A process for the production of an enamel coating having a goniochromatic metallic luster, the aluminum powder pigments employed and a process for their production

Abstract

The present application belongs to the technical field of enamel coating preparation, and particularly relates to a preparation method of an enamel coating with an angle-dependent color metal luster, an aluminum powder pigment used in the preparation method and a preparation method of the aluminum powder pigment. The preparation method of the enamel coating comprises (1) substrate pretreatment; (2) preparation of an enamel glaze containing the aluminum powder pigment; (3) primary dip coating; after the primary dip coating is completed, gradient temperature rising drying and solidification are adopted to obtain a metal substrate with a primary dip coating layer; (4) spray coating; after the spray coating is completed, first firing is performed; (5) secondary dip coating; after the secondary dip coating is completed, second firing is performed; and cooling to room temperature, the enamel coating with the angle-dependent color metal luster is prepared and formed on the metal substrate. The preparation method solves the problems of uneven distribution of aluminum powder in the aluminum-doped enamel coating and yellowing caused by oxidation, so that the enamel coating has a uniform metal texture, and the high-temperature resistance, corrosion resistance and weather resistance of the enamel coating are greatly improved.

Description

Technical Field

[0001] This invention belongs to the technical field of enamel coating preparation, specifically relating to a method for preparing an enamel coating with a metallic luster that varies with angle, the aluminum powder pigment used, and its preparation method. Background Technology

[0002] Enamel coating is a non-metallic protective layer formed by melting inorganic glassy materials (such as silicates) at high temperatures and coating them onto the surface of a substrate such as a metal. The preparation process of enamel coating generally includes the following steps: pretreatment of the metal blank surface for cleaning and roughening; glazing; firing; and cooling. Enamel coating effectively isolates metal from the external environment, preventing rust and corrosion, and extending the service life of metal products. It is especially suitable for humid environments or environments containing chemicals, exhibiting excellent corrosion resistance, such as enamel-lined reactors. Furthermore, it endows metal products with heat resistance, wear resistance, and ease of cleaning. For example, enamel pots and bowls have good heat resistance, making them easy to cook and clean; enamel-lined steel sheets are easy to clean and impact-resistant in construction.

[0003] With the development of the times, the requirements for the performance of enamel coatings have also increased, and the requirements for colors have become increasingly diversified. Traditional enamel coatings are mostly single-color or simple textures, and due to limitations in appearance, their application is basically confined to functional areas such as kitchenware and pipes. Current technology incorporates aluminum powder during the preparation of the enamel coating, which can give the coating a unique visual expression. The coating color exhibits gradient, flashing, or iridescent textures depending on the viewing angle, fundamentally improving the visual quality of the enamel. Compared to solid-color coatings, dynamic colors that change with the viewing angle avoid visual fatigue. At the same time, the metallic luster of aluminum powder provides a metallic texture to the coating, enhancing the "high-end" feel of the enamel. Enamel panels with colors that change with the viewing angle are more attractive than traditional solid-color panels. This allows for applications in high-end kitchen and bathroom appliances, and light luxury home decoration, replacing more expensive stainless steel and aluminum sheets. It also retains the stain-resistant and easy-to-clean properties of enamel; oil stains and dirt can be wiped directly without worrying about staining the coating. Moreover, aluminum powder is tightly bonded to the enamel glaze through high-temperature sintering in the enamel coating. Its metallic luster is not easily faded by factors such as friction and light, and it is more resistant to aging than the corner-coloring effect of ordinary organic coatings, maintaining a visual freshness for a long time.

[0004] Meanwhile, the added aluminum powder, with its physical properties, combines with the inherent advantages of enamel, such as high-temperature resistance, corrosion resistance, and easy cleaning, further enhancing the coating performance. After melting, the aluminum powder effectively fills the gaps created during the melting of the enamel glaze, improving the tensile and bending resistance of the enamel coating. Especially in thin enamel products, such as enamel steel plates and appliance liners, it reduces the risk of enamel coating cracking due to substrate deformation, extending product lifespan. It also strengthens the adhesion between the coating and the metal substrate, reducing the risk of substrate corrosion. Aluminum powder has excellent covering power for the substrate's base color, effectively concealing minor imperfections on the enamel substrate surface, reducing the precision requirements of substrate pretreatment, lowering production costs, and eliminating the need for large amounts of expensive hiding agents such as titanium dioxide, further reducing raw material costs. Moreover, aluminum powder itself has good high-temperature resistance and thermal conductivity. For example, when used in oven liners and industrial enamel equipment, aluminum-powder-containing enamel coatings can transfer heat more evenly, preventing enamel coating peeling due to localized overheating and enhancing the coating's resistance to high-temperature environments.

[0005] Currently, the preparation process of enamel coatings with added aluminum powder generally involves directly adding uncoated ordinary aluminum powder to the glaze, or using wet ball milling to grind the aluminum powder into aluminum flakes containing mineral oil before adding them to the glaze. However, both of these aluminum powder-added enamel coating preparation processes have the following drawbacks:

[0006] (1) Uncoated ordinary aluminum powder is very susceptible to moisture and clumping. It has poor dispersibility in the coating glaze and has a small specific surface area, making it easy to sink. This results in uneven distribution of aluminum powder during the coating process of enamel coating, which can easily cause uneven metallic texture. In areas where aluminum powder is less distributed in the enamel coating, the coating may still crack. Moreover, uncoated aluminum powder is very easy to be oxidized during the high-temperature firing process of enamel coating, which can cause the coating to yellow and affect the quality of enamel coating.

[0007] (2) Although aluminum sheets containing mineral oil solve the problem of small specific surface area and easy sinking, different aluminum sheet manufacturers use different types of mineral oil. The polarity difference of the mineral oil limits the enamel coating coating material due to the type of mineral oil in the aluminum sheet, making it difficult to achieve the best overall performance of the enamel coating. Aluminum sheets containing mineral oil also have the problem of oxidation and yellowing. During the curing process of coating the enamel coating, the mineral oil eventually evaporates into the atmosphere, which will also have an impact on the environment; at the same time, with the water-based nature of enamel coatings, aluminum sheets containing mineral oil will react with water in water-containing coatings, leading to serious safety risks such as can swelling.

[0008] Therefore, there is an urgent need to provide a coating preparation method that can uniformly distribute aluminum powder in the enamel coating, avoid the risk of coating cracking due to uneven aluminum powder distribution and the problem of yellowing of the coating due to aluminum powder oxidation, and ensure that the addition of aluminum powder can bring the above-mentioned performance advantages to the enamel coating.

[0009] Existing technologies address the issue of aluminum powder easily agglomerating when exposed to moisture by coating and modifying it. Common modification methods include modifying aluminum powder with oleic acid to form an aluminum oleate coating layer on the surface, or using an oxidation method to form an aluminum oxide coating layer. While these modification processes delay the problems of aluminum powder agglomeration and difficulty in dispersion due to moisture, the density of the modified layer is limited. There are always exposed aluminum hydroxyl groups that are not fully coated. If improperly stored, the aluminum powder can absorb moisture from the air, still posing a risk of agglomeration and difficulty in dispersion. Furthermore, the corrosion resistance problem remains unresolved. Summary of the Invention

[0010] The purpose of this invention is to provide a method for preparing an enamel coating with a heterochromatic metallic luster, addressing the defects of uneven aluminum powder distribution and oxidation-induced yellowing in existing aluminum-doped enamel coatings. This method solves the problems of uneven aluminum powder distribution and oxidation-induced yellowing in aluminum-doped enamel coatings, resulting in a uniform metallic texture and significantly improved high-temperature resistance, corrosion resistance, and weather resistance.

[0011] The specific technical solution is as follows:

[0012] A method for preparing an enamel coating with an angle-dependent metallic luster, comprising the following steps:

[0013] (1) Pretreatment of the substrate: The metal substrate is degreased and pickled.

[0014] Degreasing and pickling of the metal substrate can be performed using standard procedures. The specific degreasing procedure is as follows: First, remove the surface oil of the metal substrate using an organic solvent; then immerse it in an alkaline degreasing solution at 70-90℃ for 10-15 minutes to remove residual oil; finally, wash with deionized water until neutral.

[0015] The specific pickling procedure is as follows: First, immerse the degreased metal substrate in a 5-10wt% hydrochloric acid solution at 35-45℃ for 5-10 minutes to remove surface oxide scale and rust; then rinse with deionized water and dry.

[0016] (2) Blending enamel glaze containing aluminum powder pigments:

[0017] First, enamel powder is mixed with deionized water to obtain enamel powder slurry; the mass ratio of enamel powder to deionized water can be 1:(0.3-0.5).

[0018] Then, aluminum powder pigment is added to the obtained enamel slurry, and bentonite, sodium silicate, boric acid and deionized water are added to the enamel slurry. The mixture is ball-milled to obtain an enamel glaze containing aluminum powder pigment. The solid content of the obtained enamel glaze is controlled to be 60-65%.

[0019] The mass ratio of aluminum powder pigment to enamel powder is (0.15-0.25):1.

[0020] The aluminum powder pigment is prepared through the following steps:

[0021] S1. By weight, add 1 part of raw aluminum powder to the reactor, mix with alcohol ether solvent, heat and stir, filter, and obtain an aluminum powder dispersion with a solid content of 70-75 wt%.

[0022] S2. Mix the obtained aluminum powder dispersion with 4-5 parts of alcohol ether solvent, stir, and disperse evenly to obtain mixture a;

[0023] Weigh out 35-45% of the total weight of the silane coupling agent and add it dropwise to mixture a, and disperse it evenly to obtain mixture b; wherein, the total weight of the silane coupling agent is 0.02-0.1 parts.

[0024] S3. Dilute the pH adjuster with deionized water; then add it to mixture b to adjust the pH of mixture b to 7.5-11.0.

[0025] pH adjusters can be one or more of the following: ammonia, ethanolamine, ethylenediamine, ethylene glycolamine, or dimethylethanolamine.

[0026] S4. First, dilute 0.1-0.5 parts of tetraethyl orthosilicate with an equal weight of alcohol ether solvent until homogeneous; then add the diluted tetraethyl orthosilicate dropwise to a mixture b with a pH of 7.5-11.0, heat to 60-70℃ and react for 1-4 hours to obtain mixture c.

[0027] S5. Add the remaining silane coupling agent dropwise to mixture c, and continue the reaction for 1-10 hours after the addition is complete.

[0028] After the reaction is complete, the resulting silica-coated aluminum silver paste is filtered, washed, dried, and sieved to obtain the aluminum powder pigment.

[0029] (3) One-time dip coating: Control the lifting angle of the metal substrate at 30°-40°, and dip it into the enamel glaze containing aluminum powder pigment obtained in step (2) at a uniform dip coating speed of 10-12cm / min to complete one-time dip coating;

[0030] After the first dip coating is completed, the substrate is dried and cured by gradient temperature increase: first, keep it at 50-65℃ for 15-20 minutes; then increase the temperature to 75-125℃ and keep it for 25-35 minutes; finally, increase the temperature to 135-145℃ and keep it for 10-15 minutes to obtain a metal substrate with a first dip coating.

[0031] (4) Spraying: Spraying the aluminum-containing pigment enamel onto the first dip coating layer;

[0032] After the coating is completed, it is first dried at 100-120℃; then the first firing is completed at 550-560℃ in a nitrogen atmosphere, and cooled to room temperature at a rate of 2-3℃ / min.

[0033] (5) Secondary dip coating: The metal substrate that has completed the first firing is dipped into the enamel glaze containing aluminum powder pigment obtained in step (2) at a lifting angle of 15°-25° and a dipping speed of 6-8cm / min to complete the secondary dip coating.

[0034] After the second dip coating is completed, it is first dried at 110-115℃; then the second firing is completed at 500-550℃ in a nitrogen atmosphere.

[0035] After firing, the metal substrate is cooled to room temperature at a rate of 2-3℃ / min, and an enamel coating with a variegated metallic luster is prepared by coating the substrate.

[0036] During the research on the preparation process of enamel coatings, it was found that in order to simultaneously ensure that the enamel coating has both a high-end, angle-dependent metallic luster, and enhance its density, weather resistance, oil resistance, and prevent cracking, the following aspects need to be improved in a coordinated manner, as detailed below:

[0037] On the one hand, the aluminum powder pigment used in the enamel coating preparation process is key to achieving the angle-dependent metallic luster of the enamel coating. However, conventional aluminum powder pigments, after being mixed into water-based enamel slurry and subjected to high-temperature treatment, exhibit a high gloss loss rate, ultimately resulting in poor metallic luster in the enamel coating and failing to effectively showcase the high-end angle-dependent metallic luster. Therefore, in researching the enamel coating preparation process, this invention first further investigates the aluminum powder pigments suitable for the coating preparation process. During the research, it was found that the aluminum powder pigments prepared using the above method, when used in the enamel coating preparation method, not only disperse uniformly in the water-based enamel slurry, but more importantly, compared to traditional aluminum powder pigments, the gloss loss rate of these pigments is reduced by 25-30%; and compared to current aluminum powder-containing enamel coatings, the gloss of this enamel coating is increased by 24-27%. After a 3-month natural weathering test, the gloss retention rate of the enamel coating is increased by 20-25%.

[0038] On the other hand, based on the aforementioned aluminum powder pigment, the coating process for the enamel coating is also crucial to its density. This invention has discovered that the metal substrate is subjected to a series of processes: a first dip coating → drying and curing according to the aforementioned gradient temperature (instead of firing) → spraying on the first dip coating → a first firing only after spraying → a second dip coating, this time with different lifting angles and speeds compared to the first dip coating → a final second firing, resulting in the enamel coating. The enamel coating obtained by this coating process maintains an integrity of over 95% after baking at 200°C for 300 hours; compared to traditional enamel coatings, this enamel coating exhibits a 30-35% longer salt spray life in salt spray testing.

[0039] In this invention, the method for preparing an enamel coating with a metallic luster that varies with angle of view, wherein the raw material composition of the enamel powder in step (2) includes 58.5-60.5 wt% silicon dioxide, 13.5-15.5 wt% boron oxide, 5-8.5 wt% sodium oxide, 10-12.5 wt% calcium oxide, and 3-5 wt% barium oxide.

[0040] In the present invention, the method for preparing the enamel coating with an angle-dependent metallic luster, in step (2), the amount of bentonite added is 0.8-1.2% of the mass of the enamel powder; the amount of sodium silicate added is 0.5-1.0% of the mass of the enamel powder; and the amount of boric acid added is 1.0-1.5% of the mass of the enamel powder.

[0041] In this invention, the method for preparing an enamel coating with an angle-dependent metallic luster, wherein the viscosity of the enamel glaze in step (2) is 800-900 mPa·s. Controlling the enamel glaze within this viscosity range lays a good foundation for the formation of the dip coatings in the subsequent two dip coatings, especially the first dip coating formed in the first dip coating, which plays a crucial role in the effect of the subsequent spraying.

[0042] In this invention, the method for preparing an enamel coating with an angle-dependent metallic luster, in step (2), has an enamel powder particle size of 40-60 μm and an aluminum pigment particle size of 30-35 μm. The particle size is determined by a laser particle size analyzer. The particle size ratio between the aluminum pigment and the enamel powder is 1:(1.1-2). This gradation improves the dispersibility of the enamel glaze, making the aluminum pigment more uniformly dispersed and less prone to agglomeration, thus forming a uniform metallic reflective layer in the enamel coating and enhancing the metallic luster and hiding power of the enamel coating. On the other hand, this gradation reduces pinholes or bubbles caused by voids during enamel firing. Furthermore, under this gradation, the enamel powder melts to form a continuous glaze layer, while the aluminum pigment can maintain a partially lamellar structure within the glaze layer, balancing the density and metallic texture of the glaze layer.

[0043] In this invention, the method for preparing an enamel coating with an angle-dependent metallic luster involves controlling the spraying linear velocity to be 12-14 cm / s, the spraying distance to be 20-25 cm, and the spraying pressure to be 0.3-0.5 MPa in step (4). Testing revealed that achieving the desired spraying effect on the resulting single-layer coating at the specified spraying linear velocity, distance, and pressure resulted in a good coating effect.

[0044] In this invention, the method for preparing an enamel coating with an angle-dependent metallic luster, wherein the silane coupling agent in step (2) is at least one of methacryloyloxypropyltrimethoxysilane, aminopropyltrimethoxysilane, vinyltriethoxysilane, glycidyl etheroxypropyltrimethoxysilane, or octyltriethoxysilane.

[0045] In the present invention, the method for preparing the enamel coating with an angle-dependent metallic luster, wherein the alcohol ether solvent in step (2) is one or a mixture of several of ethanol, methanol, propylene glycol methyl ether or diethylene glycol monomethyl ether.

[0046] The aluminum powder pigment used in the above-mentioned method for preparing enamel coating has an aluminum powder matrix with a particle size of 1.0-70.0 μm and a coating layer with a thickness of 50-100 nm on the outside of the matrix. The coating layer is formed by first coating with silane coupling agent; coating with tetraethyl orthosilicate at 60-70℃; and second coating with silane coupling agent. The weight ratio of silane coupling agent used in the first coating to all silane coupling agent used in the second coating is (0.35-0.45):1.

[0047] While traditional aluminum powder pigments have achieved some improvement in corrosion resistance through coating, their gloss loss rate has also increased significantly. To enable aluminum powder pigments to exhibit superior metallic luster in applications, it is crucial to address the technical challenge of simultaneously enhancing corrosion resistance and other properties while maintaining metallic luster and minimizing gloss loss rate.

[0048] During the research process, this invention discovered that if the coating of aluminum powder pigment is formed by first applying a portion of the silane coupling agent for primary coating, then replacing it with tetraethyl orthosilicate for secondary coating, and finally applying the remaining silane coupling agent for secondary coating, the gloss loss rate of the resulting aluminum powder pigment is lower than that of traditional aluminum powder pigments. However, the reduction is not ideal, and the improvement in corrosion resistance is not optimal while reducing the gloss loss rate. Therefore, this invention further investigated and found that the order and amount of silane coupling agent used in the two coating processes, as well as the temperature at which tetraethyl orthosilicate is applied after the primary silane coupling agent coating, play a crucial role in reducing the gloss loss rate and simultaneously improving the corrosion resistance of the aluminum powder pigment.

[0049] This invention reveals that when the weight ratio of the silane coupling agent used in the first coating to the total silane coupling agent used in the two coatings is controlled at (0.35-0.45):1, the resulting aluminum powder pigment can simultaneously possess excellent metallic luster and corrosion resistance. If the amount of silane coupling agent used in the first coating is too low, the reduction in the gloss loss rate of the aluminum powder pigment is small; conversely, if the amount of silane coupling agent used in the first coating is too high, the density of the final aluminum powder pigment coating layer decreases, and consequently, the corrosion resistance cannot achieve a satisfactory effect.

[0050] Meanwhile, the coating temperature of tetraethyl orthosilicate is also crucial when replacing the silane coupling agent coating with a coating temperature below the specified range. If the temperature is below the specified range, the density of the resulting aluminum powder pigment coating layer does not reach an ideal state.

[0051] When the weight ratio of the silane coupling agent used in a single coating to the total weight of the silane coupling agent used in the coating, and the coating temperature when tetraethyl orthosilicate is used, are both within the aforementioned ranges, the gloss loss rate of the resulting aluminum powder pigment is reduced by 25-30% compared to traditional aluminum powder pigments. Its settling time in water-based enamel glazes is 2-3 times that of traditional aluminum powder pigments. After 3 months of weathering testing, the gloss retention rate is improved by 25-35%. In an alkaline environment with a pH of 11-12, the aluminum powder pigment maintains stable performance without gas formation, and its corrosion resistance time in salt spray testing can be extended by 30-35%.

[0052] Furthermore, the secondary silane coupling agent coating further improves the dispersion uniformity of aluminum powder pigments and water-based enamel glazes, and enhances the interfacial compatibility between aluminum powder pigments and coating systems.

[0053] The preparation method of the above-mentioned aluminum powder pigment includes the following steps:

[0054] (1) By weight, add 1 part of raw aluminum powder and 3-4 parts of alcohol ether solvent into the reactor, stir and heat to 60-65℃, keep the temperature constant for 2-3 hours, and then filter to obtain an aluminum powder dispersion with a solid content of 70-75%.

[0055] (2) The aluminum powder dispersion obtained in step (1) is mixed with 4-5 parts of alcohol ether solvent and added to a stirrer. The mixture is heated to 30-45℃ and dispersed evenly to obtain mixture a.

[0056] Then, add a portion of the silane coupling agent dropwise to mixture a, controlling the dropwise addition time to 0.5-1 h. After the dropwise addition is complete, continue to disperse for 1-4 h to obtain mixture b.

[0057] The total weight of the silane coupling agent is 0.02-0.1 parts, and the amount of silane coupling agent added in this step is 35-45% of the total weight.

[0058] The principle and chemical formula of silane coupling agent coating aluminum powder:

[0059] R-Si(OR')3+3H2O→R-Si(OH)3+3R'OH;

[0060] nR-Si(OH)3→[R-SiO 1.5 ] n +1.5nH2O;

[0061] .

[0062] (3) First, dilute the pH adjuster with deionized water; then add it to mixture b and adjust the pH of mixture b to 7.5-11.0.

[0063] (4) First, dilute 0.15-0.5 parts of tetraethyl orthosilicate with an equal weight of alcohol ether solvent; then add the diluted tetraethyl orthosilicate dropwise to a mixture b with a pH of 7.5-11.0, and control the dropwise addition time to 0.5-1h; heat to 60-70℃ and react for 1-4h to obtain mixture c.

[0064] The principle and chemical formula of tetraethyl orthosilicate-coated aluminum powder:

[0065] Si(OC2H5)4+4H2O→Si(OH)4+4C2H5OH;

[0066] .

[0067] (5) Add the remaining silane coupling agent dropwise to the mixture c, controlling the dropwise addition time to be 0.5-1h; after the dropwise addition is completed, continue the reaction for 1-10h;

[0068] After the reaction is complete, the resulting silica-coated aluminum silver paste is filtered, washed, dried, and sieved to obtain the aluminum powder pigment.

[0069] The specific operations for drying and sieving are as follows: remove the residual solvent in the aluminum powder in a vacuum drying oven at 70℃-150℃; then sieve using a 400-500 mesh screen to obtain the aluminum powder pigment.

[0070] Compared to the traditional silica-coated aluminum powder process, the coating layer obtained by adding a portion of the silane coupling agent in the above preparation method—first coating with tetraethyl orthosilicate, then coating with the remaining silane coupling agent—is more ordered.

[0071] The beneficial effects of the present invention are as follows: The preparation method of the enamel coating with angle-dependent metallic luster described in the present invention solves the problems of uneven distribution of aluminum powder and oxidation yellowing in aluminum-doped enamel coatings, so that the enamel coating has a uniform metallic texture and its high temperature resistance, corrosion resistance and weather resistance are greatly improved.

[0072] The enamel coating obtained by the preparation method described in this invention possesses both a high-end, angle-dependent metallic luster and enhanced density, improving its weather resistance, oil resistance, and preventing cracking. Compared to current aluminum-containing enamel coatings, this coating exhibits a 24-27% increase in gloss. After 3 months of natural weathering testing, the gloss retention rate of the enamel coating is increased by 20-25%. When baked at 200°C for 300 hours, the enamel coating obtained through this coating process maintains over 95% integrity; compared to traditional enamel coatings, this coating extends salt spray life by 30-35% in salt spray testing.

[0073] In recent years, environmental issues have received increasing attention. Powder coatings, due to their solvent-free nature and environmental friendliness, have garnered growing interest, making the transition from VOC-containing liquid coatings to powder coatings an inevitable trend. The aluminum powder pigment described herein effectively solves technical problems in powder coatings, such as poor dispersion, susceptibility to moisture absorption and clumping, and significant decline in corrosion resistance. Tests have shown that when the aluminum powder pigment is stored openly in a cool, dark place at room temperature for at least one year before being used to make powder coatings, problems such as powder spitting out and poor dispersibility do not occur during powder coating application. Furthermore, compared to traditional aluminum oleate-coated aluminum powder, the aluminum powder pigment of this invention exhibits significantly improved corrosion resistance, and it does not react with water, making it suitable for water-based coatings. Attached Figure Description

[0074] Figure 1 This is a scanning electron microscope image of the aluminum powder pigment described in this invention at a depth of 2 μm.

[0075] Figure 2 This is a scanning electron microscope image of the aluminum powder pigment described in this invention at 1 μm. Detailed Implementation

[0076] 1. Gloss at 60° angle: Measured using a gloss meter in accordance with standard GB / T 9754-2007.

[0077] 2. Gloss retention rate test after aging: Referring to GB / T 1865-2009, the test samples were aged for 10 days in a xenon lamp aging test chamber. Referring to standard GB / T 9754-2007, the gloss at a 60° angle was measured using a gloss meter. The gloss retention rate after aging was calculated as follows: Gloss retention rate (%) = (Gloss value of the coating after aging / Gloss value of the coating before aging) × 100%.

[0078] 3. Salt spray resistance test of enamel coating: Refer to the acetic acid accelerated salt spray test (AASS) of GB / T 10125-2021 for 1000h and observe the corrosion of the enamel coating surface.

[0079] 4. High temperature resistance: Place the test sample in an oven and bake at 200℃ for 300 hours, then observe the integrity of the enamel coating.

[0080] 5. Gloss Loss Rate Test of Aluminum Powder Pigment: Using a UV lamp (UVA-340, 340nm, irradiance 0.71W / m²), the standard black temperature was (60±3)℃, relative humidity was (50±5)%RH, and the aging time was 10 days. The gloss was measured at a 60° angle using a gloss meter, referencing standard GB / T 9754-2007. The gloss loss rate of the aluminum powder pigment was calculated as: gloss loss rate = (initial gloss - gloss after test) / initial gloss × 100%.

[0081] 6. Salt spray resistance test of aluminum powder pigments: Refer to GB / T 10125-2021 for 168h of neutral salt spray (NSS) and observe the appearance of aluminum powder pigments.

[0082] 7. Figure 1 In the text, SU8010 indicates the model of the scanning electron microscope; 5.0kV indicates that the accelerating voltage of the electron beam is 5.0 kilovolts; 9.0mm is the working distance, i.e., the distance from the sample stage to the objective lens; x20.0k indicates that the magnification is 20,000 times; in SE(U), SE represents secondary electron imaging, and U represents the mode without acceleration or deceleration voltage; 2.00μm is the length of the scale bar at the bottom of the image.

[0083] 8. Figure 2 In the text, SU8010 indicates the model of the scanning electron microscope; 5.0kV indicates that the accelerating voltage of the electron beam is 5.0 kilovolts; 9.0mm is the working distance, i.e., the distance from the sample stage to the objective lens; x50.0k indicates that the magnification is 50,000 times; in SE(U), SE represents secondary electron imaging, and U represents the mode without acceleration or deceleration voltage; 1.00μm is the length of the scale bar at the bottom of the image.

[0084] Example 1

[0085] The preparation method of the enamel coating with angle-dependent metallic luster includes the following specific steps:

[0086] (1) Substrate pretreatment: The metal substrate is first degreased with an organic solvent; then it is immersed in an alkaline degreasing solution at 70°C for 15 minutes to remove residual oil; finally, it is washed with deionized water until neutral. The degreased metal substrate is immersed in a 5wt% hydrochloric acid solution at 35°C for 10 minutes to remove surface oxide scale and rust; it is then rinsed with deionized water and dried; the degreasing and pickling pretreatment of the metal substrate is completed.

[0087] (2) Blending enamel glaze containing aluminum powder pigments:

[0088] First, enamel powder with a particle size of 50 μm is mixed with deionized water at a mass ratio of 1:0.5 to obtain enamel powder slurry. The raw material composition of the enamel powder is 60.5 wt% silicon dioxide, 15.5 wt% boron oxide, 8.5 wt% sodium oxide, 10.5 wt% calcium oxide, and 5 wt% barium oxide.

[0089] Then, aluminum powder pigment with a particle size of 35μm is added to the obtained enamel powder slurry, wherein the mass ratio of aluminum powder pigment to enamel powder is 0.25:1.

[0090] 1.2% bentonite, 1.0% sodium silicate, 1.0% boric acid, and deionized water by weight of enamel powder were added to the enamel powder slurry. The mixture was ball-milled and mixed to control the solid content of the enamel glaze to be 65% and the viscosity of the enamel glaze to be 900 mPa·s, thus obtaining an enamel glaze containing aluminum powder pigment.

[0091] (3) One-time dip coating: Control the lifting angle of the metal substrate at 40°, and dip it into the enamel glaze containing aluminum powder pigment obtained in step (2) at a uniform dip coating speed of 10cm / min to complete one-time dip coating.

[0092] After the first dip coating is completed, the substrate is dried and cured by gradient temperature increase: first, it is kept at 50℃ for 20 minutes; then the temperature is increased to 75℃ and kept for 25 minutes; finally, the temperature is increased to 135℃ and kept for 10 minutes to obtain a metal substrate with a first dip coating.

[0093] (4) Spraying: Spray the enamel glaze containing aluminum powder pigment onto the first dip coating layer; control the spraying linear speed to be 14cm / s, the spraying distance to be 20cm, and the spraying pressure to be 0.5MPa.

[0094] After the coating is completed, it is first dried at 100℃; then it is fired for the first time at 550℃ in a nitrogen atmosphere, and cooled to room temperature at a rate of 2℃ / min.

[0095] (5) Secondary dip coating: The metal substrate that has completed the first firing is immersed in the enamel glaze containing aluminum powder pigment obtained in step (2) at a lifting angle of 25° and a dipping speed of 8cm / min to complete the secondary dip coating.

[0096] After the second dip coating is completed, it is first dried at 115℃; then the second firing is completed at 500℃ in a nitrogen atmosphere.

[0097] After firing, the metal substrate is cooled to room temperature at a rate of 2℃ / min, and an enamel coating with a metallic luster that varies with angle is applied to the metal substrate.

[0098] The aluminum powder pigment is prepared through the following steps:

[0099] (1) By weight, 1 part of raw aluminum powder and 3 parts of diethylene glycol monomethyl ether were added to the reactor, stirred and heated to 60°C, kept at a constant temperature for 3 hours, and then filtered to obtain an aluminum powder dispersion with a solid content of 72%.

[0100] (2) The aluminum powder dispersion obtained in step (1) is mixed with 5 parts of diethylene glycol monomethyl ether and added to a stirrer. The mixture is heated to 30°C and dispersed evenly to obtain mixture a.

[0101] Then, 0.03 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane (accounting for 37.5% of the total weight of 0.08 parts of silane coupling agent) were added dropwise to mixture a, and the dropping time was controlled at 0.5h. After the dropping was completed, the mixture was dispersed for another 1h to obtain mixture b.

[0102] (3) Dilute the pH adjuster ethylenediamine with deionized water; then add it to mixture b to adjust the pH of mixture b to 8.5.

[0103] (4) Dilute 0.15 parts of tetraethyl orthosilicate with 0.15 parts of diethylene glycol monomethyl ether until uniform; then add the diluted tetraethyl orthosilicate dropwise to the mixture b with a pH of 8.5, and control the dropwise addition time to 1 h; heat to 60 °C and react for 2 h to obtain mixture c.

[0104] (5) Add the remaining 0.05 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane dropwise to the mixture c, and control the dropwise addition time to 1 h; after the dropwise addition is completed, continue the reaction for 8 h.

[0105] After the reaction was completed, the resulting silica-coated aluminum-silver paste was filtered using a plate filter and washed twice with diethylene glycol monomethyl ether. It was then dried in an oven at 140°C for 2 hours and sieved using a 500-mesh vibrating sieve to obtain the aluminum powder pigment with a coating thickness of 70 nm.

[0106] Example 2

[0107] The preparation method of the enamel coating with angle-dependent metallic luster includes the following specific steps:

[0108] (1) Substrate pretreatment: The metal substrate is first degreased with an organic solvent; then it is immersed in an alkaline degreasing solution at 70°C for 15 minutes to remove residual oil; finally, it is washed with deionized water until neutral. The degreased metal substrate is immersed in a 5wt% hydrochloric acid solution at 35°C for 10 minutes to remove surface oxide scale and rust; it is then rinsed with deionized water and dried; the degreasing and pickling pretreatment of the metal substrate is completed.

[0109] (2) Blending enamel glaze containing aluminum powder pigments:

[0110] First, enamel powder with a particle size of 60 μm is mixed with deionized water at a mass ratio of 1:0.5 to obtain enamel powder slurry. The raw material composition of the enamel powder is 60.5 wt% silicon dioxide, 15.5 wt% boron oxide, 8.5 wt% sodium oxide, 10.5 wt% calcium oxide, and 5 wt% barium oxide.

[0111] Then, aluminum powder pigment with a particle size of 30μm is added to the obtained enamel powder slurry, wherein the mass ratio of aluminum powder pigment to enamel powder is 0.15:1.

[0112] Then, 1.2% bentonite, 1.0% sodium silicate, 1.0% boric acid and deionized water by weight of enamel powder were added to the enamel powder slurry, and the mixture was ball-milled and mixed. The solid content of the resulting enamel glaze was controlled to be 60%, and the viscosity of the enamel glaze was 800 mPa·s, thus obtaining an enamel glaze containing aluminum powder pigment.

[0113] (3) One-time dip coating: control the lifting angle of the metal substrate at 30°, and dip it into the enamel glaze containing aluminum powder pigment obtained in step (2) at a uniform dip coating speed of 12cm / min to complete one-time dip coating.

[0114] After the first dip coating is completed, the substrate is dried and cured by gradient temperature increase: first, it is kept at 65℃ for 15 minutes; then the temperature is increased to 125℃ and kept for 35 minutes; finally, the temperature is increased to 145℃ and kept for 15 minutes to obtain a metal substrate with a first dip coating.

[0115] (4) Spraying: Spray the enamel glaze containing aluminum powder pigment onto the first dip coating layer; control the spraying linear speed to be 12cm / s, the spraying distance to be 25cm, and the spraying pressure to be 0.3MPa.

[0116] After the coating is completed, it is first dried at 120℃; then it is fired for the first time at 560℃ in a nitrogen atmosphere, and cooled to room temperature at a rate of 3℃ / min.

[0117] (5) Secondary dip coating: The metal substrate that has completed the first firing is immersed in the enamel glaze containing aluminum powder pigment obtained in step (2) at a lifting angle of 15° and a dipping speed of 6cm / min to complete the secondary dip coating.

[0118] After the second dip coating is completed, it is first dried at 110℃; then the second firing is completed at 550℃ in a nitrogen atmosphere.

[0119] After firing, the metal substrate is cooled to room temperature at a rate of 3°C / min, and an enamel coating with a variegated metallic luster is prepared by coating the substrate.

[0120] The aluminum powder pigment in this embodiment is the same as the aluminum powder pigment obtained in Example 1.

[0121] Example 3

[0122] The difference from Example 1 is that in step (4) of the preparation of aluminum powder pigment described in this example, 0.1 parts of tetraethyl orthosilicate are added; and the temperature is raised to 70°C and reacted for 2 hours.

[0123] The others are the same as in Example 1.

[0124] Example 4

[0125] The difference from Example 1 is that in step (3) of preparing the aluminum powder pigment described in this example, the pH value of the mixed solution b is adjusted to 10.0.

[0126] In step (4) of the preparation of the aluminum powder pigment described in this embodiment, 0.4 parts of tetraethyl orthosilicate are added; and the temperature is raised to 70°C and reacted for 2 hours. An aluminum powder pigment with a coating layer of 80 nm thickness is obtained.

[0127] The others are the same as in Example 1.

[0128] Example 5

[0129] The difference from Example 1 is that in step (2) of the preparation of aluminum powder pigment described in this example, 0.036 parts of silane coupling agent γ-glycidyl etheroxypropyltrimethoxysilane (accounting for 45% of the total weight of 0.08 parts of silane coupling agent) are added dropwise to mixture a.

[0130] In step (5) of the preparation of aluminum powder pigment described in this embodiment, the remaining 0.044 parts of silane coupling agent γ-glycidyl etheroxypropyltrimethoxysilane are added dropwise to the mixture c.

[0131] The others are the same as in Example 1.

[0132] Example 6

[0133] The difference from Example 1 is that in step (2) of the preparation of aluminum powder pigment described in this example, 0.03 parts of silane coupling agent γ-glycidyl etheroxypropyltrimethoxysilane (accounting for 37.5% of the total weight of 0.08 parts of silane coupling agent) are added dropwise to mixture a.

[0134] In step (5) of the preparation of aluminum powder pigment described in this embodiment, the remaining 0.05 parts of silane coupling agent octyltriethoxysilane are added dropwise to the mixture c.

[0135] The others are the same as in Example 1.

[0136] Comparative Example 1

[0137] The difference from Example 1 is that the aluminum powder pigment described in this comparative example is first coated with tetraethyl orthosilicate and then coated with a silane coupling agent.

[0138] The specific steps are as follows:

[0139] (1) By weight, 1 part of raw aluminum powder and 3 parts of diethylene glycol monomethyl ether were added to the reactor, stirred and heated to 60°C, kept at a constant temperature for 3 hours, and then filtered to obtain an aluminum powder dispersion with a solid content of 72%.

[0140] (2) The aluminum powder dispersion obtained in step (1) is mixed with 5 parts of diethylene glycol monomethyl ether and added to a stirrer. The mixture is heated to 30°C and dispersed evenly to obtain mixture a.

[0141] (3) Dilute the pH adjuster ethylenediamine with deionized water; then add it to the mixture a to adjust the pH of the mixture a to 8.5.

[0142] (4) Dilute 0.15 parts of tetraethyl orthosilicate with 0.15 parts of diethylene glycol monomethyl ether until uniform; then add the diluted tetraethyl orthosilicate dropwise to a mixture a with a pH of 8.5, and control the dropwise addition time to 1 h; heat to 60 °C and react for 2 h to obtain mixture b.

[0143] (5) Add 0.03 parts of silane coupling agent γ-glycidyl etheroxypropyltrimethoxysilane (accounting for 37.5% of the total weight of 0.08 parts of silane coupling agent) dropwise to mixture b, control the dropwise addition time to 0.5 h, and continue to disperse for 1 h after the dropwise addition is completed to obtain mixture c.

[0144] (6) Add the remaining 0.05 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane dropwise to the mixture c, and control the dropwise addition time to 1 h; after the dropwise addition is completed, continue the reaction for 8 h.

[0145] After the reaction was completed, the resulting silica-coated aluminum-silver paste was filtered using a plate filter and washed twice with diethylene glycol monomethyl ether. It was then dried in an oven at 140°C for 2 hours and sieved using a 500-mesh vibrating sieve to obtain the aluminum powder pigment with a coating thickness of 70 nm.

[0146] The others are the same as in Example 1.

[0147] Comparative Example 2

[0148] The difference from Example 1 is that the aluminum powder pigment described in this comparative example is prepared by mixing tetraethyl orthosilicate and a silane coupling agent together for coating.

[0149] The specific steps are as follows:

[0150] (1) By weight, 1 part of raw aluminum powder and 3 parts of diethylene glycol monomethyl ether were added to the reactor, stirred and heated to 60°C, kept at a constant temperature for 3 hours, and then filtered to obtain an aluminum powder dispersion with a solid content of 72%.

[0151] (2) The aluminum powder dispersion obtained in step (1) is mixed with 5 parts of diethylene glycol monomethyl ether and added to a stirrer. The mixture is heated to 30°C and dispersed evenly to obtain mixture a.

[0152] (3) Dilute the pH adjuster ethylenediamine with deionized water; then add it to the mixture a to adjust the pH of the mixture a to 8.5.

[0153] (4) Mix 0.15 parts of tetraethyl orthosilicate, 0.15 parts of diethylene glycol monomethyl ether, and 0.08 parts of silane coupling agent γ-glycidyl etheroxypropyltrimethoxysilane and add them to a mixture with a pH of 8.5. Control the dropping time to 1 h; heat to 60 °C and react for 10 h.

[0154] After the reaction was completed, the resulting silica-coated aluminum-silver paste was filtered using a plate filter and washed twice with diethylene glycol monomethyl ether. It was then dried in an oven at 140°C for 2 hours and sieved using a 500-mesh vibrating sieve to obtain the aluminum powder pigment with a coating thickness of 70 nm.

[0155] The others are the same as in Example 1.

[0156] Comparative Example 3

[0157] The difference from Example 1 is that the aluminum powder pigment described in this comparative example is prepared by first coating with a silane coupling agent and then coating with tetraethyl orthosilicate.

[0158] The specific steps are as follows:

[0159] (1) By weight, 1 part of raw aluminum powder and 3 parts of diethylene glycol monomethyl ether were added to the reactor, stirred and heated to 60°C, kept at a constant temperature for 3 hours, and then filtered to obtain an aluminum powder dispersion with a solid content of 72%.

[0160] (2) The aluminum powder dispersion obtained in step (1) is mixed with 5 parts of diethylene glycol monomethyl ether and added to a stirrer. The mixture is heated to 30°C and dispersed evenly to obtain mixture a.

[0161] Then, 0.03 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane (accounting for 37.5% of the total weight of 0.08 parts of silane coupling agent) were added dropwise to mixture a, and the dropping time was controlled at 0.5h. After the dropping was completed, the mixture was dispersed for another 1h to obtain mixture b.

[0162] (3) Add the remaining 0.05 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane dropwise to the mixture b, and control the dropwise addition time to 1 h; after the dropwise addition is completed, continue the reaction for 8 h; to obtain mixture c.

[0163] (4) Dilute the pH adjuster ethylenediamine with deionized water; then add it to the mixture c to adjust the pH of the mixture c to 8.5.

[0164] (5) Dilute 0.15 parts of tetraethyl orthosilicate with 0.15 parts of diethylene glycol monomethyl ether until uniform; then add the diluted tetraethyl orthosilicate dropwise to the mixed solution c with a pH of 8.5, and control the dropwise addition time to 1 h; heat to 60 °C and react for 2 h.

[0165] After the reaction was completed, the resulting silica-coated aluminum-silver paste was filtered using a plate filter and washed twice with diethylene glycol monomethyl ether. It was then dried in an oven at 140°C for 2 hours and sieved using a 500-mesh vibrating sieve to obtain the aluminum powder pigment with a coating thickness of 70 nm.

[0166] The others are the same as in Example 1.

[0167] Comparative Example 4

[0168] The difference from Example 1 is that in step (2) of the preparation of aluminum powder pigment described in this comparative example, 0.05 parts of silane coupling agent γ-glycidyl etheroxypropyltrimethoxysilane (accounting for 62.5% of the total weight of 0.08 parts of silane coupling agent) are added dropwise to mixture a.

[0169] In step (5) of the preparation of aluminum powder pigment described in this embodiment, the remaining 0.03 parts of silane coupling agent octyltriethoxysilane are added dropwise to the mixture c.

[0170] The others are the same as in Example 1.

[0171] Comparative Example 5

[0172] The difference from Example 1 is that in step (4) of the preparation of aluminum powder pigment described in this comparative example, the temperature is raised to 50°C and reacted for 2 hours.

[0173] The others are the same as in Example 1.

[0174] The obtained aluminum powder pigment was stored in an open, cool place at room temperature for one year before being made into powder coating. During the application of the powder coating, powder spitting occurred, and the dispersibility was extremely poor.

[0175] Comparative Example 6

[0176] The difference from Example 1 is that in step (3) of the enamel coating preparation method described in this comparative example, after the first dip coating is completed, the first firing is carried out: first, it is dried at 100°C; then, the first firing is completed at 550°C in a nitrogen atmosphere, and cooled to room temperature at a rate of 2°C / min.

[0177] In step (4) of the enamel coating preparation method, after the coating is completed, it is first dried at 100°C; then it is fired a second time at 550°C in a nitrogen atmosphere and cooled to room temperature at a rate of 2°C / min.

[0178] In step (5) of the enamel coating preparation method, after the second dip coating is completed, it is first dried at 115°C; then the third firing is completed at 500°C under a nitrogen atmosphere.

[0179] Comparative Example 7

[0180] The difference from Example 1 is that in step (2) of the enamel coating preparation method described in this comparative example, the particle size of the enamel powder is 35 μm and the particle size of the aluminum powder pigment is 50 μm.

[0181] Comparative Example 8

[0182] The difference from Example 1 is that in steps (3) and (5) of the enamel coating preparation method described in this comparative example, the lifting angle of the metal substrate is controlled at 40° and the dipping speed is controlled at 10cm / min during dipping. That is, the lifting angle and dipping speed are the same for the first and second dipping.

[0183] The performance indicators of the enamel coatings obtained in each embodiment and comparative example are shown in Table 1.

[0184] Table 1 Performance Indicators of Various Enamel Coatings

[0185]

[0186] The performance indicators of the aluminum powder pigments obtained in each embodiment and comparative example are shown in Table 2.

[0187] Table 2 Performance Indicators of Various Aluminum Powder Pigments

[0188]

[0189] It is evident that the enamel coating obtained by the preparation method described in this invention not only improves corrosion resistance but also ensures excellent metallic luster.

Claims

1. A method for producing an enamel coating having a goniochromatic metallic luster, characterized in that It comprises the following steps: (1) substrate pretreatment: the metal substrate is subjected to degreasing and pickling respectively; (2) preparing enamel containing aluminum powder pigment: First, mix the enamel powder with deionized water to obtain an enamel powder slurry; Then, add the aluminum powder pigment to the obtained enamel powder slurry, and add bentonite, sodium silicate, boric acid and deionized water to the enamel powder slurry, and mix by ball milling to obtain the enamel containing aluminum powder pigment; wherein the solid content of the obtained enamel is controlled to be 60-65%; The mass ratio of the aluminum powder pigment to the enamel powder is (0.15-0.25):1; The aluminum powder pigment is prepared by the following steps: S1, according to the weight parts, 1 part of raw aluminum powder is added to the reaction kettle, mixed with alcohol ether solvent, heated and stirred, and filtered to obtain an aluminum powder dispersion liquid with a solid content of 70-75wt%; S2, mix the obtained aluminum powder dispersion liquid with 4-5 parts of alcohol ether solvent, stir and disperse uniformly to obtain a mixed liquid a; Add 35-45% of the total weight parts of the silane coupling agent to the mixed liquid a, and disperse uniformly to obtain a mixed liquid b; wherein the total weight parts of the silane coupling agent is 0.02-0.1 parts; S3, dilute the pH regulator with deionized water; then add it to the mixed liquid b to adjust the pH value of the mixed liquid b to 7.5-11.0; S4, first dilute 0.1-0.5 parts of tetraethyl orthosilicate uniformly with the same weight parts of alcohol ether solvent; then add the diluted tetraethyl orthosilicate to the mixed liquid b with a pH value of 7.5-11.0, and react at 60-70℃ for 1-4h to obtain a mixed liquid c; S5, add the remaining silane coupling agent to the mixed liquid c, and continue to react for 1-10h after the addition is completed; After the reaction is completed, the obtained silica-coated aluminum silver paste is filtered, washed, dried and sieved to obtain the aluminum powder pigment; (3) primary dip coating: control the pulling angle of the metal substrate to be 30-40°, and immerse the obtained enamel containing aluminum powder pigment in step (2) at a dipping speed of 10-12cm / min to complete the primary dip coating; After the primary dip coating is completed, gradient temperature drying and curing are adopted: first, heat at 50-65℃ for 15-20min; then heat to 75-125℃ for 25-35min; finally, heat to 135-145℃ for 10-15min; to obtain a metal substrate with a primary dip coating layer; (4) spraying: spray the obtained enamel containing aluminum powder pigment on the primary dip coating layer; After the spraying is completed, first dry at 100-120℃; then complete the first firing at 550-560℃ in a nitrogen atmosphere, and cool to room temperature at a rate of 2-3℃ / min; (5) secondary dip coating: immerse the metal substrate after the first firing into the obtained enamel containing aluminum powder pigment in step (2) at a pulling angle of 15-25° and a dipping speed of 6-8cm / min to complete the secondary dip coating; After the secondary dip coating is completed, first dry at 110-115℃; then complete the second firing at 500-550℃ in a nitrogen atmosphere; After the sintering is completed, the enamel coating with the metal luster of goniochromaticity is prepared by coating on the metal substrate after cooling to room temperature at a rate of 2-3 ℃ / min.

2. Process for the production of an enamel coating with goniochromatic metallic lustre according to claim 1, characterised in that The raw material composition of the enamel powder in the step (2) comprises 58.5-60.5 wt% of silicon dioxide, 13.5-15.5 wt% of boron oxide, 5-8.5 wt% of sodium oxide, 10-12.5 wt% of calcium oxide, and 3-5 wt% of barium oxide.

3. A method of producing an enamel coating with goniochromatic metallic lustre according to claim 1, characterised in that The added amount of the bentonite in the step (2) is 0.8-1.2% of the mass of the enamel powder; the added amount of the sodium silicate is 0.5-1.0% of the mass of the enamel powder; and the added amount of the boric acid is 1.0-1.5% of the mass of the enamel powder.

4. A method of producing an enamel coating with goniochromatic metallic lustre according to claim 1, characterised in that The viscosity of the enamel frit in the step (2) is 800-900 mPa·s.

5. A method of producing an enamel coating with goniochromatic metallic lustre according to claim 1, characterised in that The particle size of the enamel powder in the step (2) is 40-60 μm, and the particle size of the aluminum powder pigment is 30-35 μm.

6. A method of producing an enamel coating with goniochromatic metallic lustre according to claim 1, characterised in that In the step (4), the spraying line speed is controlled to be 12-14 cm / s, the spraying distance is 20-25 cm, and the spraying pressure is 0.3-0.5 MPa.

7. A method of producing an enamel coating with goniochromatic metallic lustre according to claim 1, characterised in that The silane coupling agent in the step (2) is at least one of methacryloyloxypropyltrimethoxysilane, aminopropyltrimethoxysilane, vinyltriethoxysilane, glycidyl ether oxypropyltrimethoxysilane, and octyltriethoxysilane.

8. A method of producing an enamel coating with goniochromatic metallic lustre according to claim 1, characterised in that The alcohol-ether solvent in the step (2) is one or a mixture of several of ethanol, methanol, propylene glycol methyl ether, and diethylene glycol monomethyl ether.

9. An aluminum powder pigment for use in a process for the production of an enamel coating having a goniochromatic metallic luster according to claim 1, characterized in that The base of the aluminum powder pigment is aluminum powder with a particle size of 1.0-70.0 μm, and the aluminum powder has a coating layer with a thickness of 50-100 nm outside the base, and the coating layer is composed of a primary coating of a silane coupling agent, an ethyl silicate coating, and a secondary coating of a silane coupling agent in sequence; the weight ratio of the silane coupling agent used for the primary coating to the total silane coupling agent used for the coating is (0.35-0.45):

1.

10. A process for the production of an aluminum pigment as claimed in claim 9, characterized in that The method comprises the following steps: (1) 1 part of raw aluminum powder and 3-4 parts of alcohol-ether solvent are added into a reaction kettle, stirred, and heated to 60-65 ℃, and then the temperature is kept constant for 2-3 h, and then filtration is performed to obtain an aluminum powder dispersion liquid with a solid content of 70-75%; (2) the aluminum powder dispersion liquid obtained in the step (1) and 4-5 parts of alcohol-ether solvent are mixed and added into a stirrer, uniformly dispersed at a temperature of 30-45 ℃, and then a mixed liquid a is obtained; part of the silane coupling agent is added dropwise into the mixed liquid a, the dropping time is controlled to be 0.5-1 h, and then the mixed liquid a is continuously dispersed for 1-4 h to obtain a mixed liquid b; wherein the total weight parts of the silane coupling agent is 0.02-0.1 parts, and the added amount of the silane coupling agent in this step is 35-45% of the total weight parts; (3) the pH regulator is first diluted with deionized water; and then added into the mixed liquid b to adjust the pH value of the mixed liquid b to 7.5-11.0; (4) first, 0.15-0.5 parts of tetraethyl orthosilicate is diluted uniformly with equal weight parts of alcohol ether solvent; then the diluted tetraethyl orthosilicate is added dropwise into the mixed solution b with a pH value of 7.5-11.0, and the dropwise adding time is controlled to be 0.5-1 h; the temperature is increased to 60-70 DEG C, and the reaction is carried out for 1-4 h to obtain a mixed solution c; (5) the remaining silane coupling agent is added dropwise into the mixed solution c, and the dropwise adding time is controlled to be 0.5-1 h; after the dropwise adding is completed, the reaction is continued for 1-10 h; after the reaction is completed, the obtained silica-coated aluminum silver paste is filtered, washed, dried and sieved to obtain the aluminum powder pigment.

Images