An anti-corrosion coating for the inner lining of a chemical pipeline and its preparation method

Abstract

The present invention relates to an anti-corrosion coating for the inner lining of a chemical pipeline and a preparation method thereof, belonging to the technical field of chemical anti-corrosion. The coating consists of a primer and a topcoat. The primer contains the following components: 15-25 parts of organosilicon-modified epoxy resin, 5-15 parts of inorganic silicon hybrid epoxy resin, 20-30 parts of zinc phosphate anti-rust pigment, 10-20 parts of modified fly ash, 30-40 parts of extender pigment, and 8-12 parts of curing agent; the topcoat contains the following components: 35-45 parts of fluorocarbon-modified epoxy resin, 15-20 parts of KH-560-modified silicon carbide, 8-12 parts of molybdenum disulfide, 5-8 parts of isocyanate prepolymer, and 10-15 parts of rutile titanium dioxide. By reducing the porosity to ≤1.5% through the core-shell structure fly ash and combining with the three-roll mill directional dispersion process, the coating has high adhesion, good salt spray resistance, and good wear resistance. The process of the present invention is simple and low in cost, applicable to extreme chemical environments such as strong acids and high salt spray, and can significantly extend the service life of the pipeline.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical corrosion prevention, and specifically relates to a preparation method of an anti-corrosion coating for the inner lining of chemical pipelines. Background Art

[0002] With the rapid development of the chemical industry, anti-corrosion coating pipelines play a crucial role in the chemical production process and are widely used in the transportation of media such as petroleum, natural gas, and chemical raw materials. However, many problems have emerged in the actual application of existing anti-corrosion coating pipelines. Especially in some special chemical environments, their performance is difficult to meet the increasingly stringent requirements. Although there are already various anti-corrosion coating pipeline products on the market, which can provide anti-corrosion protection to a certain extent, in some extreme chemical environments, such as complex working conditions of strong acids, strong alkalis, high temperatures, and high salt mists, the anti-corrosion performance and adhesion of these coatings are often insufficient. This results in phenomena such as coating peeling, blistering, and cracking, which in turn cause the pipeline to lose anti-corrosion protection, affect its long-term stable operation and safety performance, and increase the risks and maintenance costs in the chemical production process.

[0003] To solve the above problems, some attempts have been made in the industry, such as improving the anti-corrosion effect by improving coating materials and coating processes. However, these methods still have limitations in actual application. On the one hand, although some coating materials have improved chemical resistance and high-temperature resistance to a certain extent, their preparation processes are complex and costly, which is not conducive to large-scale industrial production and application. On the other hand, although some coating processes can improve the adhesion of the coating, the operation is cumbersome, the construction difficulty is high, and it increases the production time and labor costs.

[0004] The patent document with the publication number CN111828777B discloses a pipeline anti-corrosion coating processing method, which uses the method of wrapping a stainless steel coil with a topcoat sprayed on the outer side around a cast iron pipe and fixing it with a barrier adhesive. Although this method improves the anti-corrosion effect and work efficiency to a certain extent, since the topcoat of the stainless steel coil is pre-sprayed, the topcoat is easily damaged during storage and transportation, thus affecting the final anti-corrosion performance. In addition, the process of wrapping and fixing the stainless steel coil is relatively complex, which not only increases the construction difficulty but also leads to an increase in cost.

[0005] The patent document with the publication number CN107629514A discloses an oil pipeline anti-corrosion coating, which is composed of various raw materials such as epoxy resin, clam shell powder, and polytetrafluoroethylene. Although this coating has good high-temperature resistance and oxidation resistance, due to its complex components, the preparation process is cumbersome, and the cost is high, which is not conducive to large-scale industrial production and application. At the same time, in some extreme chemical environments, such as strong acid-base and salt mist environments, its corrosion resistance still needs to be further improved.

[0006] In addition, the patent document with the publication number CN118066406B discloses an oil and gas transportation pipeline with an anti-corrosion coating. It uses a double-layer epoxy resin anti-corrosion coating inside the pipeline and transfers heat to the outer pipeline surface through a heat conduction mechanism to quickly evaporate the attached liquid. Although this method can effectively improve the corrosion resistance of the pipeline, its structure is complex, and the setting of the heat conduction mechanism increases the manufacturing cost. And in some special environments, uneven heat transfer may lead to coating aging and failure.

[0007] In summary, the anti-corrosion coating pipelines on the market at present are difficult to effectively meet the new requirements for anti-corrosion performance, adhesion, and cost control under complex working conditions. Therefore, the present invention provides a method for preparing an anti-corrosion coating for the inner lining of a chemical pipeline, aiming to overcome the above deficiencies in the prior art. By optimizing the coating material and coating process, the chemical resistance and high-temperature resistance of the coating are improved, while the construction steps are simplified and the production cost is reduced to meet the requirements of modern chemical pipelines for high-quality anti-corrosion coatings, providing a more reliable technical guarantee for the safe production and stable operation of the chemical industry. Summary of the Invention

[0008] The purpose of the present invention is to provide a method for preparing an anti-corrosion coating for the inner lining of a chemical pipeline. The coating prepared by this method has excellent chemical resistance, high-temperature resistance, and adhesion, is suitable for various extreme chemical environments, and at the same time has a simple preparation process and low cost, and is suitable for large-scale industrial production and application.

[0009] To achieve the above purpose, the technical solution adopted by the present invention is:

[0010] The primer contains the following components: 15 - 25 parts of organosilicon-modified epoxy resin, 5 - 15 parts of inorganic silicon hybrid epoxy resin, 20 - 30 parts of zinc phosphate anti-rust pigment, 10 - 20 parts of modified fly ash, 30 - 40 parts of extender pigment, and 8 - 12 parts of curing agent;

[0011] The topcoat contains the following components: 35 - 45 parts of fluorocarbon-modified epoxy resin, 15 - 20 parts of KH-560-modified silicon carbide, 8 - 12 parts of molybdenum disulfide, 5 - 8 parts of isocyanate prepolymer, and 10 - 15 parts of rutile titanium dioxide.

[0012] The modified fly ash is specifically a dense shielding type core-shell composite coated with titanium dioxide on fly ash.

[0013] Specifically, the preparation of the dense shielding type core-shell composite includes the following steps:

[0014] 1) Weigh tetrabutyl titanate and fly ash according to a mass ratio of 0.3 - 0.5:1. Mix tetrabutyl titanate and absolute ethanol according to a mass ratio of 1:3, add 0.5% hydrochloric acid aqueous solution, and the volume ratio of ethanol to water in the solution is 4:1. Then hydrolyze for 30 minutes under the conditions of pH value of 1.5 - 2 and temperature of 25 - 30 °C;

[0015] 2) Add fly ash, and perform ultrasonic dispersion at 35 - 45 kHz and 250 - 350 W for 20 - 40 min. After completion, centrifuge it at a rotational speed of 3500 - 4500 rpm, for a time of 10 - 20 minutes, and at a temperature of 50 - 70 °C. Then transfer it to a vacuum box and dry it under vacuum at 60 °C for 2 hours to obtain a dense shielding type core - shell composite.

[0016] Specifically, the curing agent is compounded by polyetheramine D230 / D400 and alicyclic amine Ancamine2280 according to a mass ratio of 4:1.

[0017] Specifically, the extender pigment is composed of talc powder, barite powder, and mica powder according to a mass ratio of 5:4:1. The zinc phosphate anti - rust pigment is modified by silane coupling agent KH - 570, and the addition amount of the modifier is 1% - 2% of the mass of zinc phosphate. The mass ratio of the modified fly ash to the extender pigment is 1:2 - 3.

[0018] Specifically, the fluorocarbon - modified epoxy resin is a tetrafluoroethylene - vinyl ether copolymer with a fluorine content of ≥55%. The isocyanate prepolymer is hexamethylene diisocyanate biuret or isophorone diisocyanate trimer, and its molar ratio to the fluorocarbon - modified epoxy resin is 1:1.2 - 1.5. Among them, the molar ratio of tetrafluoroethylene to vinyl ether is 1:0.8 - 1.2.

[0019] Specifically, the dry film thickness of the primer layer is 200 - 300 μm, the dry film thickness of the topcoat layer is 150 - 250 μm, and the total dry film thickness is 350 - 550 μm.

[0020] A preparation method of an anti - corrosion coating for the inner lining of a chemical pipeline, comprising the following steps:

[0021] Primer preparation:

[0022] a) First, mix organosilicon - modified epoxy resin, inorganic silicon - hybrid epoxy resin, KH - 570 - modified zinc phosphate, modified fly ash, and extender pigment according to the ratio;

[0023] b) Under nitrogen protection, disperse for 60 min at 2000 ± 50 rpm, add the curing agent prepared from polyetheramine D230 / D400 and Ancamine 2280 with a mass ratio of 4:1, and then continue to disperse for 30 min at 500 ± 50 rpm. Cure at 25 °C for 30 min, and then coat the mixture on the surface of the substrate and place it in an oven at 80 °C for 1 hour to form a primer layer.

[0024] Topcoat preparation:

[0025] c) Mix fluorocarbon-modified epoxy resin, KH-560-modified silicon carbide, molybdenum disulfide, isocyanate prepolymer and rutile titanium dioxide according to the ratio, pre-react at a temperature controlled at 60 ± 2 °C for 1 hour, roll and mill 3 times with a three-roll mill at a roll pressure of 0.4 - 0.6 MPa and a linear speed of 1.5 - 2.0 m / s until the fineness is ≤ 45 μm. Finally, coat the topcoat on the primer layer and cure it in an environment at 60 °C for 4 hours to form a topcoat layer.

[0026] Specifically, when the roll pressure of the three-roll mill in step c) is 0.6 MPa, the median particle size D50 of the silicon carbide particles is ≤ 20 μm.

[0027] The structural composition, implementation method and operating principle of the present invention are as follows:

[0028] 1. Synergistic effect of organic / inorganic silicon hybrid epoxy resin: The flexible chain segment of the organosilicon-modified epoxy resin and the nano-SiO of the inorganic silicon hybrid epoxy form an interpenetrating structure, enhancing the coating adhesion. 2 network forms an interpenetrating structure, enhancing the coating adhesion.

[0029] 2. Sealing and closing effect of core-shell structure modified fly ash: The TiO 2 coating layer reduces the porosity to ≤ 1.5%, blocking the penetration of corrosive media.

[0030] 3. Optimized preparation process: The present invention ensures the uniform mixing and full reaction of each component through processes such as step-by-step dispersion, pre-reaction and roll milling. Especially the optimized parameters (roll pressure, number of roll milling times and linear speed) of the three-roll mill and the directional dispersion of the roll milling process: the silicon carbide particles are refined, synergistically improving the wear resistance with molybdenum disulfide, making the fineness of the coating reach the best, improving the smoothness and compactness of the coating, thereby enhancing the anti-corrosion effect and adhesion.

[0031] The beneficial effects of the present invention are as follows:

[0032] 1. Surface modification with epoxy-based silane coupling agent significantly improves the adhesion and chemical resistance of the coating. The modified resin forms a dense cross-linked network in the coating, effectively preventing the penetration of corrosive media.

[0033] 2. Enhanced Adhesion and Mechanical Strength: The synergistic effect between the primer layer and the topcoat layer, along with the use of modified anti-rust fillers, significantly improves the adhesion and mechanical strength of the coating to the substrate, extends the service life of the coating, and reduces maintenance costs.

[0034] 3. Optimized Preparation Process: Through processes such as step-by-step dispersion, pre-reaction, and rolling milling, the present invention ensures the uniform mixing and full reaction of each component. In particular, the optimized parameters of the three-roll mill (inter-roll pressure, number of rolling passes, and linear velocity) result in the best fineness of the coating, improving the smoothness and compactness of the coating, thereby enhancing the anti-corrosion effect and adhesion. Detailed Embodiment

[0035] The present invention will be further described below in conjunction with specific embodiments, but the present invention is not limited to these embodiments. Those skilled in the art should recognize that the present invention covers all possible alternative solutions, improvement solutions, and equivalent solutions within the scope of the claims.

[0036] The following examples were tested according to the following experimental protocols. The following are the experimental conditions for all examples:

[0037] The extender pigment is composed of talc powder, barite powder, and mica powder in a mass ratio of 5:4:1.

[0038] The inorganic silicon hybrid epoxy resin is prepared by the sol-gel method. The specific steps are as follows:

[0039] (1) Mix tetraethyl orthosilicate and absolute ethanol in a mass ratio of 1:4, add 0.1 mol / L hydrochloric acid to adjust the pH to 3, and hydrolyze for 30 minutes to generate SiO 2 sol;

[0040] (2) Mix epoxy resin E-51 and SiO 2 sol in a mass ratio of 10:1, stir and react at 60 °C for 2 hours to form an inorganic silicon hybrid epoxy resin, where the particle size of SiO 2 nanoparticles is 20 - 50 nm.

[0041] Example 1

[0042] Preparation of modified fly ash: First, tetrabutyl titanate and fly ash were weighed according to a mass ratio of 0.4:1. Tetrabutyl titanate and absolute ethanol were mixed at a ratio of 1:3 and poured into 0.5% dilute hydrochloric acid water, where the volume ratio of ethanol to water was 4:1 and the pH value was adjusted to 1.8. It was stirred and hydrolyzed at 30 °C for 30 minutes to obtain a transparent sol. Then fly ash was added and treated with a 40 kHz ultrasonic machine (power 300 watts) for 30 minutes to uniformly coat titanium dioxide on the surface of fly ash. Then it was centrifuged at 4000 revolutions per minute for 15 minutes to remove the excess liquid, and then placed in a 60 °C vacuum oven and dried for 2 hours to obtain dry core-shell fly ash with a porosity ≤ 0.5%.

[0043] Preparation of primer: Weigh 20 parts of organosilicon-modified epoxy resin, 10 parts of inorganic silicon hybrid epoxy resin, 25 parts of silane-modified zinc phosphate, 15 parts of modified fly ash, and 35 parts of extender pigment prepared by mixing talc powder, barite powder and mica powder according to a ratio of 5:4:1 by weight. Under nitrogen protection, it was stirred at a high speed of 2000 revolutions per minute for 1 hour until the mixture was like thick yogurt. Then 10 parts of a curing agent prepared by mixing polyetheramine D400 and alicyclic amine Ancamine2280 in a ratio of 4:1 were added, the rotation speed was reduced to 500 revolutions per minute and stirring continued for 30 minutes, and then it was left standing for 30 minutes to defoam. Finally, it was sprayed onto the inner wall of the steel pipe and baked at 80 °C for 1 hour to form a primer layer with a thickness of 200 - 300 microns.

[0044] Preparation of topcoat: Weigh 40 parts of fluorocarbon resin, 18 parts of KH-560 modified silicon carbide, 10 parts of molybdenum disulfide, 6 parts of hexamethylene diisocyanate biuret, and 12 parts of titanium dioxide. Then after mixing, it was heated to 60 °C and pre-reacted for 1 hour to initially bond the materials. Then it was repeatedly rolled 3 times in a three-roll mill with a pressure of 0.5 MPa and a linear velocity of 1.8 m / s to grind the particles to ≤ 45 microns. Finally, it was applied on the primer and baked at 60 °C for 4 hours to form a topcoat layer with a thickness of 150 - 250 microns, and the total thickness of the topcoat and primer was 350 - 550 microns.

[0045] Example 2

[0046] Primer: 25 parts of organosilicon-modified epoxy resin, 5 parts of inorganic silicon hybrid epoxy resin, 20 parts of modified fly ash.

[0047] Topcoat: 45 parts of fluorocarbon-modified epoxy resin, 8 parts of isophorone diisocyanate trimer, where the molar ratio of isophorone diisocyanate trimer to fluorocarbon resin is 1:1.5.

[0048] In addition, during the preparation process of modified fly ash, the pH of hydrolysis was 1.5, ultrasonic dispersion was carried out at a frequency of 45 kHz, a power of 350 W, and a running time of 40 min. At the same time, centrifugal separation was carried out at 4500 rpm for 20 min, and the rest was the same as in Example 1.

[0049] Example 3

[0050] Primer: 15 parts of silicone-modified epoxy resin and 40 parts of extender pigment.

[0051] Topcoat: 35 parts of fluorocarbon-modified epoxy resin and 10 parts of rutile titanium dioxide, and the rest is the same as in Example 1.

[0052] Example 4

[0053] Topcoat: The NCO content in hexamethylene diisocyanate biuret is 22%, and at the same time, the pressure of the three-roll mill is 0.6 MPa, and the linear speed is 2.0 m / s, and the rest is the same as in Example 1.

[0054] Example 5

[0055] Primer: The curing time is shortened to 15 min.

[0056] Topcoat: The pre-reaction temperature is 62 °C, and the curing time is compressed to 3 h, and the rest is the same as in Example 1.

[0057] Comparative Example 1

[0058] Primer: Use the original fly ash without titanium dioxide coating (unmodified fly ash), and the rest is the same as in Example 1.

[0059] Comparative Example 2

[0060] Primer: The resin uses ordinary epoxy resin E-51, and the rest is the same as in Example 1.

[0061] Comparative Example 3

[0062] During the preparation process, the three-roll mill step is omitted and directly mixed and coated, and the rest is the same as in Example 1.

[0063] Adhesion (MPa) Salt spray resistance time (h) Taber abrasion (g / 1000 cycles) Porosity (%) Hydrochloric acid resistance time (h) Topcoat dry film thickness (μm) Primer dry grinding thickness (μm) Example 1 18 (cohesive failure) 2965 (rust < 0.1%) 0.021 ≤1.3 720 (no blistering) 200 250 Example 2 16 (cohesive failure) 2896 (no rust) 0.024 ≤1.5 684 (no blistering) 200 200 Example 3 14 (cohesive failure) 2637 (rust < 0.5%) 0.031 ≤1.7 572 (slight blistering) 150 300 Example 4 17 (cohesive failure) 2564 (rust < 0.2%) 0.037 ≤1.8 531 (no blistering) 250 200 Example 5 15 (cohesive failure) 2342 (rust < 0.5%) 0.038 ≤1.9 519 (edge blistering) 250 300 Comparative Example 1 8 (cohesive failure) 814 (rust > 50%) 0.056 5.2 263 (large area blistering) 200 250 Comparative Example 2 10 (cohesive failure) 1239 (rust ≈ 20%) 0.077 2.0 121 (local blistering) 200 250 Comparative Example 3 6 (cohesive failure) 1764 (rust ≈ 10%) 0.089 2.3 364 (edge blistering) 200 250

[0064] Table 1 Test Data

[0065] According to the comparison between Example 1 and Comparative Example 1, the core-shell structure reduces the porosity by 90% and increases the salt spray resistance time by 300%.

[0066] The following is the test method for specific examples:

[0067] 1. Adhesion test

[0068] Use a hydraulic pull-off tester (Elcometer 506), attach a 20-mm diameter forging die to the coating surface, cure the glue for 24 hours, then apply a vertical pulling force until the coating peels off, record the maximum pulling force (MPa), and observe the fracture type (cohesive / interface / adhesive failure), and repeat 3 times for each sample and take the average value.

[0069] 2. Salt spray resistance test

[0070] Place the coated specimen in a salt spray chamber to simulate the marine environment (5% NaCl solution, 35 °C). Check the rust area every 24 hours, record the time (hours) when rust first appears, and then test until the coating completely fails or reaches the upper limit of 4000 hours.

[0071] 3. Abrasion resistance test

[0072] Use a Taber abrasion tester with a CS-10 grinding wheel loaded (load 1 kg). After the specimen rotates 1000 times, weigh the weight loss of the coating (g / 1000 times), and at the same time observe whether the coating is worn to the substrate.

[0073] 4. Porosity test

[0074] Use the mercury intrusion method. According to the ASTM D4404-18 standard, use an AutoPore V9600 mercury intrusion porosimeter with a pressure range of 0.1 - 60000 psi to measure the porosity of the coating, analyze the sealing effect of the core-shell composite on the pores, and then compare the pore distributions of unmodified fly ash (Comparative Example 1) and modified fly ash (Example 1).

[0075] 5. Chemical resistance test

[0076] Immerse the coating in a 10% hydrochloric acid solution (25 °C), observe the blistering or peeling situation every 24 hours, and at the same time record the coating failure time (hours).

[0077] Through the synergistic strengthening effect of the organic / inorganic hybrid epoxy resin, the sealing effect of the core-shell structure modified fly ash, and the directional dispersion technology of the three-roll milling process, the present invention realizes a breakthrough improvement in the coating performance. The adhesion of Example 1 reaches 18 MPa (a 125% increase compared to 8 MPa of the traditional coating), attributed to the synergistic strengthening of the interfacial bonding by the flexible segments of the organosilicon-modified epoxy resin and the nano-SiO 2 network of the inorganic silicon hybrid epoxy, and at the same time the curing agent ratio (polyetheramine D400: alicyclic amine = 4:1) optimizes the crosslinking density; the salt spray resistance of Example 2 reaches 2896 hours (compared to 800 hours of unmodified fly ash, an increase of about 256%), because the TiO 2 coating layer (hydrolysis pH = 1.5) reduces the porosity of fly ash to ≤1.5%, effectively blocking the chloride ion penetration path, and the high-fluorine content resin forms a continuous shielding layer; due to the "hard support - lubrication and drag reduction" synergistic mechanism of the oriented arrangement of molybdenum disulfide flakes (friction coefficient ≤0.12) and the refinement of silicon carbide particles under the high pressure of 0.6 MPa of the three-roll mill.

[0078] In addition to the above embodiments, the present invention may have other implementation manners. All technical solutions formed by equivalent substitution or equivalent transformation fall within the protection scope required by the present invention.

Claims

1. A method for preparing an anti-corrosion coating for a chemical pipeline lining, characterized in that The following steps are involved: The primer contains the following components: 15-25 parts of organosilicon-modified epoxy resin, 5-15 parts of inorganic silicon hybrid epoxy resin, 20-30 parts of zinc phosphate antirust pigment, 10-20 parts of modified fly ash, 30-40 parts of body filler, and 8-12 parts of curing agent; The topcoat comprises the following components: 35-45 parts of fluorocarbon modified epoxy resin, 15-20 parts of KH-560 modified silicon carbide, 8-12 parts of molybdenum disulfide, 5-8 parts of isocyanate prepolymer, and 10-15 parts of rutile titanium dioxide; The modified fly ash is a dense shielded core-shell composite coated with fly ash titanium dioxide; The preparation of the dense shielded core-shell complex comprises the following steps: 1) Weigh tetrabutyl titanate and fly ash in a mass ratio of 0.3-0.5:1, mix tetrabutyl titanate and anhydrous ethanol in a mass ratio of 1:3, add 0.5% hydrochloric acid aqueous solution, the volume ratio of ethanol to water in the solution is 4:1, and then hydrolyze for 30 minutes at a pH of 1.5-2 and a temperature of 25-30°C; 2) adding fly ash, and ultrasonically dispersing it at 35-45kHz and 250-350W for 20-40min. After completion, centrifugation is performed at a speed of 3500-4500rpm, a time of 10-20min, and a temperature of 50-70°C. Then, the mixture is transferred to a vacuum box and vacuum dried at 60°C for 2 hours to obtain a dense shielded core-shell composite. Primer preparation: a) firstly, organic silicon modified epoxy resin, inorganic silicon hybrid epoxy resin, KH-570 modified zinc phosphate, modified fly ash and physical filler are mixed according to a proportion; the modified fly ash is specifically a dense shielded core-shell composite coated with fly ash titanium dioxide; b) Disperse at 2000±50rpm for 60min under nitrogen protection, add a curing agent prepared by polyetheramine D230 / D400 and Ancamine2280 in a mass ratio of 4:1, continue dispersing at 500±50rpm for 30min, mature at 25℃ for 30min, then apply the mixture on the surface of the substrate, and place it in an oven at 80℃ for curing for 1 hour to form a primer layer; Topcoat preparation: c) Mix fluorocarbon modified epoxy resin, KH-560 modified silicon carbide, molybdenum disulfide, isocyanate prepolymer and rutile titanium dioxide according to the proportion, pre-react at a temperature of 60±2°C for 1 hour, and grind for 3 times with a three-roll mill at a speed of 1.5-2.0m / s at 0.4-0.6MPa to a fineness of ≤45μm. Finally, apply the topcoat on the primer layer and cure it at 60°C for 4 hours to form a topcoat layer; The fluorocarbon modified epoxy resin is a tetrafluoroethylene-vinyl ether copolymer with a fluorine content of ≥55%. The isocyanate prepolymer is hexamethylene diisocyanate biuret or isophorone diisocyanate trimer, and the molar ratio of the isocyanate prepolymer to the fluorocarbon modified epoxy resin is 1:1.2-1.

5.

2. The preparation method according to claim 1, characterized in that: The mass ratio of the modified fly ash to the physical filler is 1:2-3.

3. The preparation method according to claim 1, characterized in that: The physical filler is composed of talcum powder, barite powder and mica powder in a mass ratio of 5:4:

1.

4. The preparation method according to claim 1, characterized in that: The dry film thickness of the primer layer is 200-300 μm, the dry film thickness of the topcoat layer is 150-250 μm, and the total dry film thickness is 350-550 μm.