Preparation method of enamel coating with angle-dependent different-color metallic luster, adopted aluminum powder pigment and preparation method of aluminum powder pigment

Abstract

The invention belongs to the technical field of enamel coating preparation, and particularly relates to a preparation method of an enamel coating with angle-dependent different-color metallic luster, an adopted aluminum powder pigment and a preparation method of the aluminum powder pigment. The preparation method of the enamel coating comprises the following steps: (1) pretreating the matrix; (2) preparing enamel glaze containing aluminum powder pigment; (3) primary dip-coating; after primary dip-coating is completed, gradient heating drying and curing are adopted, and a metal matrix with a primary dip-coating layer is obtained; (4) spraying; after spraying is completed, firing for the first time; (5) secondary dip-coating; after the secondary dip-coating is completed, carrying out secondary firing; and cooling to room temperature, and coating on a metal matrix to prepare the enamel coating with angle-dependent different-color metallic luster. According to the preparation method, the problems of non-uniform distribution of aluminum powder and oxidation yellowing in the aluminum-doped enamel coating are solved, so that the enamel coating has uniform metal texture, and the high temperature resistance, corrosion resistance and weather resistance are greatly improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of enamel coating preparation, and particularly relates to a preparation method of an enamel coating with a metal luster with angle-dependent color change, an aluminum powder pigment used in the preparation method, and a preparation method of the aluminum powder pigment. BACKGROUND

[0002] An enamel coating is a non-metallic protective layer formed by melting and coating inorganic glassy materials (such as silicates) on the surface of a metal substrate through a high-temperature process. The coating preparation process of the enamel coating generally includes the following steps: pretreatment of surface cleaning and roughening of the metal blank; application of enamel; firing; and cooling. The enamel coating can effectively isolate the metal from the external environment, prevent the metal from rusting and corroding, and prolong the service life of the metal product. It is particularly suitable for use in humid or chemical environments and has excellent corrosion resistance, such as enamel reaction kettles. In addition, the enamel coating can also make the metal product have the characteristics of heat resistance, wear resistance, and easy cleaning, such as enamel pots, enamel bowls, and other kitchen utensils that are heat-resistant and easy to cook and clean; enamel steel plates are easy to clean and impact-resistant in construction.

[0003] With the development of the times, the performance requirements of enamel coatings have also been improved, and the color requirements have become more and more diverse. Traditional enamel coatings are mostly single colors or simple textures, and are basically limited to functional fields such as kitchen utensils and pipelines due to the limitations of appearance factors. The prior art adds aluminum powder during the preparation process of the enamel coating, which can give the coating unique visual appeal. The color of the coating changes with the viewing angle, showing a gradual change, a flash, or a rainbow effect. This fundamentally improves the visual level of the enamel, and compared to pure color coatings, the dynamic color with angle-dependent color change can avoid single visual fatigue. At the same time, the metal luster of the aluminum powder provides a metal texture to the coating, improving the "high-end feel" of the enamel. The enamel panel with angle-dependent color change is more attractive than the traditional pure color panel, and can be applied to high-end kitchen and bathroom appliances, light luxury home decoration, and other fields. This can replace metal panels such as stainless steel and aluminum panels, which have higher costs, while also having the characteristics of enamel, such as stain resistance and easy cleaning. Oil stains and stains can be wiped directly without worrying about staining the coating. Moreover, the aluminum powder in the enamel coating is tightly combined with the enamel glaze through high-temperature sintering, and the metal luster is not easily faded by factors such as friction and light. The angle-dependent color effect of ordinary organic coatings is more resistant to aging, and the visual freshness is maintained for a long time.

[0004] At the same time, the physical properties of the added aluminum powder are combined with the inherent advantages of enamel, such as high temperature resistance, corrosion resistance, and easy cleaning, to further strengthen the coating performance. After melting, the aluminum powder can well fill the gaps generated during the melting of the enamel glaze, improve the tensile and bending resistance of the enamel coating, especially in thin enamel products such as enamel steel plate and household appliance inner liner, reduce the risk of enamel coating cracking caused by substrate deformation, prolong the service life of the product, enhance the bonding force of the coating and the metal substrate, and reduce the risk of substrate corrosion. The aluminum powder has excellent hiding power for the base color of the substrate, can effectively cover the small defects on the surface of the enamel substrate, reduce the precision requirement of the substrate pretreatment, and reduce the production cost. Without the use of a large amount of high-priced titanium dioxide and other hiding agents, the raw material cost is further reduced. Moreover, the aluminum powder itself has good high temperature resistance and thermal conductivity. For example, when applied to oven inner liner and industrial enamel equipment, the enamel coating containing aluminum powder can more evenly transfer heat, avoid enamel coating falling off caused by local overheating, and enhance the resistance of the coating to high temperature environment.

[0005] Currently, in the preparation process of enamel coating with added aluminum powder, uncoated ordinary aluminum powder is directly added to the glaze, or aluminum powder is extended into aluminum sheet containing mineral oil by wet ball milling using steel balls, and then added to the glaze. However, the above two enamel coating preparation processes with added aluminum powder have the following defects: (1) Uncoated ordinary aluminum powder is prone to moisture and caking, has poor dispersibility in the coating glaze, and has small specific surface area, which is easy to sink, resulting in uneven distribution of aluminum powder in the coating process of enamel coating, uneven metal texture, and cracking of enamel coating in areas with less aluminum powder. Moreover, uncoated aluminum powder is easily oxidized during high temperature sintering of enamel coating, which causes yellowing of the coating and affects the quality of the enamel coating.

[0006] (2) Although the aluminum sheet containing mineral oil solves the problem of small specific surface area and easy sinking, the type of mineral oil used by different aluminum sheet manufacturers will be different, and the polarity difference of the mineral oil limits the enamel coating glaze during coating, and the comprehensive performance of the prepared enamel coating is difficult to achieve the best effect. The aluminum sheet containing mineral oil also has the problem of oxidation and yellowing. During the curing process of coating preparation of enamel coating, the mineral oil eventually volatilizes in the atmosphere, which will also affect the environment; at the same time, with the water-based of enamel coating, the aluminum sheet containing mineral oil will react with water in the coating containing water, which will cause serious safety risks such as tank swelling.

[0007] Therefore, it is urgent to provide a coating preparation method capable of uniformly distributing aluminum powder in the enamel coating, avoiding the risk of coating cracking due to uneven distribution of aluminum powder and the problem of yellowing of the coating caused by oxidation of aluminum powder, and ensuring that the addition of aluminum powder can bring the above performance advantages to the enamel coating.

[0008] The prior art modifies the aluminum powder based on the defect that the aluminum powder is easy to cake after being damp. The common modification method is to modify the aluminum powder by oleic acid to form an aluminum oleate salt coating layer on the surface of the aluminum powder, or to modify the aluminum powder by oxidation to form an aluminum oxide coating layer on the surface of the aluminum powder. Although the above modification process delays the caking of aluminum powder after being damp, and the aluminum powder is not easy to disperse, the compactness of the modification layer is limited, and the aluminum hydroxyl is always partially exposed and cannot be fully coated. Once improperly stored, the aluminum powder fully absorbs moisture in the air, and there is still a risk of caking and being difficult to disperse, and the corrosion resistance has not been effectively solved. SUMMARY

[0009] The purpose of the present application is to provide a preparation method of an enamel coating with a goniochromatic metallic luster for solving the defects of uneven distribution of aluminum powder and oxidation yellowing in the existing enamel coating with added aluminum powder during the coating preparation process. The preparation method solves the problems of uneven distribution of aluminum powder and oxidation yellowing in the aluminum-containing enamel coating, so that the enamel coating has a uniform metallic texture, and the high temperature resistance, corrosion resistance and weather resistance are greatly improved.

[0010] The specific technical solutions are as follows: A preparation method of an enamel coating with a goniochromatic metallic luster, comprising the following steps: (1) Pretreatment of the substrate: degreasing and pickling the metal substrate.

[0011] The degreasing and pickling of the metal substrate can be performed according to the conventional operation. The specific operation of degreasing is as follows: first, remove the surface oil of the metal substrate with an organic solvent; then immerse the metal substrate in an alkaline degreasing solution at 70-90°C for 10-15 min to remove residual oil stains; finally, wash with deionized water until neutral.

[0012] The specific operation of pickling is as follows: first, immerse the degreased metal substrate in a hydrochloric acid solution with a concentration of 5-10 wt% at 35-45°C for 5-10 min to remove surface oxide scale and rust; then rinse with deionized water; and finally dry.

[0013] (2) Preparation of enamel glaze containing aluminum powder pigment: First, mix the enamel powder with deionized water to obtain an enamel powder slurry. The mass ratio of the enamel powder to deionized water can be 1:(0.3-0.5).

[0014] Then, the aluminum powder pigment is added into the obtained enamel slurry, and bentonite, sodium silicate, boric acid and deionized water are added into the enamel slurry, and ball milling is performed to obtain the enamel glaze containing the aluminum powder pigment; wherein the solid content of the obtained enamel glaze is controlled to be 60-65%.

[0015] The mass ratio of the aluminum powder pigment to the enamel powder is (0.15-0.25):1.

[0016] The aluminum powder pigment is prepared by the following steps: S1, 1 part of raw aluminum powder is added into a reaction kettle, mixed with alcohol ether solvent, heated and stirred, and then filtered to obtain an aluminum powder dispersion liquid with a solid content of 70-75wt%.

[0017] S2, the obtained aluminum powder dispersion liquid is mixed with 4-5 parts of alcohol ether solvent, stirred and uniformly dispersed to obtain a mixed liquid a. A part of silane coupling agent accounting for 35-45% of the total weight of the silane coupling agent is added dropwise into the mixed liquid a, and uniformly dispersed to obtain a mixed liquid b; wherein the total weight of the silane coupling agent is 0.02-0.1 parts.

[0018] S3, the pH adjusting agent is diluted with deionized water; then added into the mixed liquid b to adjust the pH value of the mixed liquid b to 7.5-11.0.

[0019] The pH adjusting agent can be selected from one or more of ammonia, ethanolamine, ethylenediamine, glycolamine or dimethyl ethanolamine.

[0020] S4, 0.1-0.5 parts of tetraethyl orthosilicate is uniformly diluted with the same weight of alcohol ether solvent; then the diluted tetraethyl orthosilicate is added dropwise into the mixed liquid b with a pH value of 7.5-11.0, and heated to 60-70℃ for 1-4h to obtain a mixed liquid c.

[0021] S5, the remaining silane coupling agent is added dropwise into the mixed liquid c, and after the dropwise addition is completed, the reaction is continued for 1-10h. After the reaction is completed, the obtained silica-coated aluminum silver paste is filtered, washed, dried and sieved to obtain the aluminum powder pigment.

[0022] (3) one-time dip coating: control the pulling angle of the metal substrate to be 30-40°, and immerse the obtained enamel glaze containing the aluminum powder pigment in step (2) at a uniform speed of 10-12cm / min to complete one-time dip coating. After one-time dip coating is completed, gradient temperature drying and curing are adopted: first, heat preservation at 50-65℃ for 15-20min; then, heat preservation at 75-125℃ for 25-35min; finally, heat preservation at 135-145℃ for 10-15min; to obtain a metal substrate with one-time dip coating layer.

[0023] (4) Spraying: spraying the enamel glaze containing the aluminum powder pigment on the once-dipped layer; After the spraying is completed, the enamel glaze is first dried at 100-120℃, and then first-fired at 550-560℃ in a nitrogen atmosphere, and cooled to room temperature at a rate of 2-3℃ / min.

[0024] (5) Second dipping: the metal substrate after the first firing is dipped into the enamel glaze containing the aluminum powder pigment obtained in step (2) at a pulling angle of 15-25° and a dipping speed of 6-8cm / min, to complete the second dipping; After the second dipping is completed, the enamel glaze is first dried at 110-115℃, and then second-fired at 500-550℃ in a nitrogen atmosphere; After the firing is completed, the enamel glaze is cooled to room temperature at a rate of 2-3℃ / min, to form the enamel coating with the metal luster of the angle-dependent color on the metal substrate.

[0025] In the research of the process for preparing the enamel coating, it is found that, in order to ensure that the enamel coating has the high-end metal luster of the angle-dependent color, and at the same time, the compactness of the enamel coating is enhanced, the weather resistance and the oil stain resistance are improved, and the cracking phenomenon is avoided, the following aspects need to be improved in coordination, which are specifically described as follows: On the one hand, the aluminum powder pigment used in the process for preparing the enamel coating is the key to the metal luster of the angle-dependent color of the enamel coating, but the conventional aluminum powder pigment doped in the water-based enamel slurry is subjected to high-temperature treatment, and the light loss rate of the aluminum powder pigment itself is high, which finally leads to poor metal luster in the enamel coating, and the high-end metal luster of the angle-dependent color cannot be well exhibited. Therefore, in the research of the process for preparing the enamel coating, the aluminum powder pigment used in the process for preparing the enamel coating is further studied, and it is found in the research that the aluminum powder pigment prepared by the above method is used in the process for preparing the enamel coating, the aluminum powder pigment can be uniformly dispersed in the water-based enamel slurry; more importantly, compared with the traditional aluminum powder pigment, the light loss rate of the aluminum powder pigment is reduced by 25-30%; compared with the enamel coating containing the aluminum powder, the glossiness of the enamel coating is improved by 24-27%. The natural weather resistance test is conducted for 3 months, and the glossiness retention rate of the enamel coating is improved by 20-25%.

[0026] On the other hand, on the basis of the aluminum powder pigment, the coating forming process of the enamel coating is also crucial to the compactness of the enamel coating. The present application research found that the metal substrate was sequentially subjected to one-time immersion coating, gradient temperature drying and curing instead of firing, spraying on the one-time immersion coating, first firing after spraying, two-time immersion coating, the immersion angle and immersion speed of which were different from the one-time immersion, and finally second firing, to obtain the enamel coating. The enamel coating obtained by the coating process can maintain more than 95% of the coating integrity after baking at 200 DEG C for 300 h, and the salt spray life of the enamel coating is prolonged by 30-35% compared with the traditional enamel coating.

[0027] In the present application, the raw material composition of the enamel powder in step (2) of the preparation method of the enamel coating with the metallic luster of goniochromatic color includes 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.

[0028] In the present application, the preparation method of the enamel coating with the metallic luster of goniochromatic color, the addition amount of bentonite in step (2) is 0.8-1.2% of the mass of the enamel powder, the addition amount of sodium silicate is 0.5-1.0% of the mass of the enamel powder, and the addition amount of boric acid is 1.0-1.5% of the mass of the enamel powder.

[0029] In the present application, the viscosity of the enamel frit in step (2) of the preparation method of the enamel coating with the metallic luster of goniochromatic color is 800-900 mPa·s. Controlling the viscosity of the enamel frit in this range lays a foundation for the formation of the immersion coating in the subsequent two-time immersion, especially the one-time immersion coating formed by the first-time immersion, which plays a crucial role in the spraying effect of the next step.

[0030] In the present application, the particle size of the enamel powder in step (2) of the preparation method of the enamel coating with the metallic luster of goniochromatic color is 40-60 μm, and the particle size of the aluminum powder pigment is 30-35 μm. The particle size is detected by a laser particle size analyzer. The particle size ratio between the aluminum powder pigment and the enamel powder is 1:(1.1-2). Such grading can improve the dispersibility of the enamel frit, make the aluminum powder pigment more uniformly dispersed and less prone to agglomeration, form a uniform metallic reflective layer in the enamel coating, and improve the metallic luster and hiding power of the enamel coating. On the other hand, the grading can reduce the pinhole or bubble defects caused by voids during the firing of the enamel. Moreover, under this grading, the enamel powder will form a continuous glaze layer after melting, and the aluminum powder pigment can still maintain part of the flaky structure in the glaze layer, balancing the compactness of the glaze layer and the metallic texture.

[0031] In the application, the preparation method of the enamel coating with goniochromatic metal luster, the step (4) controls the spraying line speed 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. Through testing, it is found that the spraying effect of the obtained primary dip coating can be good according to the spraying line speed, distance and pressure.

[0032] In the application, the preparation method of the enamel coating with goniochromatic metal luster, the step (2) uses at least one of methacryloyloxypropyltrimethoxysilane, aminopropyltrimethoxysilane, vinyltriethoxysilane, glycidyl ether oxypropyltrimethoxysilane or octyltriethoxysilane as the silane coupling agent.

[0033] In the application, the preparation method of the enamel coating with goniochromatic metal luster, the step (2) uses one or a mixture of several of ethanol, methanol, propylene glycol methyl ether or diethylene glycol monomethyl ether as the alcohol ether solvent.

[0034] The aluminum powder pigment used in the preparation method of the enamel coating has a substrate of aluminum powder with a particle size of 1.0-70.0 μm, and has a coating layer with a thickness of 50-100 nm outside the substrate, which is composed of a first coating of a silane coupling agent, a coating of tetraethyl orthosilicate at 60-70 ℃ and a second coating of a silane coupling agent. The weight ratio of the silane coupling agent used for the first coating to the total silane coupling agent used for the coating is (0.35-0.45):1.

[0035] Although the traditional aluminum powder pigment has been improved in corrosion resistance by coating, its light loss rate is also greatly increased. In order to make the aluminum powder pigment play a more excellent metal luster in application, how to improve the corrosion resistance and other properties while ensuring the metal luster and reducing the light loss rate as much as possible has become a technical problem to be solved.

[0036] In the research process of the application, it is found that if the coating of the coating layer of the aluminum powder pigment is formed by first using a part of the silane coupling agent for the first coating, then using tetraethyl orthosilicate for the coating, and then using the remaining silane coupling agent for the second coating, the light loss rate of the obtained aluminum powder pigment is reduced compared with the traditional aluminum powder pigment, but the reduction is still not ideal, and the improvement effect of the corrosion resistance of the aluminum powder pigment is not good. Therefore, based on this, the application further researches and finds that the amount of the silane coupling agent used for the two times of silane coupling agent coating and the temperature of the coating of tetraethyl orthosilicate based on the first coating of the silane coupling agent play a key role in reducing the light loss rate of the aluminum powder pigment and improving the corrosion resistance at the same time.

[0037] The present application finds 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 obtained aluminum powder pigment can simultaneously have relatively excellent metallic luster and corrosion resistance. If the amount of the silane coupling agent used in the first coating is too low, the light loss rate of the aluminum powder pigment decreases by a relatively small amount; and if the amount of the silane coupling agent used in the first coating is too high, the compactness of the final aluminum powder pigment coating layer decreases, and the corrosion resistance cannot achieve a relatively good effect.

[0038] Meanwhile, on the basis of the first silane coupling agent coating, the coating temperature of the tetraethyl orthosilicate is also very critical. If the temperature is lower than the range, the compactness of the obtained aluminum powder pigment coating layer does not reach a relatively ideal state.

[0039] When the weight ratio of the silane coupling agent used in the first coating to the total silane coupling agent used in the coating, and the coating temperature of the tetraethyl orthosilicate are simultaneously within the above respective ranges, the light loss rate of the obtained aluminum powder pigment is reduced by 25-30% compared with that of a traditional aluminum powder pigment. The settling time of the aluminum powder pigment in a water-based enamel is 2-3 times that of the traditional aluminum powder pigment. The luster retention rate after a 3-month weather resistance test is increased by 25-35%, the aluminum powder pigment can maintain stable performance in an alkaline environment with a pH value of 11-12 without generating swelling, and the corrosion resistance time in a salt spray test can be extended by 30-35%.

[0040] In addition, the second silane coupling agent coating further improves the dispersion uniformity of the aluminum powder pigment and the water-based enamel, and improves the interfacial compatibility of the aluminum powder pigment and the coating system.

[0041] The preparation method of the above-mentioned aluminum powder pigment comprises the following steps: (1) 1 part of raw aluminum powder and 3-4 parts of alcohol ether solvent are added to a reaction kettle, stirred and heated to 60-65°C, and then constant temperature is maintained for 2-3 h, and then filtration is controlled to obtain an aluminum powder dispersion liquid with a solid content of 70-75%.

[0042] (2) The aluminum powder dispersion liquid obtained in step (1) is mixed with 4-5 parts of alcohol ether solvent and added to a stirrer, uniformly dispersed at a temperature of 30-45°C, and a mixed liquid a is obtained. Part of the silane coupling agent is added dropwise to the mixed liquid a, the dropwise time is controlled at 0.5-1 h, and after the dropwise addition is completed, the dispersion is continued for 1-4 h to obtain a mixed liquid b. The total weight of the silane coupling agent is 0.02-0.1 parts, and the amount of the silane coupling agent added in this step is 35-45% of the total weight.

[0043] The principle chemical formula of the silane coupling agent coated aluminum powder is: R-Si(OR')3+3H2O→R-Si(OH)3+3R'OH; nR-Si(OH)3→[R-SiO 1.5 ] n +1.5nH2O; .

[0044] (3) first, the pH regulator is diluted with deionized water; then added to the mixed solution b, adjust the pH value of the mixed solution b to 7.5-11.0.

[0045] (4) first, 0.15-0.5 parts of tetraethyl orthosilicate is uniformly diluted with the same weight fraction of alcohol ether solvent; then the diluted tetraethyl orthosilicate is added dropwise to the mixed solution b with a pH value of 7.5-11.0, and the dropwise time is controlled to be 0.5-1h; the temperature is raised to 60-70 DEG C and reacted for 1-4h to obtain the mixed solution c.

[0046] The principle chemical formula of tetraethyl orthosilicate coated aluminum powder is: Si(OC2H5)4+4H2O→Si(OH)4+4C2H5OH; .

[0047] (5) the remaining silane coupling agent is added dropwise to the mixed solution c, and the dropwise time is controlled to be 0.5-1h; after the dropwise addition is completed, the reaction is continued for 1-10h; After the reaction is completed, the obtained silica coated aluminum silver paste is filtered, washed, dried and sieved to obtain the aluminum powder pigment.

[0048] The specific operation of drying and sieving is as follows: the residual solvent in the aluminum powder is removed in a vacuum drying oven at 70 DEG C-150 DEG C; then sieved using a 400-500 mesh screen to obtain the aluminum powder pigment.

[0049] Compared with the traditional silica coated aluminum powder process, by the addition order of the above preparation method, part of the silane coupling agent is coated once→tetraethyl orthosilicate is coated→the remaining silane coupling agent is coated twice, the coating layer prepared is more ordered.

[0050] The beneficial effects of the present application are: the preparation method of the enamel coating with the goniochromatic metal luster solves the problems of uneven distribution of aluminum powder in the aluminum-doped enamel coating and yellowing oxidation, so that the enamel coating has a uniform metal texture, and the high temperature resistance, corrosion resistance and weather resistance are greatly improved.

[0051] The enamel coating obtained by the preparation method has high end goniochromatic metal luster, and the enamel coating is densified, the weather resistance is improved, the oil stain resistance is improved, and the cracking phenomenon is avoided. Compared with the current enamel coating containing aluminum powder, the gloss of the enamel coating is improved by 24-27%. The gloss retention rate of the enamel coating is improved by 20-25% after natural weather resistance test for 3 months. The enamel coating obtained by the coating process is baked at 200℃ for 300h, and the coating integrity is maintained at more than 95%; compared with the traditional enamel coating, the salt spray life of the enamel coating is prolonged by 30-35% in the salt spray test.

[0052] In recent years, more and more attention has been paid to environmental problems, and powder coatings have been paid more and more attention due to the characteristics of not containing solvents, being very friendly to the environment and the like. The conversion of VOC liquid coatings to powder coatings has become an inevitable trend in future development. The aluminum powder pigment can well solve the technical problems of not easy to disperse in powder coatings, easy to be damp and caked, serious corrosion resistance and the like. After testing, the aluminum powder pigment is stored in the open air at room temperature and in a cool place for at least 1 year, and then made into a powder coating, which will not cause powder spitting, poor dispersibility and other problems in the powder coating construction process. And compared with the traditional aluminum salt coated aluminum powder of oleic acid, the corrosion resistance of the aluminum powder pigment is also significantly improved, and the aluminum powder pigment does not react with water at all, and can be applied to water-based coatings. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 The scanning electron microscope picture of the aluminum powder pigment at 2μm in the present application.

[0054] Figure 2 The scanning electron microscope picture of the aluminum powder pigment at 1μm in the present application. DETAILED DESCRIPTION

[0055] 1. 60° gloss: refer to standard GB / T 9754-2007, and detect by gloss meter.

[0056] 2. Test of gloss retention rate after aging: refer to GB / T 1865-2009, and the test sample is aged for 10 days by xenon lamp aging test box; refer to standard GB / T 9754-2007, and detect 60° gloss by gloss meter. The gloss retention rate after aging is calculated according to gloss retention rate (%)=(gloss value of coating after aging / gloss value of coating before aging)×100%.

[0057] 3. Test of salt spray resistance of enamel coating: refer to acetic acid accelerated salt spray test (AASS) 1000h specified in GB / T 10125-2021, and observe the corrosion of the surface of the enamel coating.

[0058] 4. High temperature resistance: Put the test sample into the oven, bake at 200℃ for 300h, and observe the integrity of the enamel coating.

[0059] 5. Test of light loss rate of aluminum powder pigment: UV lamp tube (UVA-340, 340nm, irradiation intensity 0.71W / m²), black standard temperature (60±3)℃, relative humidity (50±5)%RH, aging time 10 days, reference standard GB / T 9754-2007, use gloss meter to detect 60° angle gloss. According to the light loss rate = (initial gloss- after test gloss) / initial gloss × 100%, the light loss rate of aluminum powder pigment is calculated.

[0060] 6. Test of salt spray resistance of aluminum powder pigment: observe the appearance of aluminum powder pigment according to the neutral salt spray (NSS) 168h specified in GB / T 10125-2021.

[0061] 7、 Figure 1 In the figure, SU8010 represents the model of scanning electron microscope; 5.0kV represents that the acceleration voltage of electron beam is 5.0 kilovolt; 9.0mm is the working distance, i.e. the distance from sample stage to objective lens; x20.0k represents that the magnification is 20000 times; SE(U) in SE represents secondary electron imaging, and U represents unaccelerated voltage mode; 2.00μm is the length of the scale below the image.

[0062] 8、 Figure 2 In the figure, SU8010 represents the model of scanning electron microscope; 5.0kV represents that the acceleration voltage of electron beam is 5.0 kilovolt; 9.0mm is the working distance, i.e. the distance from sample stage to objective lens; x50.0k represents that the magnification is 50000 times; SE(U) in SE represents secondary electron imaging, and U represents unaccelerated voltage mode; 1.00μm is the length of the scale below the image.

[0063] Example 1 The preparation method of the enamel coating with the metal luster of goniochromaticity is as follows: (1) Pretreatment of the substrate: first remove the surface oil of the metal substrate with an organic solvent; then immerse it in an alkaline degreasing solution at 70℃ for 15min to remove the residual oil stains; finally wash it with deionized water until it is neutral. Immerse the degreased metal substrate in a 5wt% hydrochloric acid solution at 35℃ for 10min to remove the surface oxide scale and rust; rinse with deionized water; dry; complete the degreasing and pickling pretreatment of the metal substrate.

[0064] (2) Preparation of enamel glaze containing aluminum powder pigment: 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 an enamel powder slurry. The raw material composition of the enamel powder is 60.5 wt% of silicon dioxide, 15.5 wt% of boron oxide, 8.5 wt% of sodium oxide, 10.5 wt% of calcium oxide, and 5 wt% of barium oxide.

[0065] Then, aluminum powder pigment with a particle size of 35 μm is added to the obtained enamel powder slurry, wherein the mass ratio of aluminum powder pigment to enamel powder is 0.25:1.

[0066] And bentonite with a mass of 1.2% of the enamel powder, sodium silicate with a mass of 1.0% of the enamel powder, boric acid with a mass of 1.0% of the enamel powder, and deionized water are added to the enamel powder slurry, and the mixture is ball milled to control the solid content of the enamel glaze to 65% and the viscosity of the enamel glaze to 900 mPa·s, to obtain an aluminum powder pigment-containing enamel glaze.

[0067] (3) First dip coating: control the pulling angle of the metal substrate to be 40°, and immerse the obtained aluminum powder pigment-containing enamel glaze of step (2) at a uniform speed of 10 cm / min to complete the first dip coating.

[0068] After the first dip coating is completed, gradient temperature drying and curing are adopted: first, heat at 50℃ for 20 min; then, heat to 75℃ for 25 min; finally, heat to 135℃ for 10 min; to obtain a metal substrate with a first dip coating layer.

[0069] (4) Spraying: spray the aluminum powder pigment-containing enamel glaze on the first dip coating layer; control the spraying line speed to be 14 cm / s, the spraying distance to be 20 cm, and the spraying pressure to be 0.5 MPa.

[0070] After the spraying is completed, first, dry at 100℃; then, complete the first firing at 550℃ in a nitrogen atmosphere, and cool to room temperature at a rate of 2℃ / min.

[0071] (5) Second dip coating: immerse the metal substrate after the first firing into the aluminum powder pigment-containing enamel glaze of step (2) at a pulling angle of 25° and a dip coating speed of 8 cm / min to complete the second dip coating.

[0072] After the second dip coating is completed, first, dry at 115℃; then, complete the second firing at 500℃ in a nitrogen atmosphere; After the firing is completed, cool to room temperature at a rate of 2℃ / min, and coat the metal substrate to form an enamel coating with an angle-dependent color-changing metal luster.

[0073] The aluminum powder pigment is prepared by the following steps: (1) Add 1 part of raw material aluminum powder and 3 parts of diethylene glycol monomethyl ether into a reaction kettle, stir and heat to 60°C, and keep constant temperature for 3 h. Then, control the filtration to obtain an aluminum powder dispersion liquid with a solid content of 72%.

[0074] (2) Mix the aluminum powder dispersion liquid obtained in step (1) with 5 parts of diethylene glycol monomethyl ether into a stirrer, heat to 30°C, and disperse uniformly to obtain a mixed liquid a.

[0075] Then, add 0.03 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane (37.5% of 0.08 parts of total weight of silane coupling agent) dropwise into the mixed liquid a, control the dropwise time to be 0.5 h, continue to disperse for 1 h after the dropwise addition is completed, and obtain a mixed liquid b.

[0076] (3) Dilute the pH regulator ethylenediamine with deionized water, and then add it into the mixed liquid b to adjust the pH value of the mixed liquid b to 8.5.

[0077] (4) Dilute 0.15 parts of tetraethyl orthosilicate with 0.15 parts of diethylene glycol monomethyl ether uniformly, and then add the diluted tetraethyl orthosilicate into the mixed liquid b with a pH value of 8.5, control the dropwise time to be 1 h, heat to 60°C, and react for 2 h to obtain a mixed liquid c.

[0078] (5) Add the remaining 0.05 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane dropwise into the mixed liquid c, control the dropwise time to be 1 h, and continue to react for 8 h after the dropwise addition is completed.

[0079] After the reaction is completed, the obtained silica-coated aluminum paste is filtered by a flat plate, washed with diethylene glycol monomethyl ether for 2 times, dried in an oven at 140°C for 2 h, and screened by a 500-mesh vibrating screen to obtain the aluminum powder pigment with a coating layer having a thickness of 70 nm.

[0080] Example 2 The preparation method of the enamel coating with the metal luster of the goniochromatic color is specifically as follows: (1) Substrate pretreatment: first, remove the surface oil of the metal substrate by using an organic solvent; then, immerse the metal substrate in an alkaline degreasing solution at 70°C for 15 min to remove the residual oil stains; finally, wash the degreased metal substrate with deionized water until it is neutral. Immerse the degreased metal substrate in a 5wt% hydrochloric acid solution at 35°C for 10 min to remove the surface oxide skin and rust, wash with deionized water, and dry to complete the degreasing and pickling pretreatment of the metal substrate.

[0081] (2) Preparation of enamel glaze containing aluminum powder pigment: First, enamel powder with a particle size of 60 μm was mixed with deionized water at a mass ratio of 1:0.5 to obtain an enamel powder slurry. The raw material composition of the enamel powder was 60.5 wt% of silicon dioxide, 15.5 wt% of boron oxide, 8.5 wt% of sodium oxide, 10.5 wt% of calcium oxide, and 5 wt% of barium oxide.

[0082] Then, aluminum powder pigment with a particle size of 30 μm was added to the obtained enamel powder slurry, wherein the mass ratio of aluminum powder pigment to enamel powder was 0.15:1.

[0083] And bentonite with a mass of 1.2% of the enamel powder, sodium silicate with a mass of 1.0% of the enamel powder, boric acid with a mass of 1.0% of the enamel powder, and deionized water were added to the enamel powder slurry, and the obtained enamel slurry was ball-mixed to control the solid content of the enamel slurry to be 60%, and the viscosity of the enamel slurry to be 800 mPa·s, to obtain an enamel slurry containing aluminum powder pigment.

[0084] (3) First dip coating: the pulling angle of the metal substrate was controlled to be 30°, and the metal substrate was dipped into the obtained enamel slurry containing aluminum powder pigment in step (2) at a dipping speed of 12 cm / min to complete the first dip coating.

[0085] After the first dip coating was completed, gradient temperature rising drying and curing were adopted: first, the temperature was kept at 65℃ for 15 min; then, the temperature was raised to 125℃ for 35 min; finally, the temperature was raised to 145℃ for 15 min; to obtain a metal substrate with a first dip coating layer.

[0086] (4) Spraying: the obtained enamel slurry containing aluminum powder pigment was sprayed on the first dip coating layer; the spraying line speed was controlled to be 12 cm / s, the spraying distance was 25 cm, and the spraying pressure was 0.3 MPa.

[0087] After the spraying was completed, the metal substrate was first dried at 120℃; then, the first firing was completed in a nitrogen atmosphere at 560℃, and the metal substrate was cooled to room temperature at a rate of 3℃ / min.

[0088] (5) Second dip coating: the metal substrate after the first firing was dipped into the obtained enamel slurry containing aluminum powder pigment in step (2) at a pulling angle of 15° and a dipping speed of 6 cm / min to complete the second dip coating.

[0089] After the second dip coating was completed, the metal substrate was first dried at 110℃; then, the second firing was completed in a nitrogen atmosphere at 550℃.

[0090] After the firing was completed, the metal substrate was cooled to room temperature at a rate of 3℃ / min, and a porcelain enamel coating with angle-dependent color metal luster was formed on the metal substrate.

[0091] The aluminum powder pigment of this example was the same as the aluminum powder pigment obtained in Example 1.

[0092] Example 3 The difference between this example and Example 1 is that in the preparation of the aluminum powder pigment of this example, 0.1 parts of tetraethyl orthosilicate is added in step (4) and the temperature is raised to 70°C for 2h.

[0093] The rest is the same as Example 1.

[0094] Example 4 The difference between this example and Example 1 is that in the preparation of the aluminum powder pigment of this example, the pH value of the mixed solution b is adjusted to 10.0 in step (3).

[0095] The difference between this example and Example 1 is that in the preparation of the aluminum powder pigment of this example, 0.1 parts of tetraethyl orthosilicate is added in step (4) and the temperature is raised to 70°C for 2h.

[0096] The rest is the same as Example 1.

[0097] Example 5 The difference between this example and Example 1 is that in the preparation of the aluminum powder pigment of this example, 0.036 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane (45% of 0.08 parts of total weight of silane coupling agent) is added dropwise to the mixed solution a in step (2).

[0098] The difference between this example and Example 1 is that in the preparation of the aluminum powder pigment of this example, 0.1 parts of tetraethyl orthosilicate is added in step (4) and the temperature is raised to 70°C for 2h.

[0099] The rest is the same as Example 1.

[0100] Example 6 The difference between this example and Example 1 is that in the preparation of the aluminum powder pigment of this example, 0.03 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane (37.5% of 0.08 parts of total weight of silane coupling agent) is added dropwise to the mixed solution a in step (2).

[0101] The difference between this example and Example 1 is that in the preparation of the aluminum powder pigment of this example, 0.1 parts of tetraethyl orthosilicate is added in step (4) and the temperature is raised to 70°C for 2h.

[0102] The rest is the same as Example 1.

[0103] Comparative Example 1 The difference between this example and Example 1 is that in the preparation of the aluminum powder pigment of this example, 0.1 parts of tetraethyl orthosilicate is added in step (4) and the temperature is raised to 70°C for 2h.

[0104] The specific steps are as follows: (1) 1 part of raw material aluminum powder and 3 parts of diethylene glycol monomethyl ether were added into a reaction kettle, stirred and heated to 60°C, and then constant temperature was maintained for 3 h. After filtration, an aluminum powder dispersion liquid with a solid content of 72% was obtained.

[0105] (2) The aluminum powder dispersion liquid obtained in step (1) was mixed with 5 parts of diethylene glycol monomethyl ether and added into a stirrer, uniformly dispersed at a temperature of 30°C, and a mixed liquid a was obtained.

[0106] (3) The pH regulator ethylenediamine was diluted with deionized water, and then added into the mixed liquid a to adjust the pH value of the mixed liquid a to 8.5.

[0107] (4) 0.15 parts of tetraethyl orthosilicate were uniformly diluted with 0.15 parts of diethylene glycol monomethyl ether, and then the diluted tetraethyl orthosilicate was added dropwise into the mixed liquid a with a pH value of 8.5, the dropwise adding time was controlled to be 1 h, the temperature was increased to 60°C, and the reaction was carried out for 2 h to obtain a mixed liquid b.

[0108] (5) 0.03 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane (37.5% of the total weight of the silane coupling agent, 0.08 parts) were added dropwise into the mixed liquid b, the dropwise adding time was controlled to be 0.5 h, and after the dropwise adding was completed, the dispersion was continued for 1 h to obtain a mixed liquid c.

[0109] (6) The remaining 0.05 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane were added dropwise into the mixed liquid c, the dropwise adding time was controlled to be 1 h, and after the dropwise adding was completed, the reaction was continued for 8 h.

[0110] After the reaction was completed, the obtained silica-coated aluminum paste was subjected to flat plate filtration, and was washed twice with diethylene glycol monomethyl ether. The mixture was dried in an oven at 140°C for 2 h, and was sieved with a 500-mesh vibrating screen to obtain the aluminum powder pigment with a coating layer having a thickness of 70 nm.

[0111] The other steps were the same as in Example 1.

[0112] Comparative Example 2 The difference between the present comparative example and Example 1 is that in the present comparative example, the preparation of the aluminum powder pigment was carried out by mixing and adding the tetraethyl orthosilicate and the silane coupling agent together for coating.

[0113] The specific steps are as follows: (1) 1 part of raw material aluminum powder and 3 parts of diethylene glycol monomethyl ether were added into a reaction kettle, stirred and heated to 60°C, and then constant temperature was maintained for 3 h. After filtration, an aluminum powder dispersion liquid with a solid content of 72% was obtained.

[0114] (2) The aluminum powder dispersion liquid obtained in step (1) was mixed with 5 parts of diethylene glycol monomethyl ether and added into a stirrer, uniformly dispersed at a temperature of 30°C, and a mixed liquid a was obtained.

[0115] (3) The pH regulator ethylenediamine is diluted with deionized water, and then added to the mixed solution a to adjust the pH value of the mixed solution a to 8.5.

[0116] (4) 0.15 parts of tetraethyl orthosilicate, 0.15 parts of diethylene glycol monomethyl ether, and 0.08 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane are mixed and added to the mixed solution a with a pH value of 8.5, and the dropping time is controlled to be 1 h; the temperature is raised to 60°C and reacted for 10 h.

[0117] After the reaction is completed, the obtained silica-coated aluminum silver paste is subjected to flat plate suction filtration, and is washed with diethylene glycol monomethyl ether for 2 times. Drying is performed in an oven at 140°C for 2 h, and screening is performed with a 500-mesh vibrating screen to obtain the aluminum powder pigment with a thickness of 70 nm of the coating layer.

[0118] The other steps are the same as in Example 1.

[0119] Comparative Example 3 The difference between the present comparative example and Example 1 is that the preparation of the aluminum powder pigment in the present comparative example is first coated with a silane coupling agent, and then coated with tetraethyl orthosilicate.

[0120] The specific steps are as follows: (1) 1 part of raw aluminum powder and 3 parts of diethylene glycol monomethyl ether are added to a reaction kettle, and stirring is performed to heat to 60°C, and after constant temperature for 3 h, suction filtration is controlled to obtain an aluminum powder dispersion liquid with a solid content of 72%.

[0121] (2) The aluminum powder dispersion liquid obtained in step (1) is mixed with 5 parts of diethylene glycol monomethyl ether and added to a stirrer, and the temperature is raised to 30°C to disperse uniformly to obtain a mixed solution a.

[0122] Then, 0.03 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane (37.5% of 0.08 parts of the total weight of the silane coupling agent) is added dropwise to the mixed solution a, and the dropping time is controlled to be 0.5 h, and after the dropping is completed, dispersion is continued for 1 h to obtain a mixed solution b.

[0123] (3) The remaining 0.05 parts of silane coupling agent γ-glycidoxypropyltrimethoxysilane is added dropwise to the mixed solution b, and the dropping time is controlled to be 1 h; after the dropping is completed, the reaction is continued for 8 h; and a mixed solution c is obtained.

[0124] (4) The pH regulator ethylenediamine is diluted with deionized water, and then added to the mixed solution c to adjust the pH value of the mixed solution c to 8.5.

[0125] (5) 0.15 parts of tetraethyl orthosilicate was diluted uniformly with 0.15 parts of diethylene glycol monomethyl ether; then the diluted tetraethyl orthosilicate was added dropwise into the mixed solution c with a pH value of 8.5, and the dropwise adding time was controlled to be 1 h; the temperature was raised to 60°C and reacted for 2 h.

[0126] After the reaction was completed, the obtained silica-coated aluminum silver paste was subjected to flat plate suction filtration; and was washed with diethylene glycol monomethyl ether for 2 times. Drying was performed in an oven at 140°C for 2 h, and screening was performed with a 500-mesh vibrating screen to obtain the aluminum powder pigment with a thickness of 70 nm of the coating layer.

[0127] The other steps were the same as those in Example 1.

[0128] Comparative Example 4 The difference between the present comparative example and Example 1 is that in the preparation step (2) of the aluminum powder pigment of the present comparative example, 0.05 parts of the silane coupling agent γ-glycidoxypropyltrimethoxysilane (62.5% of 0.08 parts of the total weight of the silane coupling agent) was added dropwise into the mixed solution a.

[0129] In the preparation step (5) of the aluminum powder pigment of the present example, the remaining 0.03 parts of the silane coupling agent octyltriethoxysilane was added dropwise into the mixed solution c.

[0130] The other steps were the same as those in Example 1.

[0131] Comparative Example 5 The difference between the present comparative example and Example 1 is that in the preparation step (4) of the aluminum powder pigment of the present comparative example, the temperature was raised to 50°C and reacted for 2 h.

[0132] The other steps were the same as those in Example 1.

[0133] After the obtained aluminum powder pigment was stored in an open place at room temperature for 1 year, the powder paint was prepared again, and the powdering problem occurred in the powder paint construction process, and the dispersibility was extremely poor.

[0134] Comparative Example 6 The difference between the present comparative example and Example 1 is that in the step (3) of the enamel coating preparation method of the present comparative example, after the first dip coating was completed, the first firing was performed: drying at 100°C first; and then the first firing was completed in a nitrogen atmosphere at 550°C, and the temperature was cooled to room temperature at a rate of 2°C / min.

[0135] In the step (4) of the enamel coating preparation method, after the spraying was completed, drying was performed at 100°C first; and then the second firing was completed in a nitrogen atmosphere at 550°C, and the temperature was cooled to room temperature at a rate of 2°C / min.

[0136] In the step (5) of the enamel coating preparation method, after the second dip coating was completed, drying was performed at 115°C first; and then the third firing was completed in a nitrogen atmosphere at 500°C.

[0137] Comparative Example 7 The difference between the present comparative example and Example 1 is that the particle size of the enamel powder in step (2) of the preparation method of the enamel coating is 35 μm, and the particle size of the aluminum powder pigment is 50 μm.

[0138] Comparative Example 8 The difference between the present comparative example and Example 1 is that the pulling angle of the metal substrate is controlled to be 40° and the dipping speed is controlled to be 10 cm / min in the dipping process in step (3) and step (5) of the preparation method of the enamel coating.

[0139] The performance indexes of the enamel coatings obtained in each of the examples and comparative examples are shown in Table 1.

[0140] Table 1 Performance indexes of each of the enamel coatings

[0141] The performance indexes of the aluminum powder pigments obtained in each of the examples and comparative examples are shown in Table 2.

[0142] Table 2 Performance indexes of each of the aluminum powder pigments

[0143] It can be seen that the enamel coating obtained by the preparation method according to the present application has excellent metal luster while improving the corrosion resistance.

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) first 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 first dip coating; After the first 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 first dip coating layer; (4) spraying: spray the obtained enamel containing aluminum powder pigment on the first 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) second 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 second dip coating; After the second 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, characterized by, 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.

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