A method for preparing a metal surface protective coating

Abstract

The present invention belongs to the field of metal corrosion protection technology and relates to a method for preparing a metal surface protective coating. The preparation method comprises the following steps: (1) preparing a metal sol-gel solution; (2) preparing a silica sol-gel solution; (3) preparing an acrylate mixed solution; (4) preparing a silica sol-gel acrylate mixed solution; (5) preparing a metal sol-gel acrylate mixed solution; (6) preparing a functional additive solution; (7) spraying for the first time; (8) spraying for the second time; (9) spraying for the third time; and (10) spraying for the fourth time. The preparation method of the metal surface protective coating of the present invention can be used to prepare a metal surface protective coating more efficiently without damaging the underlying metal, and the prepared protective coating has good chemical stability, wear resistance, and corrosion resistance.

Description

Technical Field

[0001] The invention belongs to the technical field of metal anticorrosion and relates to a method for preparing a metal surface protective coating. Background Art

[0002] Lightweight metal materials such as magnesium alloys usually have a thin protective oxide film, i.e., a protective coating, on their surface to provide corrosion protection for the metal materials. However, the presence of this protective oxide film limits the practical use of lightweight metal materials such as magnesium alloys and hinders the metallic luster of lightweight metal materials such as magnesium alloys. Although sol-gel protective coatings exhibit excellent chemical stability, oxidation control, and enhanced corrosion resistance for metal substrates such as magnesium alloys, the protective coatings may damage the metal substrate and other coloring compounds during high-temperature annealing. In addition, traditional sol-gel coating methods require long solvent evaporation and curing times. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for preparing a metal surface protective coating, so as to prepare the metal surface protective coating more efficiently and without damaging the underlying metal, and to obtain a protective coating with good chemical stability, wear resistance and corrosion resistance.

[0004] To achieve this object, in a basic embodiment, the present invention provides a method for preparing a metal surface protective coating, the preparation method comprising the following steps:

[0005] (1) Preparation of metal sol-gel solution: A metal sol-gel solution is prepared by mixing an acetone organic solvent, an alcohol, and a metal alkoxide;

[0006] (2) Preparation of silica sol-gel solution: alcohol, linear silane, and branched silane are mixed to prepare silica sol-gel solution;

[0007] (3) Preparation of an acrylate mixed solution: an acrylate compound is added to a sodium hydroxide solution, shaken until it becomes milky, and then hydrochloric acid is added to prepare an acrylate mixed solution;

[0008] (4) Preparation of silica sol-gel acrylate mixed solution: mixing the silica sol-gel solution prepared in step (2) with the acrylate mixed solution prepared in step (3) to prepare a silica sol-gel acrylate mixed solution;

[0009] (5) Preparation of a metal sol-gel acrylate mixed solution: mixing the metal sol-gel solution prepared in step (1) with a portion of the silica sol-gel acrylate mixed solution prepared in step (4) to prepare a metal sol-gel acrylate mixed solution;

[0010] (6) Preparation of functional additive solution: The mixed solution of cerium solution and water-based dye is mixed with alcohol to prepare a functional additive solution;

[0011] (7) First spraying: the silica sol-gel acrylate mixed solution prepared in step (4) is mixed with alcohol and then sprayed on the metal surface for the first time;

[0012] (8) Second spraying: the metal sol-gel acrylate mixed solution prepared in part of step (5) is mixed with alcohol and then sprayed on the metal surface for a second time;

[0013] (9) spraying for the third time: mixing the silica sol-gel acrylate mixed solution obtained in part of step (4), the metal sol-gel acrylate mixed solution obtained in part of step (5), and the functional additive solution obtained in step (6), and spraying the metal surface for the third time;

[0014] (10) The fourth spraying: the silica sol-gel acrylate mixed solution obtained in part of step (4), the metal sol-gel acrylate mixed solution obtained in part of step (5), and the alcohol are mixed and then sprayed on the metal surface for the fourth time.

[0015] The relevant principles of the present invention are as follows:

[0016] The present invention forms a protective coating on the metal surface through an aerosol spraying process by regulating the chemical reaction properties, molecular size, concentration and sol mixing steps of the sol components.

[0017] The adhesion of sol-gel coatings to metal substrates can be controlled by adjusting the ratio of metal alkoxides (such as zirconium n-propoxide, aluminum isopropoxide, and titanium isopropoxide) to silanes (such as tetramethoxysilane, tetraethoxysilane, 3-aminopropyltriethoxysilane, (3-glycidoxypropyl)trimethoxysilane, and 3-(trimethoxysilyl)propyl methacrylate) and acrylates (such as 2-hydroxymethylacrylate, methyl acrylate, and ethyl acrylate). Furthermore, the organic content and hardness of the sol-gel coating can be effectively controlled to prevent cracking.

[0018] The performance of sol-gel coatings depends on the following parameters:

[0019] 1. Concentration of sol solution

[0020] The concentration of sol solutions containing silanes (such as tetramethoxysilane, tetraethoxysilane, 3-aminopropyltriethoxysilane, (3-glycidoxypropyl)trimethoxysilane, and 3-(trimethoxysilyl)propyl methacrylate) significantly affects the coating processing time. Dilution with low vapor pressure alcohols (such as ethanol and isopropanol) facilitates the drying of the sol on the metal surface and accelerates the film formation process.

[0021] 2. Curing temperature

[0022] The presence of organic components (such as epoxy, methacrylate, or vinyl) can reduce the required curing temperature to below 150° C. The lower curing temperature and the introduction of flexible organics reduce the possibility of cracking during thermal processing.

[0023] 3. Drying and curing period

[0024] The drying process is aided by the dilution of the sol-gel solution and the large amount of dry compressed air used in the aerosol spray. The dilution of the solution, the water content of the sol-gel, and the gas flow rate should be carefully adjusted to allow the sprayed solution to dry immediately and prevent residual organic solvent from washing away the coating. Preheating the metal object before each spraying process can accelerate the drying process, resulting in a uniformly distributed coating. Furthermore, the amount of sol applied to the metal surface should be controlled to prevent an excess of gel solvent, which can affect the coating's performance. The use of zirconium alkoxide and organic components allows the curing temperature of the sol-gel coating to be reduced to around 150°C.

[0025] 4. Metal alkoxide ratio

[0026] The organic-inorganic network of the sol-gel coating is controlled by the ratio of metal alkoxides (such as zirconium n-propoxide, aluminum isopropoxide, titanium isopropoxide, etc.), acrylates (such as 2-hydroxymethylacrylate, methyl acrylate, ethyl acrylate, etc.), linear silanes (such as tetramethoxysilane, tetraethoxysilane, etc.) and branched silanes (such as 3-(trimethoxysilyl)propyl methacrylate, 3-aminopropyltriethoxysilane, (3-glycidoxypropyl)trimethoxysilane, etc.).

[0027] 5. Metal surface pretreatment

[0028] The metal surface of the sample should be cleaned or polished before coating. It is recommended to first perform wet sandblasting and then solvent cleaning to remove metal oxides and dirt on the metal surface of the sample.

[0029] 6. Chemical properties of dyes

[0030] Sol-gel coatings use alcohols (e.g., ethanol, isopropanol, etc.) as solvents, so the dye chemicals selected should be soluble in these alcohols. Controlling the dye content in the sol-gel coating prevents excessive dye usage, which can affect the coating's wear and corrosion resistance.

[0031] In a preferred embodiment, the present invention provides a method for preparing a metal surface protective coating, wherein in step (1),

[0032] The acetone organic solvent is selected from acetone and / or acetylacetone;

[0033] The alcohol is selected from ethanol and / or isopropanol;

[0034] The metal alkoxide is selected from one or more of zirconium n-propoxide, aluminum isopropoxide, and titanium isopropoxide;

[0035] The volume ratio of the alcohol, acetone organic solvent and metal alkoxide is (20-60):1:(5-15).

[0036] In a preferred embodiment, the present invention provides a method for preparing a metal surface protective coating, wherein in step (2),

[0037] The alcohol is a mixture of ethanol and isopropanol;

[0038] The linear silane is selected from tetramethoxysilane and / or tetraethoxysilane;

[0039] The branched silane is selected from one or more of 3-(trimethoxysilyl)propyl methacrylate, 3-aminopropyltriethoxysilane, and (3-glycidoxypropyl)trimethoxysilane;

[0040] The volume ratio of the alcohols, linear silane and branched silane is (10-30):1:(1-5).

[0041] In a preferred embodiment, the present invention provides a method for preparing a metal surface protective coating, wherein in step (3),

[0042] The acrylic acid ester compound is selected from one or more of 2-hydroxymethylacrylate, methyl acrylate, and ethyl acrylate;

[0043] The concentration of the sodium hydroxide solution is 0.1-1 wt%;

[0044] The volume percentage concentration of the hydrochloric acid is 20-37%;

[0045] The volume ratio of the acrylic acid ester compound, sodium hydroxide solution and hydrochloric acid is (1-3):(10-30):1.

[0046] In a preferred embodiment, the present invention provides a method for preparing a metal surface protective coating, wherein in step (4), the volume ratio of the silica sol-gel solution to the acrylic ester mixed solution is (1-3):1.

[0047] In a preferred embodiment, the present invention provides a method for preparing a metal surface protective coating, wherein in step (5), the volume ratio of the metal sol-gel solution to a portion of the silica sol-gel acrylate mixed solution is (1-3):1.

[0048] In a preferred embodiment, the present invention provides a method for preparing a metal surface protective coating, wherein in step (6),

[0049] The cerium liquid is selected from one or more of cerium nitrate, cerium chloride, and cerium sulfate;

[0050] The water-based dye is one or more of ultramarine blue;

[0051] The mass ratio of the cerium liquid to the water-based dye is 1:(10-100);

[0052] The alcohol is ethanol and / or isopropanol;

[0053] The volume ratio of the mixed solution to the alcohol is 1:(1-5);

[0054] The mixed solution of the cerium liquid and the water-based dye is mixed with alcohol and then centrifuged to remove solid particles.

[0055] In a preferred embodiment, the present invention provides a method for preparing a metal surface protective coating, wherein in step (7) and step (8),

[0056] Preheating the metal in an oven to 120-200°C for 20-60 seconds before performing the first spraying;

[0057] The alcohol is ethanol and / or isopropanol;

[0058] The volume ratio of the silica sol-gel acrylate mixed solution obtained in step (4) of the first spraying to the alcohol is 1:(0.5-5);

[0059] The volume ratio of the metal sol-gel acrylate mixed solution obtained in step (5) of the second spraying to the alcohol is 1:(0.5-5);

[0060] After the first spraying and the second spraying, the coating is heated in an oven to 120-200° C. for 20-60 seconds;

[0061] The spraying thickness of the first spraying and the second spraying is independently 1 to 2 microns.

[0062] In a preferred embodiment, the present invention provides a method for preparing a metal surface protective coating, wherein in step (9),

[0063] The volume ratio of the silica sol-gel acrylate mixed solution, the metal sol-gel acrylate mixed solution, and the functional additive solution is 1:(0.1-5):(0.1-5);

[0064] After the third spraying, the coating is heated in an oven to 120-200° C. for 20-60 seconds;

[0065] The spraying thickness of the third spraying is 5 to 6 microns.

[0066] In a preferred embodiment, the present invention provides a method for preparing a metal surface protective coating, wherein in step (10),

[0067] The volume ratio of the silica sol-gel acrylate mixed solution, the metal sol-gel acrylate mixed solution, and the alcohol is 1:(0.1-5):(0.1-5);

[0068] The alcohol is ethanol and / or isopropanol;

[0069] After the fourth spraying, the coating is heated in an oven to 120-200°C for 20-60 seconds. After the coating is stable, the coating is heated in an oven again to 120-200°C for 3-10 minutes.

[0070] The spraying thickness of the fourth spraying is 5 to 6 microns.

[0071] The beneficial effect of the present invention is that the preparation method of the metal surface protective coating of the present invention can prepare the metal surface protective coating more efficiently without damaging the underlying metal, and the protective coating obtained has good chemical stability, wear resistance and corrosion resistance.

[0072] The preparation method of the present invention can be used to prepare protective coatings for various metal samples such as magnesium alloys, aluminum alloys, titanium alloys, etc., and is suitable for flat and curved samples, and is suitable for single-layer and multi-layer spraying. The glossiness of the protective coating is higher than 70GU (it can retain the metallic luster and brilliant color performance), and has enhanced wear resistance and corrosion resistance (the protective coating has excellent organic solvent resistance and exhibits corrosion resistance for more than 72 hours).

[0073] The preparation method of the present invention utilizes an organic-inorganic hybrid compound of siloxane and metal oxide. Through polymerization of the organic components and coordination with the inorganic alkoxide, a dense microstructure of the protective coating is produced. This reduces the curing temperature of the protective coating to 150°C and reduces the likelihood of cracking and damage to the coloring compound. By selecting appropriate metal alkoxides, silanes, and acrylates and carefully adjusting their ratios, the adhesion of the protective coating to the metal substrate can be controlled. The resulting glassy, ​​transparent sol-gel protective coating produces a bright, shiny coating on the metal surface. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Figure 1 This is a flow chart of the method for preparing the metal surface protective coating of Example 1.

[0075] Figure 2 This is a scanning electron micrograph of a cross section of the metal with the protective coating prepared in Example 1, showing that the thickness of the sol-gel coating is 14.2 microns.

[0076] Figure 3 This is a graph showing the glossiness test results of the protective coating prepared in Example 1.

[0077] Figure 4 This is a graph showing the corrosion resistance test results of the protective coating prepared in Example 1.

[0078] Figure 5 This is an appearance diagram of a protective coating prepared on a flat or curved metal surface using the preparation method of the present invention. DETAILED DESCRIPTION

[0079] The specific embodiments of the present invention are further described below through examples.

[0080] Example 1: Preparation of metal surface protective coating

[0081] according to Figure 1 The process of preparing metal surface protective coating:

[0082] (1) Preparation of metal sol-gel solution: Alcohol (ethanol), acetone organic solvent (acetylacetone), and zirconium n-propoxide were mixed in a volume ratio of 40:1:10 to prepare a metal sol-gel solution.

[0083] (2) Preparation of silica sol-gel solution: Ethanol, linear silane (tetraethoxysilane), and branched silane (3-(trimethoxysilyl)propyl methacrylate) were mixed in a volume ratio of 20:1:4 to prepare a silica sol-gel solution.

[0084] (3) Preparation of acrylic acid ester mixed solution: Acrylic acid ester compound (2-hydroxymethyl acrylate) was added to sodium hydroxide solution (0.6 wt%), shaken to a milky state, and then hydrochloric acid (37% v / v) was added to prepare acrylic acid ester mixed solution. The volume ratio of acrylic acid ester compound, sodium hydroxide solution and hydrochloric acid was 2:20:1.

[0085] (4) Preparation of silica sol-gel acrylate mixed solution: The silica sol-gel solution prepared in step (2) and the acrylate mixed solution prepared in step (3) were mixed in a volume ratio of 5:2.3 to prepare a silica sol-gel acrylate mixed solution.

[0086] (5) Preparation of metal sol-gel acrylate mixed solution: The metal sol-gel solution prepared in step (1) and the silica sol-gel acrylate mixed solution prepared in step (4) are mixed in a volume ratio of 1:1 to prepare a metal sol-gel acrylate mixed solution.

[0087] (6) Preparation of functional additive solution: A mixed solution of cerium solution (cerium nitrate) and water-based dye (ultramarine) at a mass ratio of 1:20 was mixed with alcohol (ethanol) (the volume ratio of the mixed solution to the alcohol was 1:4) and centrifuged at 4000 rpm to remove solid particles to prepare a functional additive solution.

[0088] (7) First spraying: The silica sol-gel acrylate mixed solution prepared in step (4) is mixed with alcohol (ethanol) at a volume ratio of 1:1 and then sprayed onto the metal surface for the first time. Before spraying, the metal is preheated in an oven to 120-200°C for 20-60 seconds; after spraying, the metal is heated in an oven to 120-200°C for 20-60 seconds. The thickness of the first spraying is 1-2 μm.

[0089] (8) Second spraying: The metal sol-gel acrylate mixed solution prepared in step (5) is mixed with alcohol (ethanol) at a volume ratio of 1:1 and then sprayed onto the metal surface for a second time. After spraying, the mixture is heated in an oven to 120-200°C for 20-60 seconds. The second spraying thickness is 1-2 μm.

[0090] (9) Third spraying: The silica sol-gel acrylate mixed solution prepared in part of step (4), the metal sol-gel acrylate mixed solution prepared in part of step (5), and the functional additive solution prepared in step (6) are mixed in a volume ratio of 1:1:1 and then sprayed on the metal surface for the third time. After spraying, heat in an oven to 120-200°C for 20-60 seconds. Repeat the spraying and heating operation three times. The spray thickness of the third spraying is 5-6 μm.

[0091] (10) Fourth spraying: The silica sol-gel acrylate mixed solution obtained in part of step (4), the metal sol-gel acrylate mixed solution obtained in part of step (5), and alcohol (ethanol) are mixed in a volume ratio of 1:1:1 and then sprayed on the metal surface for the fourth time. After spraying, heat in an oven to 120-200°C for 20-60 seconds. Repeat the spraying and heating operation three times. After the final coating is stable, heat in an oven to 120-200°C again for 3-10 minutes. The spray thickness of the fourth spraying is 5-6 microns.

[0092] Example 2: Testing of the protective coating prepared in Example 1

[0093] The protective coating prepared in Example 1 was subjected to various tests. The test items, methods and results are as follows.

[0094] 1. Coating thickness

[0095] The coating thickness was measured by scanning electron microscopy, and the results showed that the thickness of the sol-gel coating was 14.2 μm (see Figure 2 ).

[0096] 2. Coating composition

[0097] Energy dispersive X-ray spectroscopy was used to detect the mass percentage content of the coating composition, and the results are shown in Table 1 below.

[0098] Table 1 Coating composition test results

[0099] element Element content (%) oxygen 65 silicon 30 zirconium 5 Total content 100

[0100] The results in Table 1 show that the mass ratio of silicon to zirconium in the sol-gel coating is 30:5.

[0101] 3. Glossiness

[0102] The glossiness of the protective coating was tested using a gloss meter and the result showed that the glossiness was greater than 70GU (see Figure 3 ).

[0103] 4. Corrosion resistance

[0104] The corrosion resistance of the protective coating was tested using a salt spray test (ASTM-B117 method). The results showed that the surface of the sol-gel coating was intact and there was no sign of corrosion (see Figure 4 ).

[0105] In addition, the preparation method of Example 1 was used to prepare protective coatings on the surfaces of various metal materials. The appearance of the prepared protective coatings is shown in FIG. Figure 5 As shown, it shows that each protective coating has good gloss.

[0106] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these changes and variations. The above embodiments or implementation methods are merely illustrative of the present invention, and the present invention may also be implemented in other specific ways or other specific forms without departing from the gist or essential features of the present invention. Therefore, the described implementation methods should be regarded as illustrative and not restrictive in any respect. The scope of the present invention should be described by the appended claims, and any changes that are equivalent to the intent and scope of the claims should also be included within the scope of the present invention.

Claims

1. A method for preparing a metal surface protective coating, characterized in that: The preparation method comprises the following steps: (1) Preparation of metal sol-gel solution: A metal sol-gel solution is prepared by mixing an acetone organic solvent, an alcohol, and a metal alkoxide; (2) Preparation of silica sol-gel solution: alcohol, linear silane and branched silane are mixed to prepare silica sol-gel solution; (3) Preparation of acrylic acid ester mixed solution: acrylic acid ester compound is added to sodium hydroxide solution, shaken to milky state, and then hydrochloric acid is added to prepare acrylic acid ester mixed solution; (4) Preparation of silica sol-gel acrylate mixed solution: the silica sol-gel solution prepared in step (2) and the acrylate mixed solution prepared in step (3) are mixed to prepare a silica sol-gel acrylate mixed solution; (5) Preparation of a metal sol-gel acrylate mixed solution: the metal sol-gel solution prepared in step (1) is mixed with part of the silica sol-gel acrylate mixed solution prepared in step (4) to prepare a metal sol-gel acrylate mixed solution; (6) Preparation of functional additive solution: The mixed solution of cerium solution and water-based dye is mixed with alcohol to prepare a functional additive solution; the cerium solution is selected from one or more of cerium nitrate, cerium chloride, and cerium sulfate; (7) First spraying: the silica sol-gel acrylate mixed solution prepared in part of step (4) is mixed with alcohol and then sprayed on the metal surface for the first time; (8) Second spraying: the metal sol-gel acrylate mixed solution prepared in part of step (5) is mixed with alcohol and then sprayed on the metal surface for the second time; (9) Third spraying: the silica sol-gel acrylate mixed solution prepared in part of step (4), the metal sol-gel acrylate mixed solution prepared in part of step (5), and the functional additive solution prepared in step (6) are mixed and then sprayed on the metal surface for the third time; (10) The fourth spraying: the silica sol-gel acrylate mixed solution prepared in part of step (4), the metal sol-gel acrylate mixed solution prepared in part of step (5), and the alcohol are mixed and then sprayed on the metal surface for the fourth time.

2. The preparation method according to claim 1, wherein: In step (1), The acetone organic solvent is selected from acetone and / or acetylacetone; The alcohol is selected from ethanol and / or isopropanol; The metal alkoxide is selected from one or more of zirconium n-propoxide, aluminum isopropoxide, and titanium isopropoxide; The volume ratio of the alcohol, acetone organic solvent and metal alkoxide is (20-60):1:(5-15).

3. The preparation method according to claim 1, wherein: In step (2), The alcohol is a mixture of ethanol and isopropanol; The linear silane is selected from tetramethoxysilane and / or tetraethoxysilane; The branched silane is selected from one or more of 3-(trimethoxysilyl)propyl methacrylate, 3-aminopropyltriethoxysilane, and (3-glycidoxypropyl)trimethoxysilane; The volume ratio of the alcohol, linear silane and branched silane is (10-30):1:(1-5).

4. The preparation method according to claim 1, wherein: In step (3), The acrylic acid ester compound is selected from one or more of 2-hydroxymethylacrylate, methyl acrylate, and ethyl acrylate; The concentration of the sodium hydroxide solution is 0.1-1 wt %; The volume percentage concentration of the hydrochloric acid is 20-37%; The volume ratio of the acrylic acid ester compound, sodium hydroxide solution and hydrochloric acid is (1-3):(10-30):

1.

5. The preparation method according to claim 1, wherein: In step (4), the volume ratio of the silica sol-gel solution to the acrylic ester mixed solution is (1-3):

1.

6. The preparation method according to claim 1, wherein: In step (5), the volume ratio of the metal sol-gel solution to the silica sol-gel acrylate mixed solution is (1-3):

1.

7. The preparation method according to claim 1, wherein: In step (6), The water-based dye is one or more of ultramarine blue; The mass ratio of the cerium liquid to the water-based dye is 1:(10-100); The alcohol is ethanol and / or isopropanol; The volume ratio of the mixed solution to the alcohol is 1:(1-5); The mixed solution of the cerium liquid and the water-based dye is mixed with alcohol and then centrifuged to remove solid particles.

8. The preparation method according to claim 1, wherein: In steps (7) and (8), Preheat the metal in an oven to 120-200°C for 20-60 seconds before performing the first spraying; The alcohol is ethanol and / or isopropanol; The volume ratio of the silica sol-gel acrylate mixed solution obtained in step (4) of the first spraying to the alcohol is 1:(0.5-5); The volume ratio of the metal sol-gel acrylate mixed solution obtained in step (5) of the second spraying to the alcohol is 1: (0.5-5); After the first spraying and the second spraying, the coating is heated in an oven to 120-200° C. for 20-60 seconds; The spraying thickness of the first spraying and the second spraying is independently 1 to 2 microns.

9. The preparation method according to claim 1, wherein: In step (9), The volume ratio of the silica sol-gel acrylate mixed solution, the metal sol-gel acrylate mixed solution, and the functional additive solution is 1: (0.1-5): (0.1-5); After the third spraying, the coating is heated in an oven to 120-200° C. for 20-60 seconds; The third spraying has a thickness of 5 to 6 microns.

10. The preparation method according to claim 1, characterized in that: In step (10), The volume ratio of the silica sol-gel acrylate mixed solution, the metal sol-gel acrylate mixed solution, and the alcohol is 1: (0.1-5): (0.1-5); The alcohol is ethanol and / or isopropanol; After the fourth spraying, the coating is heated in an oven to 120-200°C for 20-60 seconds. After the coating is stable, the coating is heated in an oven again to 120-200°C for 3-10 minutes. The fourth spraying has a thickness of 5 to 6 microns.

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