Preparation method and application of modified hyperbranched polyurethane for rail transit field finish paint

Abstract

The application discloses a preparation method and application of modified hyperbranched polyurethane for track traffic field finish paint, and comprises the following raw materials: 2,2-dimethylol propionic acid, dicyclohexyl methane-4,4'-diisocyanate, modified chain extender and catalyst. The modified hyperbranched polyurethane containing double benzimidazole structures and curcumin and the phosphorus-nitrogen six-membered ring functional agent A containing a large number of hindered phenol structures are used in the preparation of the track traffic finish paint, so that the finish paint has excellent ultraviolet aging resistance, heat aging resistance and mechanical properties, and also has the effect of flame retardation.

Description

Technical Field

[0001] This invention relates to a method for preparing modified hyperbranched polyurethane for topcoats used in the field of rail transportation and its application. Background Technology

[0002] Rail transit vehicles are high-speed public transportation vehicles operating outdoors. Not only do they require a highly decorative appearance, but the requirements for their protective coating are also extremely stringent. Due to their long-term outdoor exposure, rail transit vehicles are subject to corrosion from rain, moisture, salt, and other corrosive substances. During operation, the vehicles also experience friction with air and dust, and may even be impacted by foreign objects such as stones. These complex external conditions undoubtedly pose a severe challenge to rail transit vehicles, and the performance requirements for the topcoat, as the final layer of protection, are becoming increasingly stringent.

[0003] To address the aforementioned issues, existing technologies have attempted to improve the situation by adding waterborne polyurethane and single functional agents. However, in practical applications, waterborne polyurethane offers very limited improvement to the topcoat. Under harsh conditions such as high temperatures and ultraviolet radiation, it is prone to aging cracks, which can affect its protective performance for rail transit vehicles. Single functional agents, on the other hand, are prone to precipitation during use, which is detrimental to the long-term protective effect of the topcoat.

[0004] Therefore, a multifunctional hyperbranched polyurethane for the field of rail transit topcoats is needed to solve the above-mentioned technical defects. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing modified hyperbranched polyurethane for topcoats in the field of rail transportation and its application.

[0006] One of the objectives of this invention is to provide a method for preparing modified hyperbranched polyurethane for topcoats used in rail transportation, which can be achieved through the following technical solution:

[0007] The preparation method of modified hyperbranched polyurethane includes the following steps:

[0008] Dicyclohexylmethane-4,4'-diisocyanate and 2,2-dimethylolpropionic acid were dissolved in N,N-dimethylformamide, heated to 75-85℃, and reacted for 1.5-3 hours. After the reaction was completed, the temperature was controlled at 0-5℃, a chain extender was added, and stirring was continued for 20-30 minutes. A catalyst was added, the temperature was raised to 70-80℃, and the reaction was carried out for 4-6 hours. Acetone was added, and the mixture was stirred for 5-10 minutes to obtain modified hyperbranched polyurethane.

[0009] Preferably, the composition comprises 1.4-2 parts by weight of 2,2-dimethylolpropionic acid, 110-120 parts by weight of dicyclohexylmethane-4,4'-diisocyanate, 60-70 parts by weight of modified chain extender, 0.1-0.2 parts by weight of catalyst, 45-60 parts by weight of N,N-dimethylformamide, and 15-20 parts by weight of acetone; wherein the catalyst is dibutyltin dilaurate.

[0010] Preferably, the preparation method of the modified chain extender includes the following steps:

[0011] Step SS1: Add 2-benzimidazole methanol, 4-nitrophthalic acid and concentrated sulfuric acid to toluene solvent, stir and disperse, and reflux for 8-10 h. After the reaction is completed, distill under reduced pressure, wash with 5% sodium carbonate and saturated brine in sequence, and dry to obtain intermediate.

[0012] Step SS2: Add the intermediate, zinc powder, and concentrated hydrochloric acid to toluene solvent, stir at room temperature for 10-12 hours. After the reaction is complete, add deionized water, neutralize with 1 mol / L sodium hydroxide solution, separate, extract, and dry to obtain the chain extender intermediate.

[0013] Step SS3: Add the chain extender intermediate to toluene, stir for 20-30 min under inert gas protection, then add curcumin, heat to 70-80℃, react for 4-6 h under light-protected conditions, cool, and evaporate under reduced pressure to obtain the modified chain extender.

[0014] Reaction principle: An intermediate containing a bisbenzimidazole structure is prepared by esterification of 2-benzimidazole methanol and 4-nitrophthalic acid. Then, a chain extender intermediate is prepared by reduction of the nitro group under the catalysis of a catalyst. Finally, the chain extender intermediate is substituted with curcumin to prepare a modified chain extender.

[0015] Preferably, in step SS1, the ratio of 2-benzimidazole methanol to 4-nitrophthalic acid is (4-4.5) mol: 1 mol.

[0016] Preferably, in step SS2, the ratio of the intermediate to concentrated hydrochloric acid is 1 mol: (10-12) mol.

[0017] Preferably, in step SS3, the ratio of the chain extender intermediate to curcumin is 1 mol: (0.45-0.5) mol.

[0018] The second objective of this invention is the application of the modified hyperbranched polyurethane prepared by the aforementioned method in topcoats for rail transit applications:

[0019] Step C1: Add the modified hyperbranched polyurethane and acrylic resin for rail transit field topcoat to the mixer and stir for 15-20 minutes at a speed of 500-700 r / min to obtain the premix.

[0020] Step C2: Next, add the dispersant, defoamer, titanium dioxide, functional agent A, curing agent, and deionized water to the premix in sequence and stir. Adjust the speed to 800-1000 r / min and stir for 0.5-1 h. After standing, discharge the material to obtain the topcoat for rail transit. The topcoat contains 20-25 parts by weight of modified hyperbranched polyurethane, 40-50 parts by weight of acrylic resin, 1-3 parts by weight of dispersant, 0.5-0.8 parts by weight of defoamer, 20-30 parts by weight of titanium dioxide, 3-5 parts by weight of functional agent A, 15-20 parts by weight of curing agent, and 30-40 parts by weight of deionized water.

[0021] Preferably, the preparation method of the functional agent A includes the following steps:

[0022] Step S1: Add 2-hydroxyterephthalic acid to tetrahydrofuran, stir until homogeneous, then add sodium hydroxide and stir for 1 hour. Slowly add hexachlorocyclotriphosphazene, heat to reflux, and react for 20 hours. Cool, filter, distill off half of the filtrate, then add deionized water, let stand, filter, wash, and dry to obtain intermediate 1. The synthetic route is as follows: ,in ;

[0023] Step S2: Intermediate 1,3,5-di-tert-butyl-4-hydroxybenzyl alcohol was added to an N-methylpyrrolidone solution, stirred until homogeneous, then the catalyst was added, and the temperature was raised to 180-200℃. The reaction was carried out for 3-5 hours. After the reaction, the temperature was cooled to 60-80℃, washed with alkaline solution, then washed with deionized water, and vacuum distilled to obtain functional agent A. The synthetic route is as follows:

[0024] ,in .

[0025] Preferably, the ratio of 2-hydroxyterephthalic acid, sodium hydroxide, and hexachlorocyclotriphosphazene is 1 mol: (1.1-1.2) mol: (0.14-0.16) mol.

[0026] Preferably, in step S2, the catalyst is tetrabutyl titanate, and its amount is 0.5% of the total mass of the reactants; the ratio of intermediate 1 to 3,5-di-tert-butyl-4-hydroxybenzyl alcohol is 1 mol: (12.5-13) mol; and the alkaline solution is a 10% sodium carbonate aqueous solution.

[0027] The beneficial effects of this invention are:

[0028] (1): This invention prepares a modified hyperbranched polyurethane containing a bisbenzimidazole structure and curcumin to participate in the preparation of a topcoat for rail transit. The conjugated system of benzimidazole and curcumin contains a large number of rigid aromatic rings and rigid structures, and contains long-chain branched structures to form a tough structure. The two form a rigid-toughness balanced complementary structure to synergistically improve the mechanical properties and UV resistance of the topcoat. The benzene rings contained in benzimidazole and curcumin can also coordinate with the metal surface to form a dense protective film, thereby improving the adhesion and corrosion resistance of the topcoat.

[0029] (2): This invention prepares phosphorus and nitrogen six-membered ring functional agent A containing a large number of hindered phenolic structures to participate in the preparation of rail transit topcoat. The large number of hindered phenolic structures can not only synergistically improve the anti-ultraviolet performance with curcumin, but also form a good binding force with a large number of ester groups contained in the topcoat through hydrogen bonding and polar interaction. This force can not only improve the phenomenon of easy precipitation of small molecule functional agents, but also form an interactive cross-linking force network structure, further improving the adhesion and anti-corrosion performance of the topcoat. In addition, the phosphorus and nitrogen six-membered ring structure can also provide phosphorus and nitrogen sources for the topcoat, and promote char formation after heating, giving it flame retardant effect.

[0030] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0032] The dispersant is polyvinylpyrrolidone;

[0033] The curing agent is aziridine;

[0034] The defoamer is tributyl phosphate.

[0035] Preparation Example 1

[0036] Preferably, a modified hyperbranched polyurethane for topcoat in the rail transit field is prepared by the following steps:

[0037] 110 parts by weight of dicyclohexylmethane-4,4'-diisocyanate and 2 parts by weight of 2,2-dimethylolpropionic acid were dissolved in 60 parts by weight of N,N-dimethylformamide. The mixture was heated to 85°C and reacted for 3 hours. After the reaction was completed, the temperature was controlled at 0°C. 60 parts by weight of a chain extender were added and the mixture was stirred for 30 minutes. 0.1 parts by weight of dibutyltin dilaurate catalyst were added, and the mixture was heated to 80°C and reacted for 6 hours. 15 parts by weight of acetone were added and the mixture was stirred for 10 minutes to obtain the modified hyperbranched polyurethane.

[0038] Preferably, the preparation method of the modified chain extender includes the following steps:

[0039] Step SS1: 0.4 mol of 2-benzimidazole methanol, 0.1 mol of 4-nitrophthalic acid, and concentrated sulfuric acid (1% of the total mass of 2-benzimidazole methanol and 4-nitrophthalic acid) were added to 100 mL of toluene solvent, stirred and dispersed, and the mixture was heated to reflux for 10 h. After the reaction was completed, the mixture was distilled under reduced pressure, washed successively with 5% sodium carbonate and saturated brine, and dried to obtain the intermediate.

[0040] Step SS2: Add 0.06 mol of intermediate, 30 g of zinc powder, and 0.6 mol of concentrated hydrochloric acid to 240 mL of toluene solvent, stir at room temperature for 12 h. After the reaction is complete, add 200 mL of deionized water, neutralize with 1 mol / L sodium hydroxide solution, separate, extract, and dry to obtain the chain extender intermediate.

[0041] Step SS3: Add 0.11 mol of the chain extender intermediate to 120 mL of toluene, stir for 30 min under inert gas protection, then add 0.05 mol of curcumin, heat to 80 °C, react for 6 h under light-protected conditions, cool, and evaporate under reduced pressure to obtain the modified chain extender (this step uses twice the amount of the chain extender intermediate mentioned above).

[0042] Preferably, the preparation method of the functional agent A includes the following steps:

[0043] Step S1: Add 1 mol of 2-hydroxyterephthalic acid to 200 mL of tetrahydrofuran, stir well, add 1.1 mol of sodium hydroxide, stir for 1 h, slowly add 0.14 mol of hexachlorocyclotriphosphazene, heat to reflux, react for 20 h, cool, filter, distill off half of the filtrate, then add deionized water, let stand, filter, wash, dry, to obtain intermediate 1;

[0044] Step S2: Add 0.01 mol of intermediate 1 and 0.13 mol of 3,5-di-tert-butyl-4-hydroxybenzyl alcohol to 200 mL of N-methylpyrrolidone solution, stir well, and then add tetrabutyl titanate catalyst (0.5% of the total mass of intermediate 1 and 3,5-di-tert-butyl-4-hydroxybenzyl alcohol). Heat to 180 °C and react for 3 h. After the reaction, cool to 60 °C, wash with 10% sodium carbonate aqueous solution, wash with deionized water, and then distill under vacuum to obtain functional agent A.

[0045] Example 1

[0046] The modified hyperbranched polyurethane prepared by the aforementioned method is used in topcoats for rail transit applications as follows:

[0047] Step C1: Add the modified hyperbranched polyurethane and acrylic resin for rail transit field topcoat to the mixer and stir for 15 minutes at a speed of 500 r / min to obtain the premix.

[0048] Step C2: Next, add the dispersant, defoamer, titanium dioxide, functional agent A, curing agent, and deionized water to the premix in sequence and stir. Adjust the speed to 800 r / min and stir for 0.5 h. After standing, discharge the material to obtain the topcoat for rail transit. The topcoat contains 20 parts by weight of modified hyperbranched polyurethane, 40 parts by weight of acrylic resin, 1 part by weight of dispersant, 0.5 parts by weight of defoamer, 20 parts by weight of titanium dioxide, 3 parts by weight of functional agent A, 15 parts by weight of curing agent, and 30 parts by weight of deionized water.

[0049] The modified hyperbranched polyurethane and functional agent A are the same as in preparation example 1.

[0050] Example 2

[0051] The application of the modified hyperbranched polyurethane in topcoats for rail transit is as follows:

[0052] Step C1: Add the modified hyperbranched polyurethane and acrylic resin for rail transit field topcoat to the mixer and stir for 20 minutes at a speed of 700 r / min to obtain the premix.

[0053] Step C2: Next, add the dispersant, defoamer, titanium dioxide, functional agent A, curing agent, and deionized water to the premix in sequence and stir. Adjust the speed to 1000 r / min and stir for 1 hour. After standing, discharge the material to obtain the topcoat for rail transit. The topcoat contains 24 parts by weight of modified hyperbranched polyurethane, 41 parts by weight of acrylic resin, 2 parts by weight of dispersant, 0.5 parts by weight of defoamer, 20 parts by weight of titanium dioxide, 4 parts by weight of functional agent A, 120 parts by weight of curing agent, and 30 parts by weight of deionized water.

[0054] The modified hyperbranched polyurethane and functional agent A are the same as in preparation example 1.

[0055] Example 3

[0056] The application of the modified hyperbranched polyurethane in topcoats for rail transit is as follows:

[0057] Step C1: Add the modified hyperbranched polyurethane and acrylic resin for rail transit field topcoat to the mixer and stir for 20 minutes at a speed of 700 r / min to obtain the premix.

[0058] Step C2: Next, add the dispersant, defoamer, titanium dioxide, functional agent A, curing agent, and deionized water to the premix in sequence and stir. Adjust the speed to 1000 r / min and stir for 1 hour. After standing, discharge the material to obtain the topcoat for rail transit. The topcoat contains 25 parts by weight of modified hyperbranched polyurethane, 50 parts by weight of acrylic resin, 3 parts by weight of dispersant, 0.8 parts by weight of defoamer, 30 parts by weight of titanium dioxide, 5 parts by weight of functional agent A, 20 parts by weight of curing agent, and 40 parts by weight of deionized water.

[0059] The modified hyperbranched polyurethane and functional agent A are the same as in preparation example 1.

[0060] Comparative Example 1

[0061] The application of the modified hyperbranched polyurethane in topcoats for rail transit is as follows:

[0062] Step C1: Add the modified hyperbranched polyurethane and acrylic resin for rail transit field topcoat to the mixer and stir for 20 minutes at a speed of 700 r / min to obtain the premix.

[0063] Step C2: Next, add the dispersant, defoamer, titanium dioxide, curing agent, and deionized water to the premix in sequence and stir. Adjust the speed to 1000 r / min and stir for 1 hour. After standing, discharge the material to obtain the topcoat for rail transit. The topcoat contains 24 parts by weight of modified hyperbranched polyurethane, 41 parts by weight of acrylic resin, 2 parts by weight of dispersant, 0.5 parts by weight of defoamer, 20 parts by weight of titanium dioxide, 120 parts by weight of curing agent, and 30 parts by weight of deionized water.

[0064] The modified hyperbranched polyurethane was prepared in the same way as in Example 1.

[0065] Comparative Example 2

[0066] The application of the modified hyperbranched polyurethane in topcoats for rail transit is as follows:

[0067] The topcoat for rail transit applications comprises the following raw materials in parts by weight: 24 parts by weight of modified hyperbranched polyurethane, 43 parts by weight of acrylic resin, 1 part by weight of dispersant, 0.6 parts by weight of defoamer, 22 parts by weight of titanium dioxide, 4 parts by weight of functional agent A, 17 parts by weight of curing agent, and 33 parts by weight of deionized water.

[0068] The preparation method of the topcoat for rail transit includes the following steps:

[0069] Step C1: Add the modified hyperbranched polyurethane for rail transit field topcoat, acrylic resin 1, and acrylic resin 2 to a mixer and stir for 20 minutes at a speed of 700 r / min to obtain a premix.

[0070] Step C2: Next, add the dispersant, defoamer, titanium dioxide, functional agent A, curing agent, and deionized water to the premix in sequence and stir. Adjust the speed to 1000 r / min and stir for 1 hour. After standing, discharge the material to obtain the topcoat for use in the rail transit field.

[0071] Functional agent A is the same as in Preparation Example 1. The difference between the modified hyperbranched polyurethane and Preparation Example 1 is that 1,6-hexanediol is used instead of chain extender.

[0072] Comparative Example 3

[0073] The application of the modified hyperbranched polyurethane in topcoats for rail transit is as follows:

[0074] Step C1: Add the modified hyperbranched polyurethane and acrylic resin for rail transit field topcoat to the mixer and stir for 20 minutes at a speed of 700 r / min to obtain the premix.

[0075] Step C2: Next, add the dispersant, defoamer, titanium dioxide, curing agent, and deionized water to the premix in sequence and stir. Adjust the speed to 1000 r / min and stir for 1 hour. After standing, discharge the material to obtain the topcoat for rail transit. The topcoat contains 24 parts by weight of modified hyperbranched polyurethane, 41 parts by weight of acrylic resin, 2 parts by weight of dispersant, 0.5 parts by weight of defoamer, 20 parts by weight of titanium dioxide, 120 parts by weight of curing agent, and 30 parts by weight of deionized water.

[0076] In the preparation of the topcoat, no functional agent A was added. The difference between the modified hyperbranched polyurethane and the preparation example 1 is that the chain extender was replaced with an equimolar amount of 1,6-hexanediol.

[0077] Performance testing

[0078] The topcoat obtained above was applied to the surface of the ABS board, with the following specific parameters: the construction environment was maintained at a temperature of 25℃ and a humidity of 50-60%; an air spray gun was used for spraying; and the final coating thickness was controlled at 50 micrometers. Then, the sprayed coating was flash-dried at ambient temperature for 30 minutes, followed by baking at 100℃ for 20 minutes. The cured coating was then subjected to the following performance tests.

[0079] (1) Pencil hardness test: The pencil hardness of the coating was tested according to the national standard GB / T6739-1966, "Determination of Pencil Hardness of Coating".

[0080] (2) The adhesion test of the grid method is carried out by drawing grids using a multi-purpose dry film tester. Adhesive tape is applied to the center of the grid, and then the tape is pulled off smoothly. The phenomenon of coating peeling is observed. The standard corresponding to the peeling state of the grid in the grid is calculated to make a judgment.

[0081] (3) Light aging resistance test: Take three 150mm×100mm samples and conduct the test according to the test method specified in Q / JLYJ7110279. The test time is 1200h. After the test, check the appearance and adhesion.

[0082] (4) Heat resistance aging test: Take three 150mm×100mm samples and put them in an 80℃ temperature chamber for 500h for high temperature heat aging test. After the test, check the appearance of the samples after restoring at room temperature for at least 30 minutes, and measure the appearance and adhesion of the samples after heat aging;

[0083] (5) Limiting oxygen index: The oxygen index of the coating is tested using a limiting oxygen index tester. The sample size is 120mm×6mm×3mm.

[0084] Table 1:

[0085]

[0086] Table 2:

[0087]

[0088] From the test results in the table above, it can be concluded that Examples 1-3 performed excellently in all tests, exhibiting superior hardness, adhesion, UV aging resistance, and heat aging resistance. Comparative Example 1, compared to Example 2, did not contain functional agent A during its preparation process, therefore the topcoat lacked nitrogen-phosphorus six-membered rings and a large amount of hindered phenols. After UV aging, the adhesion and hardness of the topcoat decreased significantly, and it lacked excellent flame retardant properties. The appearance also showed obvious changes. Due to the presence of modified chain extenders, the curcumin and benzimidazole structures also had UV-resistant effects, resulting in poor performance in UV aging resistance. The performance was excellent. Compared with Example 2, Comparative Example 2 used diol instead of modified chain extender in its preparation, so it lacked a large number of resistant aromatic rings and benzene rings in curcumin. Therefore, it performed worse than Example 2 in hardness and adhesion tests, but performed excellently in UV resistance tests and poorly in heat aging resistance tests. However, the large number of hindered phenolic structures it contained could also improve the heat aging resistance of the topcoat to a limited extent, making it better than Comparative Example 3. It also had excellent combustion-supporting properties. Compared with Example 2, Comparative Example 1, and Comparative Example 2, Comparative Example 3 did not contain modified chain extender and functional agent A, so its performance was the worst in all tests.

[0089] The above description is merely an example and illustration of the concept of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the concept of the invention or exceed the scope defined in the claims, they should all fall within the protection scope of the present invention.

Claims

1. A process for the preparation of modified hyperbranched polyurethane for the face coat in the field of rail transport, characterized by, The preparation method of the modified hyperbranched polyurethane comprises the following steps: The modified hyperbranched polyurethane is prepared by the following steps: dissolving dicyclohexylmethane-4,4'-diisocyanate and 2,2-dimethylol propionic acid in N,N-dimethylformamide, heating to 75-85 DEG C, reacting for 1.5-3 hours, after the reaction is completed, controlling the temperature at 0-5 DEG C, adding a modified chain extender, continuing to stir for 20-30 minutes, adding a catalyst, heating to 70-80 DEG C, reacting for 4-6 hours, adding acetone, and stirring for 5-10 minutes; The preparation method of the modified chain extender comprises the following steps: In step SS1, 2-benzimidazole methanol and 4-nitrophthalic acid are added to toluene solvent, stirred and dispersed, heated to reflux for 8-10 hours, after the reaction is completed, distilled under reduced pressure, washed with 5% sodium carbonate aqueous solution and saturated brine in sequence, and dried to obtain an intermediate; In step SS2, the intermediate and concentrated hydrochloric acid are added to toluene solvent, stirred at room temperature for 10-12 hours, after the reaction is completed, deionized water is added, neutralized with 1 mol / L sodium hydroxide solution, separated, extracted, and dried to obtain a chain extender intermediate; In step SS3, the chain extender intermediate is added to toluene, stirred for 20-30 minutes under inert gas protection, then curcumin is added, heated to 70-80 DEG C, reacted for 4-6 hours in dark, cooled, and distilled under reduced pressure to obtain the modified chain extender.

2. A process for the preparation of modified hyperbranched polyurethane for face coat in rail transportation sector as claimed in claim 1, wherein, 2,2-dimethylol propionic acid 1.4-2 parts by weight, dicyclohexylmethane-4,4'-diisocyanate 110-120 parts by weight, modified chain extender 60-70 parts by weight, catalyst 0.1-0.2 parts by weight, N,N-dimethylformamide 45-60 parts by weight, and acetone 15-20 parts by weight; the catalyst is dibutyltin dilaurate.

3. A process for the preparation of modified hyperbranched polyurethane for face coat in rail transportation sector as claimed in claim 1 wherein, In step SS1, the amount ratio of 2-benzimidazole methanol to 4-nitrophthalic acid is (4-4.5) mol:1 mol.

4. The process for the preparation of modified hyperbranched polyurethane for face coat in rail transportation sector as claimed in claim 1 wherein, In step SS2, the amount ratio of the intermediate to concentrated hydrochloric acid is 1 mol:(10-12) mol.

5. The process for the preparation of modified hyperbranched polyurethane for face coat in rail transportation sector as claimed in claim 1 wherein, In step SS3, the amount ratio of the chain extender intermediate to curcumin is 1 mol:(0.45-0.5) mol. ​ 6. The modified hyperbranched polyurethane prepared by the preparation method of claim 1 is applied in a topcoat in the field of rail transit. In step C1, the modified hyperbranched polyurethane and acrylic resin are added to a stirrer, stirred for 15-20 minutes at a rotation speed of 500-700 r / min to obtain a premix. Step C2: Then the dispersant, defoaming agent, titanium white, functional agent A, curing agent, deionized water are added into the premix in turn, stirring, and adjusting the speed to 800-1000r / min, stirring for 0.5-1h, standing, discharging, to prepare the topcoat for rail transit field, wherein the modified hyperbranched polyurethane is 20-25 parts by weight, the acrylic resin is 40-50 parts by weight, the dispersant is 1-3 parts by weight, the defoaming agent is 0.5-0.8 parts by weight, the titanium white is 20-30 parts by weight, the functional agent A is 3-5 parts by weight, the curing agent is 15-20 parts by weight, and the deionized water is 30-40 parts by weight; The preparation method of the functional agent A comprises the following steps: Step S1: 2-hydroxyterephthalic acid is added to tetrahydrofuran, stirred uniformly, then sodium hydroxide is added, stirred for 1h, hexachlorocyclotriphosphazene is slowly added, heated to reflux, reacted for 20h, cooled, filtered, distilled off half of the filtrate, then deionized water is added, stood, filtered, washed, dried, to prepare intermediate 1; Step S2: Intermediate 1 and 3,5-di-tert-butyl-4-hydroxybenzyl alcohol are added to an N-methylpyrrolidone solution, stirred uniformly, then a catalyst is added, heated to 180-200℃, reacted for 3-5h, after reaction, cooled to 60-80℃, washed with an alkali solution, then washed with deionized water, vacuum reduced pressure distilled, to prepare the functional agent A.

7. Use according to claim 6, characterized in that, In step S1, the amount ratio of 2-hydroxyterephthalic acid, sodium hydroxide and hexachlorocyclotriphosphazene is 1mol:(1.1-1.2)mol:(0.14-0.16)moL.

8. Use according to claim 6, characterized in that, In step S2, the catalyst is tetrabutyl titanate, and the amount is 0.5% of the total mass of the reactants; the amount ratio of intermediate 1 and 3,5-di-tert-butyl-4-hydroxybenzyl alcohol is 1mol:(12.5-13)moL; the alkali solution is a 10% mass fraction sodium carbonate aqueous solution.

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