Water-based baking varnish as well as preparation method and application thereof

Abstract

The invention relates to the technical field of coating and surface engineering, and discloses a water-based baking varnish and a preparation method and application thereof.The water-based baking varnish is prepared from hydroxyl acrylic acid dispersoid, water-based amino resin, water-based aluminum powder, a silane coupling agent, an amine closed latent acid catalyst, anti-settling slurry and the like; the preparation method comprises the following steps: grafting a silane coupling agent on the surface of water-based aluminum powder under a low-speed stirring condition to prepare modified aluminum paste; carrying out high-speed shearing dispersion on the hydroxyl acrylic dispersion, the water-based amino resin and the anti-settling slurry to prepare base paint; adding the modified aluminum paste into the base paint, uniformly mixing at a low speed, and adding a catalyst and an auxiliary agent for blending; the water-based baking varnish is mainly applied to surface coating of automobile hubs and electric vehicle hubs. Through pre-grafting of the silane coupling agent and cooperation of a latent catalytic system, aluminum powder hydrogen evolution is inhibited, a stable rheological structure is constructed, and the curing crosslinking density and the appearance effect of a paint film are improved while the storage stability is guaranteed.

Description

Technical Field

[0001] This invention relates to the field of coatings and surface engineering technology, specifically to a water-based baking paint, its preparation method, and its application. Background Technology

[0002] Water-based baking paint, as an environmentally friendly industrial coating, uses water as the main solvent and features low volatile organic compound emissions and high safety. It is widely used in the surface coating of automotive parts, machinery and equipment and electronic products. Among them, water-based metallic baking paint can form a unique metallic shimmer and angle-dependent color effect on the coating surface by introducing metallic pigments such as flake aluminum powder into the resin matrix. It is often used on the surface of workpieces such as automobile wheels and electric vehicle wheels, which have high requirements for both appearance decoration and protection.

[0003] Existing water-based metal baking paint technology typically uses hydroxyl acrylic dispersion combined with amino resin as film-forming material, and adds water-based aluminum powder paste during the preparation process. Such coatings need to be baked at high temperature after application to promote cross-linking reaction between resin functional groups and thus solidify into a film. In order to ensure the basic performance of the coating, physical mixing is often used in industrial production to disperse the resin, pigments, fillers and conventional additives evenly, which can obtain a protective coating with certain hardness and adhesion.

[0004] However, due to the high chemical reactivity of aluminum powder, direct exposure to aqueous media can easily lead to chemical reactions with water molecules and the release of hydrogen gas, causing the coating to swell during storage. This not only affects the storage stability of the product but also poses a safety hazard of packaging container bursting. At the same time, because existing systems often lack effective chemical modification and precise rheological control of the aluminum powder surface, the flake aluminum powder is prone to disordered turning or agglomeration in the paint solution. It cannot maintain a parallel orientation to the substrate surface during film formation, resulting in a weak metallic texture in the final cured paint film. Furthermore, defects caused by aluminum powder corrosion make it difficult for the coating's water resistance and salt spray resistance to meet the high standards of wheel hub coating. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a water-based baking paint, its preparation method, and its application, aiming to solve the problems of poor storage stability caused by the easy reaction of aluminum powder with water to produce hydrogen gas and the insufficient appearance, texture, and protective performance of the coating due to the poor orientation of aluminum powder in water-based metal baking paints.

[0006] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of this invention provides a water-based baking paint, comprising the following components in parts by weight: Hydroxyacrylic acid dispersion 60.0-75.0 parts; 10.0-15.0 parts of water-based amino resin; 10.0-15.0 parts of water-based aluminum powder; 0.2-0.5 parts of silane coupling agent; 0.5-1.5 parts of amine-blocked latent acid catalyst; Anti-settling slurry: 5.0-8.0 parts.

[0007] By adopting the above technical solution, the present invention improves coating performance by utilizing the interaction between components, as detailed below: The silanol groups generated from the hydrolysis of the silane coupling agent undergo a condensation reaction with the hydroxyl groups on the surface of the water-based aluminum powder, forming dense Al-O-Si covalent bonds. This process forms a hydrophobic layer on the aluminum powder surface, preventing water molecules from directly contacting the metallic aluminum and inhibiting the hydrogen evolution reaction between the aluminum powder and water, thus solving the gas expansion problem during the storage of water-based metallic paints. The organic functional groups of the silane coupling agent chemically bond with the resin matrix, enhancing the adhesion between the pigment and the resin interface.

[0008] The hydroxyl acrylic dispersion forms the main body of the paint film, providing adhesion and weather resistance; the waterborne amino resin, as a crosslinking agent, reacts with the hydroxyl acrylic resin to form a three-dimensional network structure, improving the hardness and water resistance of the paint film.

[0009] The anti-settling slurry system has a thixotropic structure, which prevents aluminum powder from settling during static storage, reduces viscosity during spraying and shearing to facilitate leveling, restricts aluminum powder tumbling during film formation, and promotes the oriented alignment of sheet-like aluminum powder parallel to the substrate surface to obtain a high metallic luster.

[0010] Preferably, the silane coupling agent is γ-glycidyl etheroxypropyltrimethoxysilane.

[0011] By adopting the above technical solution, the epoxy group in the γ-glycidyl etheroxypropyltrimethoxysilane molecule has good compatibility with the hydroxy acrylic acid system and participates in the ring-opening crosslinking reaction during the baking process, thereby improving the density and chemical resistance of the paint film.

[0012] Preferably, the preparation method of the amine-blocked latent acid catalyst includes: dissolving p-toluenesulfonic acid monohydrate in a solvent, and adding 2-amino-2-methyl-1-propanol dropwise at a temperature of 15-35℃ for neutralization reaction until the pH of the system reaches 7.5-8.0. By adopting the above technical solution, 2-amino-2-methyl-1-propanol is used to neutralize p-toluenesulfonic acid to form an amine salt. This catalyst is chemically stable at room temperature, avoiding pre-crosslinking of the paint during storage and extending its pot life. During the high-temperature baking stage, the amine salt thermally dissociates, releasing sulfonic acid groups to catalyze the de-alcoholization crosslinking of amino resin and acrylic resin, balancing low-temperature storage stability and high-temperature curing efficiency.

[0013] Preferably, the hydroxyl acrylic dispersion is a styrene-acrylate copolymer aqueous dispersion; the aqueous amino resin is fully methyl etherified hexamethoxymethyl melamine. By adopting the above technical solution, the styrene-acrylate copolymer provides film hardness and flexibility; the fully methyl etherified hexamethoxymethyl melamine has high functionality and low self-condensation tendency, providing high crosslinking density and improving the water resistance of the coating film.

[0014] Preferably, the water-based baking paint also contains 1.0-4.0 parts by weight of additives, which consist of wetting agents, dispersants, leveling agents and defoamers.

[0015] By adopting the above technical solution, compounded additives improve the surface tension of the system, reduce surface defects, and enhance the smoothness of the paint film appearance.

[0016] A second aspect of this invention provides a method for preparing a water-based baking paint, comprising the following steps: S1. Preparation of modified aluminum powder slurry: Water-based aluminum powder, silane coupling agent, co-solvent and wetting and dispersing agent are mixed under low speed stirring, so that the silane coupling agent is grafted onto the surface of water-based aluminum powder. S2. Preparation of base paint: High-speed shear dispersion of hydroxy acrylic dispersion, waterborne amino resin, anti-settling agent and defoamer; S3. Paint Mixing: Add the modified aluminum powder slurry obtained in step S1 to the base paint obtained in step S2, stir at low speed until uniform, and then add amine-blocked latent acid catalyst, wetting agent, dispersant, leveling agent, thickener and deionized water to obtain water-based baking paint.

[0017] By adopting the above technical solution, the preparation process of this invention has the following characteristics: Step S1 prioritizes mixing the silane coupling agent with water-based aluminum powder in a co-solvent environment, allowing the silane coupling agent to be fully adsorbed and grafted onto the surface of the aluminum powder. This completes the surface passivation modification before the aluminum powder enters the aqueous resin system, reducing hydrogen evolution side reactions.

[0018] Step S2 uses high-speed shearing to break up resin and prevent sediment agglomeration, and activates the thixotropic properties of the rheology modifier. Steps S1 and S3 are controlled with low-speed stirring to avoid mechanical shearing that could damage the flake-like aluminum powder structure, prevent deformation or breakage of the aluminum powder, and preserve the angle-dependent color optical effect of the aluminum powder.

[0019] Preferably, in step S1, the low-speed stirring speed is 400-500 rpm, and the stirring time is 30-40 minutes.

[0020] By adopting the above technical solution, the rotation speed and time ensure that the silane coupling agent is uniformly coated on the surface of aluminum powder, avoiding excessive stirring that could lead to solvent evaporation or damage to the aluminum powder.

[0021] Preferably, in step S2, the rotation speed of the high-speed shear dispersion is 1000-1500 rpm; in step S3, the rotation speed of the low-speed stirring is 300-500 rpm.

[0022] By adopting the above technical solution, different rotation speed settings balance the dispersion efficiency of the base material and the protection of the pigment structure.

[0023] The third aspect of the present invention provides an application of a water-based baking paint, specifically its application in the surface coating of automobile wheel hubs and electric vehicle wheel hubs.

[0024] By adopting the above technical solution, this water-based paint is suitable for complex-shaped workpieces such as wheel hubs. The orientation capability of aluminum powder gives the wheel hub a metallic textured appearance, while the cross-linked network provides water resistance and corrosion resistance against rain and gravel impacts.

[0025] This invention provides a water-based baking paint, its preparation method, and its application. It has the following beneficial effects: 1. This invention forms a chemical bond layer on the surface of aluminum powder by grafting a silane coupling agent with water-based aluminum powder. This helps to block the direct contact between water molecules and metallic aluminum, reducing the risk of aluminum powder reacting with water. This helps to improve the gas generation phenomenon of water-based metallic paint during storage and helps to improve the storage stability of the product.

[0026] 2. This invention utilizes an amine-blocked latent acid catalyst, which remains relatively inert at room temperature. This helps to delay the pre-crosslinking reaction of the paint during storage, extend the pot life, and promote the crosslinking and curing of amino resin and acrylic resin. Thus, it balances the low-temperature storage stability and high-temperature curing efficiency of the paint to a certain extent.

[0027] 3. This invention constructs a rheological system by preventing sedimentation and combines it with a graded stirring process. On the one hand, it uses high-speed dispersion to establish a rheological network, and on the other hand, it uses low-speed stirring in the mixing stage, which helps to maintain the structural integrity of the flake aluminum powder. This is beneficial to promote the aluminum powder to align parallel to the substrate surface during film formation, improve the metallic luster and angle-dependent color effect of the coating, and thus make it more suitable for surface coating of automotive and electric vehicle wheel hubs. Detailed Implementation

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0030] Preparation Examples 1-4: Preparation Example 1: This preparation example provides an amine-blocked latent acid catalyst, and the preparation steps are as follows: Add 30.0 parts of deionized water and 20.0 parts of isopropanol to a reaction vessel equipped with a condenser and a stirrer, and start stirring to mix the deionized water and isopropanol evenly; slowly add 20.0 parts of p-toluenesulfonic acid monohydrate, and continue stirring until the solid is completely dissolved; under ice-water bath cooling conditions, slowly add 2-amino-2-methyl-1-propanol, controlling the dropping rate to maintain the temperature of the reaction system below 35°C; continuously monitor the pH value of the reaction system, and stop the dropping when the pH value reaches 7.5 to 8.0; continue stirring for 30 minutes to obtain an amine-blocked latent acid catalyst that is a colorless to slightly yellow transparent liquid with a solid content of 35% to 40%.

[0031] Preparation Example 2: This preparation example provides a modified aluminum powder paste A, and the preparation steps are as follows: Add 5.0 parts of propylene glycol monomethyl ether, 0.2 parts of polyether-modified siloxane wetting agent, and 0.5 parts of polymer block copolymer dispersant to a dispersion tank, and stir at 400 rpm for 5 minutes; slowly add 10.0 parts of water-based aluminum powder, and maintain low-speed stirring at 400 rpm for 10 minutes to avoid high shear damaging the aluminum powder coating layer; dropwise add 0.2 parts of γ-glycidyl ether oxypropyltrimethoxysilane, and continue stirring at low speed for 30 minutes at room temperature of 25°C, using the water in the system to hydrolyze the silane and graft it onto the surface of the aluminum powder to obtain modified aluminum powder slurry A.

[0032] Preparation Example 3: This preparation example provides a modified aluminum powder paste B, and the preparation steps are as follows: Add 7.5 parts of propylene glycol monomethyl ether, 0.3 parts of polyether-modified siloxane wetting agent, and 0.7 parts of polymer block copolymer dispersant to a dispersion tank, and stir at 400 rpm for 5 minutes; slowly add 12.5 parts of water-based aluminum powder, and maintain low-speed stirring at 400 rpm for 10 minutes; dropwise add 0.35 parts of γ-glycidyl ether oxypropyltrimethoxysilane, and continue low-speed stirring at room temperature of 25°C for 30 minutes to obtain modified aluminum powder slurry B.

[0033] Preparation Example 4: This preparation example provides a modified aluminum powder paste C, and the preparation steps are as follows: Add 10.0 parts of propylene glycol monomethyl ether co-solvent, 0.4 parts of polyether modified siloxane wetting agent, and 1.0 part of polymer block copolymer dispersant to a dispersion tank, and stir at 500 rpm for 5 minutes; slowly add 15.0 parts of water-based aluminum powder, and keep stirring at a low speed of 500 rpm for 10 minutes; add 0.5 parts of γ-glycidyl ether oxypropyltrimethoxysilane dropwise, and continue stirring at a low speed for 30 minutes at room temperature of 25°C to obtain modified aluminum powder slurry C.

[0034] Examples 1-3: Example 1: This embodiment provides a method for preparing water-based baking paint, including the following steps: (1) Preparation of materials: Prepare 60.0 parts of hydroxyl acrylic dispersion, 10.0 parts of waterborne amino resin, 15.9 parts of modified aluminum powder slurry A (product of preparation example 2), 0.5 parts of amine-blocked latent acid catalyst (product of preparation example 1), 5.0 parts of anti-settling slurry, 0.2 parts of wetting agent, 0.5 parts of dispersant, 0.3 parts of leveling agent, 0.2 parts of defoamer, 0.1 parts of thickener and appropriate amount of deionized water; (2) Resin dispersion: Add hydroxy acrylic dispersion and water-based amino resin to the dispersion tank, mix evenly, add anti-settling slurry and defoamer, disperse at high speed of 1000 rpm for 10 minutes, and control the fineness to be less than 15 μm; (3) Paint preparation: Reduce the speed of the dispersion tank to 300 rpm, add modified aluminum powder slurry A, and stir to disperse evenly; then add wetting agent, dispersant, leveling agent and amine-blocked latent acid catalyst in sequence, and continue stirring for 10 minutes; (4) Viscosity adjustment: Add thickener and deionized water, adjust the viscosity of the Forte 4 cup to 30 seconds, filter with a 200-mesh filter to obtain water-based baking paint; (5) Coating curing: Spray water-based baking paint onto the surface of the pretreated aluminum alloy sheet, flash dry at room temperature of 25°C for 5 minutes, and then place it in an oven and bake at 170°C for 30 minutes to form a film.

[0035] Example 2: This embodiment provides a method for preparing water-based baking paint, including the following steps: (1) Preparation of materials: Prepare 67.5 parts of hydroxyl acrylic acid dispersion, 12.5 parts of waterborne amino resin, 21.35 parts of modified aluminum powder paste B (product of preparation example 3), 1.0 part of amine-blocked latent acid catalyst (product of preparation example 1), 6.5 parts of anti-settling paste, 0.35 parts of wetting agent, 0.75 parts of dispersant, 0.55 parts of leveling agent, 0.35 parts of defoamer, 0.3 parts of thickener and appropriate amount of deionized water; (2) Resin dispersion: Add hydroxyl acrylic dispersion and water-based amino resin to the dispersion tank, mix evenly, add anti-settling slurry and defoamer, disperse at high speed of 1200 rpm for 15 minutes, and control the fineness to be less than 15 μm; (3) Paint mixing: Reduce the speed of the dispersion tank to 400 rpm, add modified aluminum powder paste B, and stir to disperse evenly; then add wetting agent, dispersant, leveling agent and amine-blocked latent acid catalyst in sequence, and continue stirring for 12 minutes; (4) Viscosity adjustment: Add thickener and deionized water, adjust the viscosity of the Forte 4 cup to 35 seconds, filter with a 200-mesh filter to obtain water-based baking paint; (5) Coating and curing: Spray water-based baking paint onto the surface of the pretreated aluminum alloy sheet, flash dry at room temperature of 25°C for 8 minutes, and then place it in an oven and bake at 175°C for 30 minutes to form a film.

[0036] Example 3: This embodiment provides a method for preparing water-based baking paint, including the following steps: (1) Preparation of materials: Prepare 75.0 parts of hydroxyl acrylic acid dispersion, 15.0 parts of waterborne amino resin, 26.9 parts of modified aluminum powder slurry C (product of preparation example 4), 1.5 parts of amine-blocked latent acid catalyst (product of preparation example 1), 8.0 parts of anti-settling slurry, 0.5 parts of wetting agent, 1.0 part of dispersant, 0.8 parts of leveling agent, 0.5 parts of defoamer, 0.5 parts of thickener and appropriate amount of deionized water; (2) Resin dispersion: Add hydroxyl acrylic dispersion and water-based amino resin to the dispersion tank, mix evenly, add anti-settling slurry and defoamer, disperse at high speed of 1500 rpm for 20 minutes, and control the fineness to be less than 15 μm; (3) Paint mixing: Reduce the speed of the dispersion tank to 500 rpm, add modified aluminum powder slurry C, and stir to disperse evenly; then add wetting agent, dispersant, leveling agent and amine-blocked latent acid catalyst in sequence, and continue stirring for 15 minutes; (4) Viscosity adjustment: Add thickener and deionized water, adjust the viscosity of the Forte 4 cup to 40 seconds, filter with a 200-mesh filter to obtain water-based baking paint; (5) Coating and curing: Spray water-based baking paint onto the surface of the pretreated aluminum alloy sheet, flash dry at room temperature of 25°C for 10 minutes, and then place it in an oven and bake at 180°C for 30 minutes to form a film.

[0037] Comparative Examples 1-4: Comparative Example 1: The difference compared to Example 1 is that no silane coupling agent was used for modification.

[0038] Specifically, an equal amount of unmodified commercially available water-based aluminum powder was used to replace modified aluminum powder slurry B, and γ-glycidyl ether oxypropyltrimethoxysilane (GPTMS) was not added to the formulation. The insufficient weight was made up by deionized water, and the other raw material types, amounts and preparation processes were the same as in Example 1.

[0039] Comparative Example 2: Compared with Example 1, the difference is that the way aluminum powder is added is changed and pre-dispersion grafting is not performed.

[0040] Specifically, instead of using the modified aluminum powder paste B from Preparation Example 3, an equal amount of commercially available water-based aluminum powder and corresponding proportions of γ-glycidyl etheroxypropyltrimethoxysilane, cosolvent, dispersant, and wetting agent were added directly in the paint mixing step (step 3), with the rest being the same as in Example 1.

[0041] Comparative Example 3: Compared with Example 1, the difference lies in the change of catalyst type and curing process.

[0042] Specifically, the amine-blocked latent acid catalyst was replaced with an equimolar amount of unblocked p-toluenesulfonic acid aqueous solution; and the baking temperature in the coating curing step was adjusted from 175°C to 140°C (holding for 30 minutes), with the rest being the same as in Example 1.

[0043] Comparative Example 4: The difference compared to Example 1 is that the amount of anti-settling slurry is adjusted.

[0044] Specifically, the amount of anti-settling slurry was reduced from 6.0 parts to 2.0 parts, while the rest remained the same as in Example 1.

[0045] Test Examples 1-4 Test Example 1: Thermal Storage Stability Test Test Method: The thermal storage stability of the coating samples prepared in the examples and comparative examples was tested using the isothermal accelerated aging method. The test was conducted according to standard methods, and the specific steps are as follows: First, the liquid coating samples prepared in Examples 1 to 5 and Comparative Examples 1 to 4 were placed into 500 ml inner-coated yellow tin cans, filling them to approximately 85% of their volume, and the can lids were sealed. Before the test, all samples were placed in a water bath at 25±1℃ for 1 hour, and the initial viscosity of each sample was measured using a Forecast-4 cup viscometer, and the data were recorded.

[0046] The sealed sample was placed in a constant temperature drying oven set at 50±2℃ and stored for 336 hours.

[0047] After the storage period expires, remove the sample and place it at 25℃ for 24 hours to allow it to return to room temperature. After opening the can, visually inspect the sample surface for crusting or water separation, and observe whether the can is bulging or deformed. Stir manually with a paint mixing knife and check for hard sediment at the bottom.

[0048] For the ungelled sample, after thorough mixing, its viscosity after storage was measured again at 25°C. The viscosity growth rate was calculated using the formula: the viscosity growth rate equals the difference between the storage viscosity and the initial viscosity, divided by the initial viscosity, and then multiplied by 100%.

[0049] Test results: The thermal storage stability test results of Examples 1 to 5 and Comparative Examples 1 to 4 are shown in Table 1.

[0050] Table 1 shows the test results of thermal storage stability and viscosity change at 50℃ for 14 days. Conclusion: The test results above show that after 14 days of high-temperature storage at 50°C, the viscosity growth rate of Examples 1 to 5 was controlled within 10%, and no gelation, gas production or hard precipitation occurred, confirming that the water-based baking paint system has good chemical and physical stability.

[0051] In Comparative Example 1, no silane coupling agent was added to the formulation. The active sites on the surface of the aluminum powder were directly exposed to the aqueous medium. Water molecules attacked the aluminum metal and released hydrogen gas, causing the can to swell. At the same time, the generated aluminum hydroxide flocculents destroyed the colloidal stability of the resin, resulting in the overall gelation of the system.

[0052] Although Comparative Example 2 added an equal amount of silane coupling agent, it used a direct addition method without a pre-dispersion grafting process. Data showed that its viscosity increased by as much as 61.31%, indicating that under the competitive adsorption environment of multiple components, silane molecules failed to effectively cover the aluminum powder surface to form a dense protective layer. Some silanes underwent self-condensation or reacted with resin functional groups in the aqueous phase, resulting in insufficient protection of the aluminum powder and an abnormally increased crosslinking density of the system.

[0053] Comparative Example 3 used unblocked p-toluenesulfonic acid as a catalyst. The free strong acid accelerated the polycondensation reaction of the hydroxyl and amino resins at 50°C, leading to a sharp increase in molecular weight and a viscosity increase rate exceeding 160%. Simultaneously, the free acid damaged the passivation layer on the aluminum powder surface, triggering a reaction in the molten aluminum that generated bubbles. Example 1 used an amine-blocked latent acid catalyst. At a storage temperature of 50°C, the catalyst remained in a blocked latent state, preventing early cross-linking of the resin and acid etching of the aluminum powder.

[0054] Although the viscosity data of Comparative Example 4 showed little change, undispersible hard clumps appeared at the bottom. This was because the amount of anti-settling slurry was too low, failing to build sufficient thixotropic network yield value to support the high-density aluminum powder particles, leading to irreversible pigment sedimentation under gravity. Examples 1 to 3, through reasonable anti-settling slurry formulations, achieved a balance between the system's rheological properties and the pigment suspension requirements.

[0055] Test Example 2: Physical and Mechanical Properties and Water Resistance Tests of Coating Film Test methods: The film performance of the coatings in the examples and comparative examples was tested using standard board preparation processes and instrumental analysis. Phosphated aluminum alloy plates were selected as the substrate. The coating samples of Examples 1 to 5 and Comparative Examples 1 to 4 were adjusted to the application viscosity and automatically sprayed using an air spray gun, with the dry film thickness controlled at 15-20 μm.

[0056] After spraying, the samples of Examples 1 to 5 and Comparative Examples 1, 2 and 4 were flash-dried at room temperature of 25°C for 8 minutes and then baked in an oven at 175°C for 30 minutes. The sample of Comparative Example 3 was flash-dried at room temperature of 25°C for 8 minutes and then baked in an oven at 140°C for 30 minutes. After the samples cooled and were adjusted for 24 hours, various performance tests were conducted.

[0057] Adhesion: According to GB / T9286-1998 standard, use a cross-cutting tool to make 1mm intervals, apply special tape and tear it at 60°, and rate the grade (0 is the best and 5 is the worst).

[0058] Pencil hardness: According to GB / T6739-2006 standard, the highest hardness that does not scratch the paint film is recorded when Mitsubishi pencils are pushed under a load of 750g.

[0059] Water resistance (instrumental characterization): Water resistance tests were conducted according to GB / T1733-1993 standard. Before testing, the initial gloss (G0) of the sample at 60° was measured using a BYK miniature gloss meter, and the initial colorimetric value was measured using a spectrophotometer. The edge-sealed sample was immersed in distilled water at 40±2℃ for 240 hours, then removed, the surface moisture was blotted out, and allowed to recover for 1 hour. The gloss (G1) and colorimetric value after water resistance were measured again using the above instruments, and the gloss retention rate (G1 / G0×100%) and color difference value were calculated. The lower the gloss retention rate, The larger the value, the more severe the microscopic deformation or whitening of the paint film due to water erosion. Adhesion was then retested after water resistance testing.

[0060] Test results: The test results of the physical and mechanical properties and water resistance of the coatings of Examples 1 to 5 and Comparative Examples 1 to 4 are shown in Table 2.

[0061] Table 2 shows the test results of the physical and mechanical properties and water resistance of the paint film: Conclusion: After 240 hours of water immersion, the gloss retention rate of the paint films prepared in Examples 1 to 5 remained above 96%, and the color difference was minimal. With a viscosity less than 0.6, adhesion did not decrease after water immersion. Data shows that the coating film maintained excellent surface optical integrity and interfacial bonding under long-term water immersion conditions.

[0062] The gloss retention rate of Comparative Example 1 dropped significantly to 64.3%, and the color difference... The value was as high as 8.24, indicating that in the absence of a silane coupling agent, water molecules penetrated into the paint film, causing microscopic separation at the resin-aluminum powder interface. This resulted in the formation of numerous tiny light scattering centers (microbubbles or whitening), leading to a sharp reduction in gloss and a change in hue. Simultaneously, moisture accumulation on the aluminum powder surface caused interfacial failure, and the adhesion dropped to level 4 after water resistance was achieved.

[0063] Comparative Example 2 showed a gloss retention rate of 81.5% and a color difference of... The value was 3.67, and all indicators were inferior to the example. Direct addition of silane coupling agent failed to achieve dense coating on the aluminum powder surface, and some unmodified hydrophilic surfaces of the aluminum powder became channels for moisture penetration. Moisture intrusion caused local swelling of the paint film, resulting in decreased surface smoothness (reduced gloss) and changes in optical properties.

[0064] Comparative Example 3 had the worst performance across all categories, with a gloss retention rate of only 42.1% and color difference. The viscosity reached as high as 15.89 (severe whitening). This is due to low-temperature curing and an unsealed catalyst, resulting in excessively low crosslinking density of the paint film and a loose polymer network structure. A large amount of water molecules penetrated and swelled inside the paint film, destroying its density and leading to severe light scattering and complete loss of adhesion.

[0065] Although Comparative Example 4 exhibited normal chemical cross-linking and good adhesion after water resistance, its gloss retention rate was slightly lower than that of Example 1 (92.4%). The result is too high. This confirms the sedimentation conclusion in Test Example 1: insufficient anti-settling agent leads to uneven pigment distribution, poor microscopic smoothness of the paint film surface, and makes it easier for microscopic defects to form around the raised pigment particles during water immersion, affecting optical stability.

[0066] Test Example 3: Coating Film Appearance and Optical Anisotropy Test Test method: The aluminum alloy sample prepared in Test Example 2 was selected as the test object. The chromaticity performance of the paint film with respect to angle was measured by a multi-angle spectrophotometer. The brightness values ​​at a near mirror angle of 15° and a far mirror angle of 110° were recorded. The metal scintillation index and chromaticity index with respect to angle were also recorded. The paint film surface was scanned by an orange peel analyzer to measure the long-wavelength and short-wavelength values ​​and calculate the sharpness value. Three different positions were selected for measurement of each sample and the arithmetic mean was taken.

[0067] Test results: The test results of the coating appearance and optical anisotropy of Examples 1 to 5 and Comparative Examples 1 to 4 are shown in Table 3.

[0068] Table 3 shows the test results for the appearance and optical anisotropy of the paint film: Conclusion: The angle-dependent colorimetric index values ​​of Examples 1 to 5 remained stable between 13.8 and 15.1, and the sharpness values ​​were all greater than 86. The high brightness value at a 15-degree angle on the front and the low brightness value at a 110-degree angle on the side indicate that the aluminum powder achieved a highly parallel and oriented arrangement within the paint film. The incident light underwent strong specular reflection on the front and scattered absorption on the side, thus creating the light-dark transition effect.

[0069] Comparative Example 1 showed a color difference index of only 8.3, with a decrease in luminance at a 15-degree angle from the front and a sharpness value dropping to 72.4. The absence of a silane coupling agent in the formulation led to a mismatch between the surface energy of the aluminum powder and the resin matrix, causing the aluminum powder to agglomerate during film formation. The agglomerated aluminum powder particles were in a disordered stacking state, disrupting the parallel orientation structure, increasing the diffuse reflection ratio of light, and the micro-agglomerates also reduced surface smoothness.

[0070] Comparative Example 2 has an angle-dependent heterochromaticity index of 10.6, which falls between that of Example 1 and Comparative Example 1. The direct addition method resulted in low silane modification efficiency, poor dispersion of some aluminum powder, and microscopic flocculation. This microscopic arrangement disorder reduced the directional reflection of light, confirming the necessity of pre-dispersion grafting for improving the optical properties of metals.

[0071] Comparative Example 3 had the lowest sharpness value, at only 65.2. Low-temperature curing and low cross-linking density resulted in uneven distribution of shrinkage stress in the paint film during baking, and the solvent evaporation rate did not match the resin curing rate, leading to microscopic defects on the paint film surface. This interfered with specular reflection of light, resulting in decreased image clarity.

[0072] Comparative Example 4 showed a significant decrease in the angular heterochromatic index to 9.1, but the sharpness index remained acceptable. Insufficient anti-settling agent resulted in an excessively low wet film yield value, failing to withstand the effects of gravity during flash-drying and the initial stages of baking. Under the influence of gravity, the aluminum powder flipped or tilted, leading to a chaotic arrangement of the cured aluminum powder. Although the resin leveling properties were less affected, the disordered aluminum powder orientation directly resulted in a certain degree of loss of metallic appearance, indicating that the precise formulation of the thixotropic agent directly affects the directional arrangement of the flake pigments.

[0073] Test Example 4: Salt Spray Corrosion Resistance Test Test method: The corrosion resistance of the coating was evaluated using the neutral salt spray test. The aluminum alloy sample prepared in Test Example 2 was selected. Before testing, a sharp scratching tool was used to make intersecting cuts on the sample surface, the depth of which needed to penetrate the coating and reach the metal substrate.

[0074] The treated samples were placed in a salt spray test chamber and subjected to continuous spray testing according to GB / T1771-2007 standard. The test conditions were set as follows: sodium chloride solution mass concentration 5%±0.5%, chamber temperature 35±2℃, and pH value 6.5 to 7.2.

[0075] After 1000 hours of continuous testing, the sample was removed, rinsed with clean water to remove surface salt, and dried. The blistering level of the sample surface was evaluated according to GB / T1740-2007 standard, and the maximum width of corrosion spread on one side of the scratch was measured using calipers.

[0076] Test results: The test results of salt spray corrosion resistance of Examples 1 to 5 and Comparative Examples 1 to 4 are shown in Table 4.

[0077] Table 4 shows the test results of salt spray corrosion resistance: Conclusion: After 1000 hours of salt spray testing, the blistering level of the plates in Examples 1 to 5 was mainly level 0, and the corrosion spread width at the scratches was less than 2.0 mm. The results confirm that the modified coating system has excellent barrier properties and resistance to electrochemical corrosion.

[0078] Comparative Example 1 showed a blistering level of 4 and an erosion width of 6.5 mm. The absence of a silane coupling agent resulted in microscopic voids at the resin-aluminum powder interface, allowing chloride ions, the corrosive medium, to rapidly penetrate along the interface. Hydrogen gas generated after aluminum powder corrosion accumulated under the coating, accelerating blistering and peeling.

[0079] Comparative Example 2 showed a bubbling grade of 2 and a corrosion diffusion width of 3.8 mm. The direct addition process resulted in uneven distribution of the silane coupling agent between the resin and pigment phases, failing to form a dense aluminum powder passivation layer. Some of the active aluminum powder reacted with the salt spray, leading to galvanic cell reactions, resulting in the accumulation of corrosion products and coating damage.

[0080] Comparative Example 3 showed the most severe corrosion, reaching level 5. Low-temperature curing and the unsealed catalyst system resulted in low cross-linking density and a loose network structure in the paint film, failing to effectively block the diffusion of moisture and chloride ions. Simultaneously, the residual acidic substances from the unsealed acid catalyst may have acted as an electrolyte, accelerating the corrosion process of the substrate.

[0081] Although Comparative Example 4 exhibited acceptable chemical stability, its corrosion diffusion width was slightly wider than that of the Example, and microbubbles appeared on the board surface. Insufficient anti-settling agent resulted in disordered aluminum powder arrangement, failing to form a layered maze effect. The path of corrosive media penetrating the coating was shortened, reducing the physical shielding efficiency, indicating that the directional arrangement of pigments has an auxiliary enhancing effect on corrosion resistance.

Claims

1. A water-based baking paint, characterized in that, It contains the following components in parts by weight: Hydroxyacrylic acid dispersion: 60.0-75.0 parts; Waterborne amino resin: 10.0-15.0 parts; Water-based aluminum powder: 10.0-15.0 parts; Silane coupling agent: 0.2-0.5 parts; Amine-blocked latent acid catalyst: 0.5-1.5 parts; Anti-settling grout: 5.0-8.0 parts.

2. The water-based baking paint according to claim 1, characterized in that, It contains the following components in parts by weight: Hydroxyacrylic acid dispersion: 67.5 parts; Waterborne amino resin: 12.5 parts; Water-based aluminum powder: 12.5 parts; Silane coupling agent: 0.35 parts; Amine-blocked latent acid catalyst: 1.0 part; Anti-settling grout: 6.5 parts.

3. The water-based baking paint according to claim 1, characterized in that, The silane coupling agent is γ-glycidyl etheroxypropyltrimethoxysilane.

4. The water-based baking paint according to claim 1, characterized in that, The preparation of the amine-blocked latent acid catalyst includes: Dissolve p-toluenesulfonic acid monohydrate in a solvent, and then add 2-amino-2-methyl-1-propanol dropwise at a temperature of 15-35℃ to carry out a neutralization reaction until the pH of the system reaches 7.5-8.

0.

5. The water-based baking paint according to claim 1, characterized in that, The hydroxy acrylic acid dispersion is a styrene-acrylate copolymer aqueous dispersion; the aqueous amino resin is fully methylated hexamethoxymethyl melamine.

6. The water-based baking paint according to claim 1, characterized in that, It also contains 1.0-4.0 parts by weight of additives, which are composed of wetting agents, dispersants, leveling agents and defoamers.

7. A method for preparing a water-based baking paint as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Preparation of modified aluminum powder slurry: Water-based aluminum powder, silane coupling agent, co-solvent and wetting and dispersing agent are mixed under low-speed stirring, so that the silane coupling agent is grafted onto the surface of the water-based aluminum powder; S2. Preparation of base paint: High-speed shear dispersion of hydroxyl acrylic dispersion, aqueous amino resin and anti-settling slurry; S3. Paint Mixing: Add the modified aluminum powder slurry obtained in step S1 to the base paint obtained in step S2, stir at low speed until uniform, and then add amine-blocked latent acid catalyst, wetting agent, dispersant, leveling agent, thickener and deionized water to obtain water-based baking paint.

8. The method for preparing a water-based baking paint according to claim 7, characterized in that, In step S1, the low-speed stirring speed is 400-500 rpm, and the stirring time is 30-40 minutes.

9. The method for preparing a water-based baking paint according to claim 7, characterized in that, In step S2, the rotation speed of the high-speed shear dispersion is 1000-1500 rpm; in step S3, the rotation speed of the low-speed stirring is 300-500 rpm.

10. The application of the water-based baking paint according to any one of claims 1-6 in the surface coating of automobile wheel hubs and electric vehicle wheel hubs.

Images