroxyl-containing aqueous polyurethane dispersions allows for chemical cross-linking between hydroxyl groups (-OH) and isocyanate (-NCO) curing agents, further enhancing film adhesion. The synergistic effect of the aqueous polyurethane dispersion and the hydroxyl-containing aqueous dispersion can compensate for the shortcomings of individual film-forming resins (single aqueous polyurethane dispersions have poor self-thickening properties, low solid content, and poor water resistance, heat resistance, and gloss; single aqueous acrylic resin emulsions are hot-sticky and cold-brittle, have poor flexibility, are not solvent-resistant, and are difficult to color-match).

[0014] This invention, through the synergistic effect of various raw material components and specific proportions, yields a water-based two-component polyurethane topcoat with excellent comprehensive performance. The results of the examples show that the water-based two-component polyurethane topcoat provided by this invention can be applied over large areas in high-temperature, low-temperature, or high-humidity environments. It mixes well with unsanded intermediate coats, exhibiting excellent anti-sagging properties, color blending ability, and interlayer adhesion without affecting application fluidity. It does not produce fly ash and also possesses excellent mechanical properties and corrosion resistance. Furthermore, the topcoat after application does not require a topcoat clear varnish, exhibiting excellent gloss and reflectivity, comparable to the film appearance of a solvent-based 1K base coat + topcoat clear varnish coating system. Therefore, it has high practical and promotional value.

[0015] It should be noted that the hydroxyl content of the hydroxyl-containing aqueous dispersion is calculated based on the calculated value of the solid resin.

[0016] Preferably, the viscosity of the hydroxyl-containing aqueous dispersion is 100 mPa·s to 3000 mPa·s.

[0017] Preferably, the solid content of the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50°C and the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C are both 33%-40%.

[0018] Preferably, the hydroxyl-containing aqueous dispersion is at least one of Covestro Bayhydrol A 2470, Covestro Bayhydrol A 2770, or Covestro Bayhydrol U XP 2766.

[0019] Preferably, the waterborne polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50°C is Covestro Bayhydol UH 2606.

[0020] Preferably, the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C is at least one of Covestro Bayhydrol UH 2648 / 1, Covestro Bayhydrol UH 2110, Covestro Bayhydrol UH 2621 or Covestro Bayhydrol UH 2952 / 1.

[0021] In this invention, the aliphatic anionic polyurethane dispersion has advantages such as light color, high transparency, good compatibility, slightly slower drying speed, softer film, moderate film volume shrinkage, strong corrosion resistance, and strong interlayer adhesion. It can minimize interference with the color of the wet paint film, has the rapid color matching required for repair paint, and exhibits excellent leveling, chemical resistance, and weather resistance after film formation, as well as high toughness and good elasticity. Among them, the low glass transition temperature waterborne aliphatic anionic polyurethane dispersion (especially Covestro Bayhydrol UH 2952 / 1) and the high glass transition temperature waterborne polycarbonate-containing aliphatic anionic polyurethane dispersion (Covestro Bayhydol UH 2606) can adhere to the surface of the vehicle body (or OEM product surface) without sanding, with excellent adhesion and chemical resistance. It can not only meet the needs of large-area spraying of the entire body of rail transit vehicles, but also has good compatibility with intermediate coat coating and is also suitable for repair paint under infrared baking lamps (without clear coat). Hydroxyl-containing aqueous dispersions have a higher hydroxyl content within their molecules, fewer intermolecular chemical bonds, and a more ordered molecular arrangement, resulting in lower viscosity. Furthermore, higher hydroxyl values ​​lead to better reactivity with polyisocyanate curing agents, greater crosslinking density, and better adhesion and hardness after film formation. Adding a certain amount of high-hydroxyl-value resin to the film-forming resin and compounding it with an aqueous polyurethane dispersion can not only improve the thixotropic index and anti-sagging properties of the coating, but also synergistically enhance the flexibility and hardness of the film, giving the coating high gloss, high fullness, high reflectivity, as well as excellent water resistance, solvent resistance, and thermal stability.

[0022] Preferably, the pigment is an organic pigment or an inorganic pigment.

[0023] Preferably, when the pigment is an organic pigment, the A component comprises: 10% to 20% of the water-based polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40 to 50°C and 15% to 25% of the water-based color paste.

[0024] When the pigment is an inorganic pigment, component A comprises: 5% to 12% of the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40 to 50°C, 0% to 12% of the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤ 5°C, and 25% to 45% of the aqueous color paste.

[0025] More preferably, when the pigment is an inorganic pigment, component A comprises 25% to 35% aqueous pigment.

[0026] Preferably, based on component A as 100%, the aqueous color paste comprises 5%–10% water, 0.8%–1.4% dispersant, 0.2%–0.5% second defoamer, 0.2%–1% second surfactant, 5%–13% organic pigment, 0.9%–4.5% color paste stabilizer, and 0.1%–0.3% neutralizer.

[0027] Preferably, based on component A as 100%, the water-based pigment paste comprises 7%–12% water, 1.2%–1.8% dispersant, 0.2%–0.5% second defoamer, 0.2%–0.5% second surfactant, 15%–25% inorganic pigment, 1.5%–5.4% pigment paste stabilizer, and 0.1%–0.3% neutralizer.

[0028] Preferably, the dispersant is at least one of a nonionic wetting dispersant or an anionic wetting dispersant.

[0029] More preferably, the nonionic wetting and dispersing agent is at least one of BYK DISPERBYK-190, BYK DISPERBYK-LPN 23160, BYK DISPERBYK-2010, BYK DISPERBYK-2012, ZENX VXW6208-60, or BASF Dispex Ultra PX 4585.

[0030] More preferably, the anionic wetting and dispersing agent is at least one of TEGO Dispers 747W, TEGO Dispers 757W, TEGO Dispers 750W, or TEGO Dispers 755W.

[0031] More preferably, the dispersant comprises a nonionic wetting and dispersing agent and an anionic wetting and dispersing agent in a mass ratio of 1:1 to 2.5:1.

[0032] Preferably, the mass ratio of the non-volatile components of the dispersant to the organic pigment is 0.02:1 to 0.2:1.

[0033] Preferably, the mass ratio of the non-volatile components of the dispersant to the inorganic pigment is 0.03:1 to 0.05:1.

[0034] Preferably, the second defoamer is at least one of BYK-011, BYK-024, BYK-1711, or TEGO-810.

[0035] Preferably, the second surfactant is at least one of an alkynyl glycol surfactant or an alcohol alkoxy compound.

[0036] More preferably, the second surfactant is at least one of Air Chemical Surfynol AD01, Air Chemical Surfynol 440, Air Chemical Surfynol 104BC, or BYK-DYNWET 800.

[0037] In this invention, Surfynol AD01 is a Gemini alkynyldiol surfactant with an HLB value (hydrophilic-lipophilic balance) of 4, possessing defoaming and wetting functions, providing defoaming, dynamic wetting, and film-forming aid effects; Surfynol 440 is a nonionic alkynyldiol ethoxylated compound with low dynamic tension, good compatibility, and an HLB value of 8, possessing Gemini wetting and low-foaming effects, reducing the dynamic and static surface tension of the system, and preventing the occurrence of pinholes, craters, and fisheyes; Surfynol 104BC is a Gemini tetramethyldecynyldiol nonionic surfactant with wetting, leveling, and defoaming functions; BYK-DYNWET 800 is an alcohol alkoxylated compound with a slight foam-stabilizing function, reducing the dynamic surface tension of the system, and is particularly suitable for high-speed grinding slurries.

[0038] Preferably, the organic pigment is at least one of maroon, phthalocyanine green, phthalocyanine blue, organic yellow, or carbon black.

[0039] Preferably, the inorganic pigment is titanium dioxide.

[0040] Preferably, the color paste stabilizer includes at least three of the following: water-based fumed silica, modified urea rheology modifier, resin-free color paste humectant, or anti-drying agent for water-based pigment concentrate.

[0041] More preferably, the color paste stabilizer includes at least three of the following: Degussa AEROSIL R972, BYK RHEOBYK-420, TEGO Humectant 7005, or BYKETOL-PC.

[0042] More preferably, the color paste stabilizer comprises Degussa AEROSIL R972, BYK RHEOBYK-420 and TEGO Humectant 7005 in a mass ratio of 1:1:8 to 1:1:12.

[0043] More preferably, the color paste stabilizer comprises Degussa AEROSIL R972, BYK RHEOBYK-420 and BYKETOL-PC in a mass ratio of 1:1:8 to 1:1:12.

[0044] In this invention, AEROSIL R972 is an aqueous fumed silica with silanol groups on its surface. After thorough stirring and dispersion, hydrogen bonds are formed between the silanol groups, creating a three-dimensional network structure. This increases the viscosity of the dispersion system, produces thixotropic flow properties, and acts as an anti-settling agent, improving the stability, corrosion resistance, and thixotropy of the pigment. RHEOBYK-420 is a modified urea rheology modifier with anti-settling and anti-sagging properties, producing a thixotropic effect. TEGO Humectant 7005 is a resin-free pigment paste humectant. BYKETOL-PC is an anti-drying agent for aqueous pigment concentrates, which is also a modified urea solution. Based on its specific molecular structure, it allows for optimal control of water evaporation, ensuring that the aqueous pigment concentrate does not dry out too quickly.

[0045] Preferably, the neutralizing agent is an 8wt% to 12wt% aqueous solution of dimethylethanolamine.

[0046] Preferably, the method for preparing the water-based pigment includes the following steps:

[0047] Dispersant, second defoamer and second surfactant are added to water and mixed evenly. Pigment and color paste stabilizer are added, ground, and neutralizer is added to obtain water-based color paste.

[0048] For example, the preparation method of the aqueous pigment includes the following steps:

[0049] Add water to a clean container and, while stirring at 280 rpm to 320 rpm, add the dispersant and the second defoamer in sequence. Adjust the stirring speed to 800 rpm to 1500 rpm, add the second surfactant, and stir for 5 to 10 minutes to make it uniform. Adjust the stirring speed to 1500 rpm to 2000 rpm, add the pigment and color paste stabilizer in sequence, and adjust the stirring speed to 2000 rpm to 3000 rpm, dispersing and stirring for 25 to 40 minutes. Grind using a horizontal grinder or a basket grinder. Finally, add the neutralizing agent and mix evenly to obtain the water-based color paste.

[0050] Because the second surfactant has low solubility in water (e.g., Surfynol AD01 has an HLB of 4 and a solubility of 0.1% in water), low molecular weight, and is volatile, it needs to be fully dispersed in order to fully adsorb onto various particles in the system and achieve good compatibility. It is recommended that the dispersion speed be no less than 800 rpm to avoid water sensitivity problems.

[0051] More preferably, the pH of the aqueous pigment is 8 to 8.5.

[0052] More preferably, the fineness of the aqueous pigment is 10μm to 20μm.

[0053] It should be noted that there is no need to adjust the pH before or during grinding, nor to add other components such as resin, amines, organic cosolvents, or thickeners, otherwise the grinding effect of the pigment paste will be affected.

[0054] Preferably, the first surfactant is a Geminid acetylenic diol surfactant.

[0055] More preferably, the first surfactant is at least one of Air Chemical Surfynol AD01, Air Chemical Surfynol 104BC, or Air Chemical Surfynol 440.

[0056] More preferably, the first surfactant is at least two of Air Chemical Surfynol AD01, Air Chemical Surfynol 104BC, or Air Chemical Surfynol 440.

[0057] For example, the first surfactant is Air Chemical Surfynol AD01, Air Chemical Surfynol 104BC and Air Chemical Surfynol 440 in a mass ratio of 2:1:1.

[0058] In this invention, Surfynol AD01 is a Gemini acetylenic diol surfactant with an HLB value (hydrophilic-lipophilic balance) of 4. It has defoaming and wetting functions, providing defoaming, dynamic wetting, and film-forming aid effects. Surfynol 104BC is a Gemini tetramethyldecynyl diol nonionic surfactant. The dynamic surface tension of the wet film is slightly lower than that of the substrate, which can effectively wet the substrate and form a continuous and stable paint film. It has low water sensitivity and functions such as wetting, leveling, defoaming, improving dispersibility, reducing surface tension, and stabilizing consistency and viscosity. Surfynol 440 is a nonionic acetylenic diol ethoxylated compound with low dynamic tension, good compatibility, and an HLB value of 8. It has Gemini wetting and low-foaming effects, which can reduce the dynamic and static surface tension of the system and prevent the occurrence of pinholes, craters, and fisheyes.

[0059] Preferably, the first defoamer is at least one of BYK-093, BYK-011, or TEGO-830.

[0060] Preferably, the substrate wetting agent is a polyether-modified organosilicon surfactant.

[0061] More preferably, the substrate wetting agent is at least one of BYK-345, BYK-346, BYK-347, Tego-4100, Tego-280, or Air Chemical Surfynol 104BC.

[0062] The preferred substrate wetting agent of this invention can effectively reduce the surface tension of the system, significantly improve the substrate wettability of the system, while having good recoatability, not increasing surface slip, and is suitable for all water-based systems without cosolvents.

[0063] Preferably, the leveling agent is at least one of BYK-378, BYK-381, BYK-333, BYK-331, Tego-4100, or Air Chemical Surfynol 104BC.

[0064] During the leveling process, coating molecules are constantly in motion. As the viscosity of the coating gradually increases during leveling, the system molecules gradually form Bénard vortices, leading to defects such as pinholes and sagging in the coating film during film formation. In the preferred leveling agents of this invention, BYK-378 is a polyether-modified dimethyl polysiloxane, which can significantly reduce the surface tension of the system, improve surface smoothness, and thus improve the scratch resistance of the coating. It also has excellent substrate wetting properties and good anti-cratering effect, even at low addition levels. Furthermore, it has low foam stability, good compatibility, is easy to recoat, and does not cause oil spots. BYK-381 is an ionic polyacrylate solution-type surface additive that can improve the leveling and substrate wetting of the system, preventing surface defects (craters and pinholes). Tego-4100 is a leveling agent with a twin structure, exhibiting high activity and low foam stability. Even at extremely low concentrations, it can effectively reduce surface tension and improve wetting and leveling properties.

[0065] The dynamic surface tension of the wet film needs to be slightly lower than that of the substrate so that the paint film can effectively wet the substrate and form a continuous and stable paint film. The surfactant, substrate wetting agent and leveling agent preferred in this invention have low water sensitivity, which can effectively reduce surface tension and achieve a balance between static and dynamic surface tension. Their special structure gives them wetting and defoaming effects, which can improve the dispersibility of the system, improve fluidity and leveling, control foam, and stabilize consistency and viscosity.

[0066] Preferably, the aqueous ultraviolet absorber is at least one of BASF Tinuvin 5151, Zhonghao Chemical UV-DL2, Yongguang Chemical EVERSORB AQ1, Yongguang Chemical EVERSORB AQ2, or Yongguang Chemical EVERSORB AQ3.

[0067] In this invention, Tinuvin 5151 light stabilizer is a synergistic mixture of hydrophilic 2-(2-hydroxyphenyl)-benzotriazole ultraviolet absorber (UVA) and basic hindered amine radical scavenger (HALS), exhibiting excellent spectral coverage in the ultraviolet region. The hydroxyl-containing UV absorber can react with isocyanate and melamine crosslinking agents to reduce migration. UV-DL2 is a compound product of highly efficient ultraviolet absorber and light stabilizer, which can slow down the aging phenomena of coatings caused by light exposure, such as discoloration, loss of gloss, cracking, and peeling, and improve gloss retention, thereby enhancing the durability of the coating. EVERSORB AQ1, EVERSORB AQ2, and EVERSORB AQ3 are high-efficiency light stabilizers that can inhibit film aging and are suitable for transparent varnishes.

[0068] Preferably, the first cosolvent comprises at least three of the following: propylene glycol methyl ether propionate, propylene glycol butyl ether, dipropylene glycol dimethyl ether, ethylene glycol butyl ether, dipropylene glycol methyl ether, dimethyl sulfoxide, N-methylpyrrolidone, dipropylene glycol butyl ether, or diethylene glycol monobutyl ether.

[0069] More preferably, based on component A as 100%, the first co-solvent comprises 1% to 2% of a first solvent, 0.75% to 1.5% of a second solvent, and 1.25% to 2.5% of a third solvent; the first solvent comprises at least one of propylene glycol methyl ether propionate (PMP), dipropylene glycol dimethyl ether (DMM), or propylene glycol butyl ether (PNB); the second solvent comprises at least one of ethylene glycol butyl ether (BCS), dipropylene glycol methyl ether (DPM), or dimethyl sulfoxide (DMSO); and the third solvent comprises at least one of N-methylpyrrolidone (NMP), diethylene glycol monobutyl ether (BDG), or dipropylene glycol butyl ether (DPNB).

[0070] More preferably, the first cosolvent comprises propylene glycol methyl ether propionate (PMP), dipropylene glycol methyl ether (DPM), and N-methylpyrrolidone (NMP) in a mass ratio of 1:1:1.

[0071] This invention, by controlling the boiling point (with a boiling point gradient) and water miscibility of the first co-solvent (including the solubility of the solvent in water and the solubility of water in the solvent), facilitates a balanced evaporation rate during film formation, increases water solubility, and is more conducive to coating the paint film under high temperature or high humidity conditions. It also results in moderate drying speed, no fly ash, no sagging, and minimal blistering marks. As long as the on-site ventilation is good, normal coating can be performed. The specific first co-solvent can couple different phases, exhibiting strong solubility for both polar and non-polar substances. It interacts with the solution system in water-based coatings, causing slight phase separation, which helps reduce the surface tension of the water-based system, enhances the fluidity of the coating, thereby adjusting its performance, increasing the viscosity of the water-based coating, changing the drying time, improving the wettability and coating effect, and enhancing the elasticity, abrasion resistance, water resistance, solvent resistance, and mechanical strength of the paint film. The preferred first co-solvent can also be used as a film-forming aid. Its unique hydrophilic and oleophilic balance properties enable it to exert different film-forming effects on different resins, helping the coating to form a uniform and smooth coating. In particular, it can significantly reduce the surface tension of the system, has an auxiliary leveling effect, and prevents cracking and orange peel formation.

[0072] Preferably, the neutralizing agent is an 8wt% to 12wt% aqueous solution of dimethylethanolamine.

[0073] Preferably, the thickener is a polyurethane associative thickener.

[0074] More preferably, the thickener is at least one of Hemings RHEOLATE 299, BYK RHEOBYK-H 7625VF, Rohm and Haas RM5000, or Rohm and Haas RM-8W.

[0075] More preferably, the thickener is at least two of the following: Hemings RHEOLATE 299, BYK RHEOBYK-H 7625VF, Rohm and Haas RM5000, or Rohm and Haas RM-8W.

[0076] In this invention, RHEOLATE 299, RHEOBYK-H 7625VF, and RM5000 are the same type of polyurethane associative thickeners, unaffected by pH value. RHEOLATE 299 provides efficient thickening in the low to medium shear rate range, RHEOBYK-H 7625VF can rapidly increase viscosity at low shear, and RM5000 can increase viscosity in the high shear rate range; the three complement each other. RM-8W is a low-odor, solvent-free hydrophobically modified polyurethane nonionic associative thickener with excellent flow and leveling properties, uniform film formation, gloss performance, and high thickening efficiency.

[0077] Preferably, the solid content of component A is 35% to 45%.

[0078] In this invention, the solid content of component A refers to the mass percentage of all solid components (including resin dispersion and pigment) in component A.

[0079] Preferably, the mass ratio of component A to component B is 2.2:1 to 5.7:1, and more preferably 3:1 to 5:1.

[0080] Preferably, based on component B as 100%, the curing agent comprises 25%–30% low-viscosity hydrophobic hexamethylene diisocyanate, 25%–30% high-viscosity hydrophilic hexamethylene diisocyanate, and 18%–22% hydrophilic modified isophorone diisocyanate.

[0081] It should be noted that the hydrophilic modified isophorone diisocyanate in this invention refers to a hydrophilic modified aliphatic polyisocyanate based on isophorone diisocyanate (IPDI).

[0082] More preferably, the low-viscosity hydrophobic hexamethylene diisocyanate is at least one of Covestro Desmodur N3900, Covestro Desmodur N3600, or Wanhua WANNATE HT-600.

[0083] More preferably, the high-viscosity hydrophilic hexamethylene diisocyanate is at least one of Covestro Bayhydrol XP2655, Covestro Bayhydur XP 2547, Covestro Bayhydur XP 2487 / 1, Covestro Bayhydur 304 or Covestro Bayhydur 305.

[0084] Preferably, the solid content of the hydrophilic aliphatic polyisocyanate is 60% to 70%.

[0085] Preferably, the hydrophilic aliphatic polyisocyanate has an -NCO content of 8.5% to 12%.

[0086] More preferably, the hydrophilic aliphatic polyisocyanate is at least one of Covestro Bayhydur 401-70, Covestro Bayhydur 401-60, or Evonik Vestanet T1890E.

[0087] By limiting the type of curing agent, this invention can further improve the curing effect and enhance the application fluidity, viscosity, and gloss of the coating.

[0088] Preferably, the second co-solvent is at least two of propylene glycol methyl ether acetate (PMA), propylene glycol methyl ether acrylate (PMP), propylene glycol diacetate (PGDA), ethylene glycol butyl ether acetate (BGA), dipropylene glycol dimethyl ether (DMM), propylene carbonate (PC), or divalent ester (DBE).

[0089] More preferably, based on 100% of component B, the second co-solvent comprises 6%–9% of a fourth solvent, 8%–12% of a fifth solvent, and 6%–9% of a sixth solvent; the fourth solvent comprises at least one of propylene glycol methyl ether acetate (PMA) or propylene glycol methyl ether propionate (PMP); the fifth solvent comprises at least one of propylene glycol diacetate (PGDA), ethylene glycol butyl ether acetate (BGA), or dipropylene glycol dimethyl ether (DMM); and the sixth solvent comprises at least one of propylene carbonate (PC) or divalent ester (DBE).

[0090] Table 1 Summary of the physicochemical properties of the second co-solvent

[0091]

[0092]

[0093] The present invention adds a certain amount of second co-solvent to the curing component, which can improve the compatibility between the curing agent and the hydroxyl-containing aqueous dispersion, extend the open time of the paint film, and facilitate leveling. The specific second co-solvents are all environmentally friendly solvents (see Table 1 for specific properties), and have the following characteristics: (1) They contain multiple functional groups and have strong dissolving power for both polar and non-polar substances, which can improve the strength of the coating film; (2) They have low surface tension, strong solvent power, low viscosity and good volatility, which can provide good solubility and flowability, ensure uniform coating, and be easy to apply. At the same time, they can improve the adhesion and durability of the coating, regulate the evaporation rate of the entire system, regulate the drying speed of the paint film, reduce pinholes and fish eyes, and increase gloss, wetting, leveling and releveling properties; (3) Acrylic carbonate has good film-forming properties and strong carbon dioxide absorption capacity. When used in conjunction with hydrophilic isocyanate curing agents, the effect is excellent, which can improve prickly heat and reduce bubbles; (4) The combination of multiple second co-solvents can complement each other and achieve better overall results. In addition, by compounding second co-solvents with different boiling points (with gradient boiling points), it is beneficial to balance the evaporation rate during the film-forming process and improve the solubility in water. For example, the synergistic effect of propylene glycol methyl ether propionate (PMP), propylene glycol diacetate (PGDA), and propylene carbonate (PC) can not only improve the strength of the coating film, but also facilitate the stable storage of the coating. It can also absorb carbon dioxide during the coating and film-forming process, and is also excellent for developing water-based isocyanate curing agents.

[0094] Preferably, the molar ratio of -NCO in component B to -OH in component A is 1.4:1 to 1.6:1, and more preferably 1.5:1.

[0095] The preferred molar ratio of -NCO to -OH in this invention not only satisfies the combined application of components A and B, but also, after adding a specific activating diluent during subsequent use, the ratio of -NCO to -OH in the system is more conducive to the formation of the paint film.

[0096] Secondly, the present invention provides a method for preparing a water-based two-component polyurethane topcoat, comprising the following steps:

[0097] S1. Weigh each raw material according to the design ratio. Add the hydroxyl-containing aqueous dispersion, the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃, and the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃ to water and mix evenly. Add the first surfactant, the first defoamer, the substrate wetting agent, the leveling agent, the aqueous ultraviolet absorber, the aqueous color paste, and the first cosolvent and mix evenly. Then add the neutralizer and the thickener in sequence and mix evenly to obtain component A.

[0098] S2, add the second co-solvent to the curing agent and mix evenly to obtain component B.

[0099] It should be noted that steps S1 and S2 are not in any particular order.

[0100] The method for preparing water-based two-component polyurethane topcoat provided by this invention does not require special treatment of the raw materials and is simple and easy to implement.

[0101] For example, the preparation method of component A includes the following steps:

[0102] Weigh each raw material according to the designed proportions. Add the hydroxyl-containing aqueous dispersion, the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃, and the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃ to a clean container. Add water at 600-800 rpm and stir for 4-6 minutes. Then, add the first surfactant, the first defoamer, the substrate wetting agent, the leveling agent, the aqueous ultraviolet absorber, the aqueous color paste, and the first cosolvent in sequence. Continue stirring at 600-800 rpm for 10-15 minutes. Then, add the neutralizing agent to adjust the pH of the system to 8-8.5. Finally, add the thickener and mix evenly to obtain component A.

[0103] Thirdly, the present invention provides the application of the above-mentioned waterborne two-component polyurethane topcoat in automotive coatings.

[0104] Preferably, the automotive coating is a coating for rail transit vehicles.

[0105] Fourthly, the present invention provides a method for using the above-mentioned water-based two-component polyurethane topcoat, comprising the following steps:

[0106] Mix components A and B thoroughly, add the activating diluent, mix thoroughly to obtain the application coating, and apply it evenly to the intermediate coat.

[0107] The activating diluent includes a hydroxyl-containing aqueous dispersion, an aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50°C, an aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C, an aqueous synthetic flake silicate solution, an aqueous modified urea solution, a first cosolvent, a stabilizer, a first defoamer, a leveling agent, and water.

[0108] In the application of waterborne two-component polyurethane topcoat, a unique activating diluent is used to dilute the waterborne two-component polyurethane topcoat (component A + component B), which has the following beneficial effects: (1) it improves the system's tolerance to high content of surfactants or extreme pH values; (2) it can prevent the coating from sagging or miscibility; (3) when spraying large areas, it can accelerate the formation of the structure after construction, thereby improving the adhesion of vertical surfaces (improving the interlayer adhesion between waterborne intermediate coat and waterborne two-component polyurethane topcoat).

[0109] Preferably, the activating diluent comprises the following raw material components by mass percentage: 8%–11.5% hydroxyl-containing aqueous dispersion, 3%–4.5% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40–50°C, 3%–4.5% aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C, 25%–33.5% aqueous synthetic flake silicate solution, 0.5%–0.7% aqueous modified urea solution, 7.5%–8.5% first cosolvent, 7.5%–8.5% stabilizer, 0.5%–0.7% first defoamer, 0.5%–0.7% leveling agent, and 27%–42% water, the sum of which is 100%.

[0110] Preferably, the mass ratio of component A to the activating diluent is 100:(20-30).

[0111] The above-mentioned mass ratio ensures the solid content of the coating material. The specific amount of activating thinner can be adjusted according to the ambient temperature and humidity, and whether sagging occurs during spraying. Generally, the higher the temperature or humidity, the more activating thinner should be added; the more severe the sagging, the more activating thinner should be added.

[0112] Preferably, the hydroxyl content of the hydroxyl-containing aqueous dispersion is 3.5% to 5%.

[0113] Preferably, the viscosity of the hydroxyl-containing aqueous dispersion is 100 mPa·s to 3000 mPa·s.

[0114] Preferably, the solid content of the hydroxyl-containing aqueous dispersion is 35% to 50%.

[0115] Preferably, the solid content of the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50°C and the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C are both 33%-40%.

[0116] Preferably, the hydroxyl-containing aqueous dispersion is at least one of Covestro Bayhydrol A 2470, Covestro Bayhydrol A 2770, or Covestro Bayhydrol U XP 2766.

[0117] Preferably, the waterborne polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50°C is Covestro Bayhydol UH 2606.

[0118] Preferably, the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C is at least one of Covestro Bayhydrol UH 2648 / 1, Covestro Bayhydrol UH 2110, Covestro Bayhydrol UH 2621 or Covestro Bayhydrol UH 2952 / 1.

[0119] Preferably, the water-based synthetic sheet silicate solution comprises the following raw material components by mass percentage: 1.5%–2.5% organic-modified synthetic layer silicate, 1.5%–2.5% emulsifying dispersant, and 95%–97% water, with the sum of all components being 100%.

[0120] More preferably, the organic-modified layered silicate is at least one of BYK Laponite-RD or BYK Laponite-RDS.

[0121] Laponite-RD and Laponite-RDS are synthetic sheet-like silicates that are insoluble in water but can hydrate and swell in water to form a colorless and transparent colloid. When their concentration in water is above 2%, they can form a colloid that enhances thixotropy, thus playing an activating and regulating role in preventing sagging. This invention utilizes the synergistic effect of the combination of laponite and polymers to improve the system's tolerance to high concentrations of surfactants, electrolytes, or extreme pH values.

[0122] More preferably, the emulsifying dispersant is at least one of polyethylene glycol or polypropylene glycol. If the emulsifying dispersant includes polyethylene glycol and polypropylene glycol, the mass ratio of polyethylene glycol to polypropylene glycol can be 1:1.

[0123] More preferably, the emulsifying dispersant is at least one of PEG-1000, PEG-1500, PPG-200, PPG-400 or PPG-600.

[0124] Preferably, the method for preparing the water-based synthetic sheet silicate solution includes the following steps:

[0125] An emulsifying dispersant is added to water and mixed evenly. Then, an organic compound is added to modify and synthesize layered silicates. The mixture is then mixed evenly to obtain a water-based synthetic sheet silicate solution.

[0126] For example, while stirring at 400 rpm to 600 rpm, an emulsifying dispersant is added to water. The stirring speed is adjusted to 1000 rpm to 1500 rpm, and organic matter is slowly added to modify and synthesize layered silicates. Stirring continues until the solution is clear and no dissolved particles are visible, thus obtaining a water-based synthetic sheet silicate solution.

[0127] Preferably, the water-based modified urea solution is at least one of BYK RHEOBYK-7420CA, BYK RHEOBYK-7420ET, or BYK RHEOBYK-7420ES.

[0128] In this invention, the water-based modified urea solution is a liquid rheology modifier. A specific amount of water-based modified urea solution can produce a strong thixotropic effect, thereby improving the anti-settling and anti-sagging properties of the system.

[0129] Preferably, the first cosolvent comprises at least three of the following: propylene glycol methyl ether propionate, propylene glycol butyl ether, dipropylene glycol dimethyl ether, ethylene glycol butyl ether, dipropylene glycol methyl ether, dimethyl sulfoxide, N-methylpyrrolidone, dipropylene glycol butyl ether, or diethylene glycol monobutyl ether.

[0130] More preferably, based on component A as 100%, the first co-solvent comprises 1% to 2% of a first solvent, 0.75% to 1.5% of a second solvent, and 1.25% to 2.5% of a third solvent; the first solvent comprises at least one of propylene glycol methyl ether propionate (PMP), dipropylene glycol dimethyl ether (DMM), or propylene glycol butyl ether (PNB); the second solvent comprises at least one of ethylene glycol butyl ether (BCS), dipropylene glycol methyl ether (DPM), or dimethyl sulfoxide (DMSO); and the third solvent comprises at least one of N-methylpyrrolidone (NMP), diethylene glycol monobutyl ether (BDG), or dipropylene glycol butyl ether (DPNB).

[0131] Preferably, the stabilizer is at least one of propylene glycol methyl ether acetate (PGMEA) or propylene glycol methyl ether propionate (PMP).

[0132] In this invention, the preferred stabilizer can provide good solubility and flowability, ensuring a uniform coating that is easy to apply, while improving the adhesion and durability of the coating.

[0133] Preferably, the first defoamer is at least one of BYK-093, BYK-011, or TEGO-830.

[0134] Preferably, the leveling agent is at least one of BYK-378, BYK-381, BYK-333, BYK-331, Tego-4100, or Air Chemical Surfynol 104BC.

[0135] For example, the preparation method of the activating diluent includes the following steps:

[0136] Weigh each raw material according to the design ratio, and add the following to water at 400 rpm to 600 rpm: a hydroxyl-containing aqueous dispersion, an aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40 to 50°C, an aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤ 5°C, a first defoamer, a leveling agent, a first cosolvent, a stabilizer, a water-based synthetic flake silicate solution, and a water-based modified urea solution. Continue stirring at 600 rpm to 800 rpm for 8 min to 12 min to obtain the activated diluent.

[0137] This invention incorporates the corresponding film-forming resin from component A into the activating diluent, increasing both the application solids content and the hydroxyl content, further promoting the reactivity of the film-forming resin with the isocyanate curing agent. This enhances adhesion and improves the gloss, brightness, and fullness of the paint film. Through the synergistic effect of the hydroxyl-containing aqueous dispersion and the aqueous polyurethane dispersion with different glass transition temperatures, the system's tolerance to the maximum film thickness of two-component polyurethane topcoats is improved, accelerating the formation of an ideal paint film surface after application. It is particularly suitable for large-area spraying on rail transit vehicle bodies and can be successfully applied in high-temperature, low-temperature, or high-humidity environments. The water-based modified urea solution is a liquid rheology modifier that produces a strong thixotropic effect, improving the anti-settling and anti-sagging properties of water-based and highly polar systems. Furthermore, the stabilizer is a high-performance industrial solvent with strong dissolving power and fluidity for both polar and non-polar substances, ensuring a uniform coating and ease of application. This further improves the adhesion and durability of the coating. It also enhances the solubility of water-based synthetic flake silicate solutions, ensuring the coating does not flocculate or clump during long-term storage. During application, the addition of an activating diluent significantly improves interlayer adhesion and allows for precise adjustment of rheological properties, thereby enhancing the interlayer adhesion between the two-component polyurethane topcoat and the water-based intermediate coat.

[0138] A synergistic effect can be achieved between water-based synthetic flake silicate solution and water-based modified urea solution, increasing the low shear viscosity of the coating, preventing sagging or mixing during application, improving the workability and storage stability of the coating, and effectively preventing the sedimentation and dehydration shrinkage of solids in the water-based coating system, thereby accelerating post-application structure formation. Extensive testing revealed that when water-based synthetic flake silicate solution and water-based modified urea solution are combined, excessive amounts of the rheology modifier can lead to inability to store the activated dilution for extended periods (caking and flocculation may occur with prolonged storage), and can also reduce the water resistance of water-based two-component polyurethane topcoats. Excessive amounts of the surfactant can cause defects such as pinholes, crazing, and orange peel in the paint film. Therefore, combining the rheology modifier with surface additives, stabilizers, and other additives with a portion of the film-forming resin in a specific ratio further improves the storage stability and water resistance of the coating, achieving optimal film appearance. Detailed Implementation

[0139] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0140] In the embodiments of this invention, all water used is deionized water.

[0141] The experimental instruments used in this invention include:

[0142] Grid dividing tool, Guangzhou Biaogeda;

[0143] Pencil hardness tester, impact instrument, bending tester, cupping tester, sharpness tester, BYK Germany;

[0144] MA85 Multi-Angle Photometer, X-Rite (USA);

[0145] ISO 6 cup, Co-4 cup, solvent-resistant wiper, pH meter, film thickness gauge, Shanghai Modern Environmental Engineering Technology Co., Ltd.;

[0146] Salt spray resistance testing equipment, artificial weathering resistance (QUA / SPARY), Q-Lab, USA.

[0147] To better illustrate the present invention, further examples are provided below.

[0148] Example 1

[0149] This embodiment provides a water-based two-component polyurethane topcoat, composed of component A and component B in a mass ratio of 4.5:1.

[0150] Component A consists of the following raw material components by mass percentage: 44% hydroxyl-containing aqueous dispersion, 11% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃ (denoted as PUD with Tg = 40-50), 12% aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤ 5℃ (denoted as PUD with Tg ≤ 5), 0.4% first surfactant, 0.4% first defoamer, 0.4% substrate wetting agent, 0.3% leveling agent, 0.3% aqueous ultraviolet absorber, 19.1% aqueous color paste (of which, water 7%, dispersant 1%, second defoamer 0.3%, second surfactant 0.6%, organic pigment 8%, color paste stabilizer 2% and neutralizer 0.2%), 3.5% first cosolvent, 1.3% neutralizer, 0.3% thickener and 7% water.

[0151] Component B consists of the following raw material components by mass percentage: 75% curing agent (28% low viscosity hydrophobic hexamethylene diisocyanate, 27% high viscosity hydrophilic hexamethylene diisocyanate, and 20% hydrophilic modified isophorone diisocyanate) and 25% second cosolvent.

[0152] The specific components of the above raw materials are shown in Table 2. The calculated solid content of component A is 37.9%.

[0153] The preparation method of the above-mentioned water-based two-component polyurethane topcoat includes the following steps:

[0154] S1. Weigh each raw material according to the designed ratio. Add the hydroxyl-containing aqueous dispersion, the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃, and the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃ to a clean container. Add water at 700 rpm and stir for 5 min. Then add the first surfactant, the first defoamer, the substrate wetting agent, the leveling agent, the aqueous ultraviolet absorber, the aqueous color paste, and the first cosolvent in sequence, and continue stirring at 700 rpm for 13 min. Then add the neutralizing agent to adjust the pH of the system to 8.5. Finally, add the thickener and mix evenly to obtain component A.

[0155] S2, add the second co-solvent to the curing agent and mix evenly to obtain component B.

[0156] Example 2

[0157] This embodiment provides a water-based two-component polyurethane topcoat, composed of component A and component B in a mass ratio of 4.9:1.

[0158] Component A consists of the following raw material components by mass percentage: 40% hydroxyl-containing aqueous dispersion, 12% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃ (denoted as PUD with Tg = 40-50), 20% aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤ 5℃ (denoted as PUD with Tg ≤ 5), 0.2% first surfactant, 0.2% first defoamer, 0.2% substrate wetting agent, 0.1% leveling agent, 0.1% aqueous ultraviolet absorber, 18% aqueous color paste (of which, water 10%, dispersant 0.8%, second defoamer 0.2%, second surfactant 1%, organic pigment 5%, color paste stabilizer 0.9% and neutralizer 0.1%), 3% first cosolvent, 1% neutralizer, 0.2% thickener and 5% water.

[0159] Component B consists of the following raw material components by mass percentage: 73% curing agent (30% low viscosity hydrophobic hexamethylene diisocyanate, 25% high viscosity hydrophilic hexamethylene diisocyanate, and 18% hydrophilic modified isophorone diisocyanate) and 27% second cosolvent.

[0160] The specific components of the above raw materials are shown in Table 2. The calculated solid content of component A is 35.5%.

[0161] The preparation method of the above-mentioned water-based two-component polyurethane topcoat includes the following steps:

[0162] S1. Weigh each raw material according to the designed ratio. Add the hydroxyl-containing aqueous dispersion, the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃, and the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃ to a clean container. Add water at 800 rpm and stir for 4 min. Then add the first surfactant, the first defoamer, the substrate wetting agent, the leveling agent, the aqueous ultraviolet absorber, the aqueous color paste, and the first cosolvent in sequence, and continue stirring at 800 rpm for 10 min. Then add the neutralizing agent to adjust the pH of the system to 8.5. Finally, add the thickener and mix evenly to obtain component A.

[0163] S2 is the same as in Example 1, and will not be described again.

[0164] Example 3

[0165] This embodiment provides a water-based two-component polyurethane topcoat, composed of component A and component B in a mass ratio of 4.3:1.

[0166] Component A consists of the following raw material components by mass percentage: 45% hydroxyl-containing aqueous dispersion, 20% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion (denoted as PUD with Tg = 40-50) with a glass transition temperature of 40-50℃, 0.3% first surfactant, 0.2% first defoamer, 0.2% substrate wetting agent, 0.1% leveling agent, 0.1% aqueous ultraviolet absorber, 24.9% aqueous color paste (of which, water 5%, dispersant 1.4%, second defoamer 0.5%, second surfactant 0.2%, organic pigment 13%, color paste stabilizer 4.5% and neutralizer 0.3%), 3% first cosolvent, 1% neutralizer, 0.2% thickener and 5% water.

[0167] Component B consists of the following raw material components by mass percentage: 80% curing agent (28% low viscosity hydrophobic hexamethylene diisocyanate, 30% high viscosity hydrophilic hexamethylene diisocyanate and 22% hydrophilic modified isophorone diisocyanate) and 20% second cosolvent.

[0168] The specific components of the above raw materials are shown in Table 2. The calculated solid content of component A is 42.8%.

[0169] The preparation method of the above-mentioned water-based two-component polyurethane topcoat includes the following steps:

[0170] S1. Weigh each raw material according to the designed ratio. Add the hydroxyl-containing aqueous dispersion, the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃, and the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃ to a clean container. Add water at 600 rpm and stir for 6 min. Then add the first surfactant, the first defoamer, the substrate wetting agent, the leveling agent, the aqueous ultraviolet absorber, the aqueous color paste, and the first cosolvent in sequence, and continue stirring at 600 rpm for 15 min. Then add the neutralizing agent to adjust the pH of the system to 8. Finally, add the thickener and mix evenly to obtain component A.

[0171] S2 is the same as in Example 1, and will not be described again.

[0172] Example 4

[0173] This embodiment provides a water-based two-component polyurethane topcoat, composed of component A and component B in a mass ratio of 4.4:1.

[0174] Component A consists of the following raw material components by mass percentage: 43% hydroxyl-containing aqueous dispersion, 8% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃ (denoted as PUD with Tg=40-50), 8% aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃ (denoted as PUD with Tg≤5), 0.2% first surfactant, 0.2% first defoamer, 0.2% substrate wetting agent, 0.1% leveling agent, 0.1% aqueous ultraviolet absorber, 30.8% aqueous color paste (of which, water 8%, dispersant 1.5%, second defoamer 0.3%, second surfactant 0.3%, inorganic pigment 18%, color paste stabilizer 2.5% and neutralizer 0.2%), 3% first cosolvent, 1% neutralizer, 0.2% thickener and 5.2% water.

[0175] Component B consists of the following raw material components by mass percentage: 75% curing agent (55% high viscosity hydrophobic hexamethylene diisocyanate and 20% hydrophilic modified isophorone diisocyanate) and 25% second cosolvent.

[0176] The specific components of the above raw materials are shown in Table 2. The calculated solid content of component A is 44.9%.

[0177] The preparation method of the above-mentioned waterborne two-component polyurethane topcoat is the same as that in Example 1, and will not be repeated here.

[0178] Example 5

[0179] This embodiment provides a water-based two-component polyurethane topcoat, composed of component A and component B in a mass ratio of 5.6:1.

[0180] Component A consists of the following raw material components by mass percentage: 43% hydroxyl-containing aqueous dispersion, 6% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃ (denoted as PUD with Tg=40-50), 6% aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃ (denoted as PUD with Tg≤5), 0.2% first surfactant, 0.2% first defoamer, 0.2% substrate wetting agent, 0.1% leveling agent, 0.1% aqueous ultraviolet absorber, 32.4% aqueous color paste (of which, water 10%, dispersant 1.5%, second defoamer 0.2%, second surfactant 0.2%, inorganic pigment 15%, color paste stabilizer 5.4% and neutralizer 0.1%), 5.6% first cosolvent, 1% neutralizer, 0.2% thickener and 5% water.

[0181] Component B consists of the following raw material components by mass percentage: 75% curing agent (75% high viscosity hydrophilic hexamethylene diisocyanate) and 25% second cosolvent.

[0182] The specific components of the above raw materials are shown in Table 2. The calculated solid content of component A is 41.2%.

[0183] The preparation method of the above-mentioned waterborne two-component polyurethane topcoat is the same as that in Example 1, and will not be repeated here.

[0184] Example 6

[0185] This embodiment provides a water-based two-component polyurethane topcoat, composed of component A and component B in a mass ratio of 2.3:1.

[0186] Component A consists of the following raw material components by mass percentage: 43% hydroxyl-containing aqueous dispersion, 12% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion (denoted as PUD with Tg = 40-50) with a glass transition temperature of 40-50℃, 1% first surfactant, 1% first defoamer, 1% substrate wetting agent, 1% leveling agent, 1% aqueous ultraviolet absorber, 25.2% aqueous color paste (of which, water 7%, dispersant 1.2%, second defoamer 0.2%, second surfactant 0.2%, inorganic pigment 15%, color paste stabilizer 1.5% and neutralizer 0.1%), 3% first cosolvent, 3% neutralizer, 0.5% thickener and 8.3% water.

[0187] Component B consists of the following raw material components by mass percentage: 75% curing agent (75% hydrophilic modified isophorone diisocyanate) and 25% second cosolvent.

[0188] The specific components of the above raw materials are shown in Table 2. The calculated solid content of component A is 41.8%.

[0189] The preparation method of the above-mentioned waterborne two-component polyurethane topcoat is the same as that in Example 1, and will not be repeated here.

[0190] Table 2. Specific raw material components of water-based two-component polyurethane topcoats in Examples 1-6.

[0191]

[0192]

[0193]

[0194]

[0195] Comparative Example 1

[0196] This comparative example provides a waterborne two-component polyurethane topcoat, similar to Example 1, except that Covestro Bayhydrol A2470 is replaced with an equal mass of Covestro Bayhydol UH 2606 (i.e., the waterborne resin does not include hydroxyl-containing waterborne dispersions). The remaining components are the same as in Example 1 and will not be described again.

[0197] Comparative Example 2

[0198] This comparative example provides a waterborne two-component polyurethane topcoat, similar to Example 1, except that Covestro Bayhydol UH 2606 is replaced with an equal mass of Covestro Bayhydrol A 2470 (i.e., the waterborne resin does not contain a waterborne polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40–50°C). The remaining components are the same as in Example 1 and will not be described again.

[0199] Comparative Example 3

[0200] This comparative example provides a waterborne two-component polyurethane topcoat, similar to Example 1, except that Covestro Bayhydol UH 2606 and Bayhydrol UH 2952 / 1 are replaced with an equal mass of Covestro Bayhydrol A 2470 (i.e., the waterborne resin contains only hydroxyl-containing waterborne dispersions). The remaining components are the same as in Example 1 and will not be described again.

[0201] Comparative Example 4

[0202] This comparative example provides a waterborne two-component polyurethane topcoat, similar to Example 1, except that Covestro Bayhydrol A 2470 and Bayhydrol UH 2952 / 1 are replaced with an equal mass of Covestro Bayhydol UH 2606 (i.e., the waterborne resin contains only a waterborne polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50°C). The remaining components are the same as in Example 1 and will not be described again.

[0203] Comparative Example 5

[0204] This comparative example provides a waterborne two-component polyurethane topcoat, similar to Example 1, except that Covestro Bayhydol UH 2606 and Bayhydrol A 2470 are replaced with an equal mass of Covestro Bayhydrol UH2952 / 1 (i.e., the waterborne resin contains only a waterborne polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C). The remaining components are the same as in Example 1 and will not be described again.

[0205] Comparative Example 6

[0206] This comparative example provides a waterborne two-component polyurethane topcoat, similar to Example 1, except that the waterborne resins (Covestro Bayhydol UH 2606, Bayhydrol A 2470, and Bayhydrol UH 2952 / 1) are replaced with LianGu Uacryl 4219 (acrylic emulsion). The remaining components are the same as in Example 1 and will not be described again.

[0207] Comparative Example 7

[0208] This comparative example provides a water-based two-component polyurethane topcoat, similar to Example 1, except that Covestro Bayhydol UH 2606 and Bayhydrol UH 2952 / 1 are replaced with an equivalent mass of Zhanxin. 6803 (waterborne acrylic resin emulsion). The remaining components are the same as in Example 1, and will not be described again.

[0209] Application Example 1

[0210] This application example provides a method for using a water-based two-component polyurethane topcoat, including the following steps:

[0211] Mix components A and B from Example 1 thoroughly, add the activating diluent, and mix thoroughly to obtain the application coating. The mass ratio of component A to the activating diluent is 100:25.

[0212] The activating diluent is composed of the following raw material components by mass percentage: 10% hydroxyl-containing aqueous dispersion, 4% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃, 4% aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃, 29% aqueous synthetic flake silicate solution, 0.6% aqueous modified urea solution, 8% primary cosolvent, 8% stabilizer, 0.6% defoamer, 0.6% leveling agent, and 35.2% water.

[0213] The water-based synthetic sheet silicate solution is composed of the following raw materials by mass percentage: 2% organic modified synthetic layer silicate, 2% emulsifying dispersant, and 96% water.

[0214] It should be noted that the hydroxyl-containing aqueous dispersion, the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃, the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃, the first cosolvent, the first defoamer, and the leveling agent in the activating diluent are the same as the corresponding raw material components in component A of Example 1. The specific components of the remaining raw materials are shown in Table 3.

[0215] Application Example 2

[0216] This application example provides a method for using a water-based two-component polyurethane topcoat, including the following steps:

[0217] Mix components A and B from Example 2 thoroughly, add the activating diluent, and mix thoroughly to obtain the application coating. The mass ratio of component A to the activating diluent is 100:20.

[0218] The activating diluent is composed of the following raw material components by mass percentage: 11.5% hydroxyl-containing aqueous dispersion, 4.5% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃, 4.5% aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃, 33.5% aqueous synthetic flake silicate solution, 0.7% aqueous modified urea solution, 8.4% primary cosolvent, 8.5% stabilizer, 0.7% defoamer, 0.7% leveling agent, and 27% water.

[0219] The water-based synthetic sheet silicate solution is composed of the following raw materials in the indicated mass percentages: 1.5% organic-modified synthetic layer silicate, 1.5% emulsifying dispersant, and 97% water.

[0220] It should be noted that the hydroxyl-containing aqueous dispersion, the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃, the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃, the first cosolvent, the defoamer, and the leveling agent in the activating diluent are the same as the corresponding raw material components in component A of Example 2. The specific components of the remaining raw materials are shown in Table 3.

[0221] Application Example 3

[0222] This application example provides a method for using a water-based two-component polyurethane topcoat, including the following steps:

[0223] Mix components A and B from Example 3 thoroughly, add the activating diluent, and mix thoroughly to obtain the application coating. The mass ratio of component A to the activating diluent is 100:30.

[0224] The activating diluent is composed of the following raw material components by mass percentage: 8.1% hydroxyl-containing aqueous dispersion, 3.1% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃, 3.1% aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃, 27% aqueous synthetic flake silicate solution, 0.5% aqueous modified urea solution, 7.6% primary cosolvent, 7.6% stabilizer, 0.5% defoamer, 0.5% leveling agent, and 42% water.

[0225] The water-based synthetic sheet silicate solution is composed of the following raw materials in the indicated mass percentages: 2.5% organic-modified synthetic layer silicate, 2.5% emulsifying dispersant, and 95% water.

[0226] It should be noted that the hydroxyl-containing aqueous dispersion, the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃, the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃, the first cosolvent, the defoamer, and the leveling agent in the activating diluent are the same as the corresponding raw material components in component A of Example 3. The specific components of the remaining raw materials are shown in Table 3.

[0227] Application Example 4

[0228] This application example provides a method for using a water-based two-component polyurethane topcoat, including the following steps:

[0229] Mix components A and B from Example 4 thoroughly, add the activating diluent, and mix thoroughly to obtain the application coating. The mass ratio of component A to the activating diluent is 100:25.

[0230] The raw material ratios of the activating diluent and the water-based synthetic flake silicate solution are the same as in Application Example 1. The hydroxyl-containing aqueous dispersion, the water-based polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40–50°C, the water-based aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C, the first cosolvent, the defoamer, and the leveling agent in the activating diluent are the same as the corresponding raw material components in Component A of Example 4. The specific components of the remaining raw materials are shown in Table 3.

[0231] Application Example 5

[0232] This application example provides a method for using a water-based two-component polyurethane topcoat, including the following steps:

[0233] Mix components A and B from Example 5 thoroughly, add the activating diluent, and mix thoroughly to obtain the application coating. The mass ratio of component A to the activating diluent is 100:25.

[0234] The raw material ratios of the activating diluent and the water-based synthetic flake silicate solution are the same as in Application Example 1. The hydroxyl-containing aqueous dispersion, the water-based polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40–50°C, the water-based aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C, the first cosolvent, the defoamer, and the leveling agent in the activating diluent are the same as the corresponding raw material components in Component A of Example 5. The specific components of the remaining raw materials are shown in Table 3.

[0235] Application Example 6

[0236] This application example provides a method for using a water-based two-component polyurethane topcoat, including the following steps:

[0237] Mix components A and B from Example 6 thoroughly, add the activating diluent, and mix thoroughly to obtain the application coating. The mass ratio of component A to the activating diluent is 100:25.

[0238] The raw material ratio and components of the activating diluent are the same as in Application Example 1, and will not be repeated here.

[0239] Table 3 shows the specific raw material components of the activating diluents in Application Examples 1-5.

[0240]

[0241] Application Comparative Examples 1-7

[0242] Comparative Examples 1-7 each provide a method for applying a water-based two-component polyurethane topcoat, comprising the following steps:

[0243] Components A and B of Comparative Examples 1-7 were mixed evenly, and then an activating diluent was added and mixed evenly to obtain 7 sets of application coatings. The mass ratio of component A to activating diluent was 100:25. The raw material components and proportions of the activating diluent were the same as in Application Example 1, and will not be repeated here.

[0244] Application Comparative Example 8

[0245] This application provides a comparative example of a method for using a water-based two-component polyurethane topcoat, including the following steps:

[0246] Mix components A and B from Example 1 thoroughly, add water, and mix thoroughly to obtain the coating. The mass ratio of component A to water is 100:25.

[0247] Application Comparison Example 9

[0248] This comparative example provides a method for using a waterborne two-component polyurethane topcoat, similar to Application Example 1, except that the waterborne polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50°C in the activating diluent is replaced with an equal amount of hydroxyl-containing waterborne dispersion. All other conditions are the same as in Application Example 1 and will not be repeated here.

[0249] Application Comparison Example 10

[0250] This application provides a comparative example of a method for using a water-based two-component polyurethane topcoat, similar to Application Example 1, except that the water-based modified urea solution in the activating diluent is replaced with an equal amount of water-based synthetic flake silicate solution. All other conditions are the same as in Application Example 1 and will not be repeated here.

[0251] Performance testing

[0252] Performance tests were conducted on the construction coatings provided in Application Examples 1-6 and Comparative Examples 1-10.

[0253] (1) The tinplate was sanded with 400-500 grit sandpaper, and the surface oil was removed with ethanol. Then, the coatings provided in Examples 1-6 and Comparative Examples 1-10 were applied. For example, the coatings provided in Examples 1-2, 6 and Comparative Examples 1-4 were applied under high temperature and high humidity (38℃, RH=85%), and the coatings provided in Examples 3-5 and Comparative Examples 5-10 were applied under low temperature and high humidity (0℃, RH=85%). The film thickness was 25μm-35μm, and the film was baked at 60℃ for 2 hours. The color and appearance of the obtained test samples, drying time, impact resistance, bending test and cupping test were tested. The test standards are shown in Table 4, and the test results are shown in Tables 5-6.

[0254] (2) Apply WB07-1 intermediate coat paint (purchased from Beijing Yulin Chemical Co., Ltd.) to a steel plate coated with cathodic electrophoretic paint (CED, model EcoTa8500, purchased from Shanghai Jinlitai Chemical Co., Ltd.), with a film thickness of 55μm to 66μm, and bake at 60℃ for 2h; then apply the construction coatings provided in Application Examples 1 to 6 and Application Comparative Examples 1 to 10 on the WB07-1 intermediate coat paint, such as: applying the construction coatings provided in Application Examples 3 to 5 and Application Comparative Examples 1 to 4 under high temperature and high humidity (38℃, RH=85%), and applying the construction coatings provided in Application Examples 1 to 2, 6 and Application Comparative Examples 5 to 10 under low temperature and high humidity (0℃, RH=85%); spray 2 to 3 coats, flash dry for 3 to 5 minutes for each coat, and pre-bake at 60℃ for 10 minutes (the paint film is surface dry). The properties of the obtained test samples, including solvent resistance, water resistance, gasoline resistance, acid resistance, alkali resistance, and resistance to accelerated aging under artificial climate, were tested respectively. The test standards are shown in Table 4, and the test results are shown in Tables 5 and 6.

[0255] Table 4 shows the performance testing standards and requirements for the construction coatings provided in the application examples and comparative examples.

[0256]

[0257]

[0258]

[0259] Note: For any undated references, the latest version applies to this invention.

[0260] Table 5 shows the performance test results of the construction coatings provided in the application examples.

[0261]

[0262] In the table:

[0263] Thixotropic index: Measured using an NDJ-1 rotational viscometer; the viscosity of component A is adjusted by the amount of thickener used (ISO 6). # The viscosity was measured at low rotational speed (6 r / min) and high rotational speed (60 r / min) for about 70 seconds. The thixotropic index was calculated using the formula: Thixotropic index (DI) = low rotational viscosity / high rotational viscosity.

[0264] Appearance evaluation: 0 is "excellent", 1 is "qualified", and the larger the number, the worse the result. 0 - smooth and delicate; 1 - not delicate enough; 2 - slight orange peel; 3 - rough paint film with many pits; 4 - obvious orange peel, paint film runs, and dullness.

[0265] Workability: 0 is "Excellent", 1 is "Acceptable", and the higher the number, the worse the workability. 0 - Smooth and even paint film, no runs; 1 - Slight orange peel; 2 - Slight runs; 3 - Slight orange peel and slight runs; 4 - Fly dust, particles, poor flowability; 5 - Severe orange peel and severe runs.

[0266] Cross-cut test: 0 is "excellent", 1 is "qualified", and the larger the number is, the worse the adhesion. 0 - no sanding of the substrate, excellent adhesion; the rest are tested according to the cross-cut test standard GB / T9286—1998 "Cross-cut test of paint and varnish film".

[0267] Table 6 shows the performance test results of the application coatings provided in the comparative examples.

[0268]

[0269]

[0270] In summary, the waterborne two-component polyurethane topcoat provided by this invention uses a hydroxyl-containing waterborne dispersion and a high glass transition temperature waterborne polycarbonate-containing aliphatic anionic polyurethane dispersion as film-forming resin in component A. A low glass transition temperature waterborne aliphatic anionic polyurethane dispersion is selectively added. Through specific proportions and the synergistic effect of each component, the characteristics of different types of waterborne resins are brought to their optimal state, resulting in a waterborne two-component polyurethane topcoat with good thixotropic properties. The paint film exhibits excellent characteristics such as high gloss, high vividness, high fullness, and minimal orange peel texture. Furthermore, it does not produce fly ash during large-area spraying, has good spray gun application, and improved thixotropic properties. Simultaneously, it balances the flexibility and hardness of the cured film, allowing for large-area application in high-temperature, low-temperature, or high-humidity environments. This invention utilizes the synergistic effect of each raw material component and a specific ratio to achieve a base coat without sanding. The water-based two-component polyurethane topcoat of this invention can be directly sprayed on. After coating, no topcoat or clear varnish is required. The paint film has excellent gloss and reflectivity, comparable to the paint film appearance of a solvent-based 1K base coat + clear varnish coating system. It has high practical and promotional value.

[0271] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A water-based two-component polyurethane topcoat, characterized in that, It is composed of component A and component B in a mass ratio of 2:1 to 6.5:1; wherein component A comprises the following raw material components in mass percentage: 40% to 45% hydroxyl-containing aqueous dispersion, 5% to 20% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40 to 50°C, 0% to 20% aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C, 0.2% to 1% first surfactant, 0.2% to 1% first defoamer, 0.2% to 1% substrate wetting agent, 0.1% to 1% leveling agent, 0.1% to 1% aqueous ultraviolet absorber, and 15% to 4% aqueous color paste. The composition comprises: 5%, a first cosolvent of 3%–6%, a neutralizing agent of 1%–3%, a thickener of 0.2%–0.5%, and water of 5%–10%; the hydroxyl content of the hydroxyl-containing aqueous dispersion is 3.5%–5%, and the solid content of the hydroxyl-containing aqueous dispersion is 35%–50%; the aqueous color paste comprises water, a dispersant, a second defoamer, a second surfactant, a pigment, a color paste stabilizer, and a neutralizing agent; component B comprises the following raw material components by mass percentage: a curing agent of 70%–80% and a second cosolvent of 20%–30%; the curing agent is at least one of hexamethylene diisocyanate or hydrophilic aliphatic polyisocyanate.

2. The water-based two-component polyurethane topcoat as described in claim 1, characterized in that, The viscosity of the hydroxyl-containing aqueous dispersion is 100 mPa·s to 3000 mPa·s; the solid content of the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40 to 50°C and the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤ 5°C are both 33% to 40%.

3. The water-based two-component polyurethane topcoat as described in claim 1, characterized in that, The hydroxyl-containing aqueous dispersion is at least one of Covestro Bayhydrol A 2470, Covestro Bayhydrol A 2770, or Covestro Bayhydrol U XP 2766; the aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50°C is Covestro Bayhydol UH 2606; the aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C is at least one of Covestro Bayhydrol UH2648 / 1, Covestro Bayhydrol UH 2110, Covestro Bayhydrol UH 2621, or Covestro Bayhydrol UH2952 / 1.

4. The water-based two-component polyurethane topcoat as described in claim 1, characterized in that, Based on component A as 100%, the water-based color paste comprises 5%–10% water, 0.8%–1.4% dispersant, 0.2%–0.5% second defoamer, 0.2%–1% second surfactant, 5%–13% organic pigment, 0.9%–4.5% color paste stabilizer, and 0.1%–0.3% neutralizer; or based on component A as 100%, the water-based color paste comprises 7%–12% water, 1.2%–1.8% dispersant, 0.2%–0.5% second defoamer, 0.2%–0.5% second surfactant, 15%–25% inorganic pigment, 1.5%–5.4% color paste stabilizer, and 0.1%–0.3% neutralizer.

5. The water-based two-component polyurethane topcoat as described in claim 1, characterized in that, Based on component B as 100%, the curing agent comprises 25%–30% low-viscosity hydrophobic hexamethylene diisocyanate, 25%–30% high-viscosity hydrophilic hexamethylene diisocyanate, and 18%–22% hydrophilic modified isophorone diisocyanate; wherein the solid content of the hydrophilic aliphatic polyisocyanate is 60%–70%, and the -NCO content of the hydrophilic aliphatic polyisocyanate is 8.5%–12%.

6. The water-based two-component polyurethane topcoat as described in claim 5, characterized in that, The low-viscosity hydrophobic hexamethylene diisocyanate is at least one of Covestro Desmodur N3900, Covestro Desmodur N3600, or Wanhua WANNATE HT-600; the high-viscosity hydrophilic hexamethylene diisocyanate is at least one of Covestro Bayhydrol XP 2655, Covestro Bayhydur XP 2547, Covestro Bayhydur XP 2487 / 1, Covestro Bayhydur 304, or Covestro Bayhydur 305; and the hydrophilic aliphatic polyisocyanate is at least one of Covestro Bayhydur 401-70 or Covestro Bayhydur 401-60.

7. The water-based two-component polyurethane topcoat according to any one of claims 1 to 6, characterized in that, The first co-solvent comprises at least three of the following: propylene glycol methyl ether propionate, propylene glycol butyl ether, dipropylene glycol dimethyl ether, ethylene glycol butyl ether, dipropylene glycol methyl ether, dimethyl sulfoxide, N-methylpyrrolidone, dipropylene glycol butyl ether, or diethylene glycol monobutyl ether; the second co-solvent comprises at least two of the following: propylene glycol methyl ether acetate, propylene glycol methyl ether acrylate, propylene glycol diacetate, ethylene glycol butyl ether acetate, dipropylene glycol dimethyl ether, propylene carbonate, or divalent ester; the dispersant is at least one of a nonionic wetting and dispersing agent or anionic wetting and dispersing agent. The second surfactant is at least one of alkynyl glycol surfactant or alcohol alkoxy compound; the color paste stabilizer includes at least three of the following: aqueous fumed silica, modified urea rheology modifier, resin-free color paste humectant, or anti-drying agent for aqueous pigment concentrate; the neutralizing agent is an 8wt% to 12wt% aqueous solution of dimethylethanolamine; the first surfactant is a Gemini alkynyl glycol surfactant; the substrate wetting agent is a polyether-modified organosilicon surfactant; the aqueous ultraviolet absorber is at least one of BASF Tinuvin 5151, Zhonghao Chemical UV-DL2, Yongguang Chemical EVERSORBAQ1, Yongguang Chemical EVERSORB AQ2, or Yongguang Chemical EVERSORB AQ3; the thickener is a polyurethane associative thickener.

8. The application of the waterborne two-component polyurethane topcoat according to any one of claims 1 to 7 in automotive coatings.

9. The method of using the water-based two-component polyurethane topcoat according to any one of claims 1 to 7, characterized in that, The process includes the following steps: mixing component A and component B evenly, adding an activating diluent, mixing evenly to obtain a coating, and applying it evenly to a water-based base coat; the activating diluent includes a hydroxyl-containing water-based dispersion, a water-based polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40-50℃, a water-based aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5℃, a water-based synthetic flake silicate solution, a water-based modified urea solution, a first co-solvent, a stabilizer, a first defoamer, a leveling agent, and water.

10. The method of using the water-based two-component polyurethane topcoat as described in claim 9, characterized in that, The activating diluent comprises the following raw material components in the following mass percentages: 8%–11.5% hydroxyl-containing aqueous dispersion, 3%–4.5% aqueous polycarbonate-containing aliphatic anionic polyurethane dispersion with a glass transition temperature of 40–50°C, 3%–4.5% aqueous aliphatic anionic polyurethane dispersion with a glass transition temperature ≤5°C, 25%–33.5% aqueous synthetic flake silicate solution, 0.5%–0.7% aqueous modified urea solution, 7.5%–8.5% primary cosolvent, 7.5%–8.5% stabilizer, 0.5%–0.7% primary defoamer, and 0.5%–0.7% leveling agent. The aqueous synthetic flake silicate solution comprises 27%–42% water, with the sum of all components being 100%; the mass ratio of component A to the activating diluent is 100:(20–30); the aqueous synthetic flake silicate solution comprises the following raw material components by mass percentage: 1.5%–2.5% organic modified synthetic flake silicate, 1.5%–2.5% emulsifying dispersant, and 95%–97% water, with the sum of all components being 100%; the aqueous modified urea solution is at least one of BYK RHEOBYK-7420CA, BYK RHEOBYK-7420ET, or BYK RHEOBYK-7420ES.

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