Water-based finish paint for outer surface of motor train unit and preparation method of water-based finish paint

Abstract

The invention discloses a water-based coating composition, a water-based coating prepared from the water-based coating composition, and application of the water-based coating composition and the water-based coating as finish paint, especially application of the water-based coating composition and the water-based coating as finish paint on the outer surface of a railway vehicle. The water-based paint composition is prepared from the following raw materials in parts by weight: 10 to 50 parts by weight of chain extension modified polyurethane prepolymer, 1 to 6 parts by weight of L-cystine dimethyl ester hydrochloride, 5 to 10 parts by weight of N, N '-dimethylformamide, 1 to 5 parts by weight of triethylamine, 0.1 to 3 parts by weight of 2-amino-4-hydroxy-6-methylpyrimidine and 40 to 100 parts by weight of deionized water. A finish paint layer formed after the water-based paint is sprayed has the characteristics of higher hardness, proper tensile property, good wear resistance, excellent self-healing capability and long service life, and can adapt to various adverse environments encountered by a motor train unit train.

Description

Technical Field

[0001] The invention belongs to the technical field of coatings, and in particular relates to a water-based topcoat coating for the outer surface of a motor vehicle unit and a preparation method thereof. Background Art

[0002] High-speed EMUs play an extremely important role in transportation. The coating used on the exterior surface of the vehicle body is currently the most widely used type of coating on EMUs. With the increase in the number of vehicles, the increase in the service life, and the repair and maintenance of vehicles, the amount of EMU coating will increase year by year and always remain at a high level. Since high-speed EMUs have long routes and cross a wide area, they are more likely to encounter various harsh climate environments. In addition to being subjected to cold and hot changes, wind, rain, snow and frost all year round, they are also often subjected to wind and sand impact corrosion, rolled up gravel impact, coastal salt spray corrosion, and strong wind pressure difference impact at the entrance and exit of tunnels. Therefore, the coating system for the exterior surface of high-speed EMUs must have better performance than conventional coatings. At present, traditional solvent-based coatings are still widely used. Solvent-based coatings must be diluted with the help of organic solvents and contain a large amount of toxic and harmful substances such as benzene, toluene, xylene, formaldehyde, etc., and their volatilization period is as long as more than ten years. Environmentally friendly water-based coatings are the trend to replace solvent-based coatings.

[0003] The Chinese invention patent "A modified waterborne polyurethane resin environmentally friendly coating and its preparation method" (patent announcement number CN115717025B, announcement date September 29, 2023) discloses a modified waterborne polyurethane resin environmentally friendly coating, including 10-50 parts of waterborne fluorinated polyurethane emulsion, 20-50 parts of filler, 5-10 parts of Au / polyurethane microspheres, 0.5-2 parts of dispersant, 0.5-2 parts of wetting agent, 0.5-3 parts of film former, 0.1-5 parts of thickener, 0.1-2 parts of defoamer, 0.1-2 parts of mildewproof agent, and 5-20 parts of water. Nano gold and titanium dioxide powder are used in the preparation process of the coating. Although it will enhance the aging resistance of the coating, the addition of nano gold has a certain inhibition on the tensile properties of the coating, and the cost of nano gold is relatively high, which is not suitable for large-scale application of the coating on the outer surface of EMUs. The coating disclosed in the Chinese invention patent "A method for preparing a non-fluorine, traceless, anti-sewage polyurethane self-stratified coating" (patent announcement number CN115074014B, announcement date April 25, 2023) avoids the biological toxicity of fluorine-containing materials and exhibits certain hydrophobic anti-fouling and self-healing properties. However, the water-based polyurethane coating has no significant improvement in hardness, and the coating needs to be cured for 72 hours at a temperature of 50-100°C after being applied to the substrate. Such a construction process is not suitable for EMUs. Summary of the invention

[0004] In order to overcome the above-mentioned deficiencies of the prior art, the present invention provides a water-based coating for the outer surface of a train set and a preparation method thereof. The coating has a suitable cost-effectiveness ratio and a simple construction process; in addition, the coating of the present invention has excellent mechanical properties and self-healing ability, which prolongs its actual service life on the outer surface of the train set.

[0005] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0006] A water-based coating composition comprises the following raw materials in parts by weight:

[0007] 10-50 parts by weight of chain-extended modified polyurethane prepolymer, 1-6 parts by weight of L-cystine dimethyl ester hydrochloride, 5-10 parts by weight of N,N'-dimethylformamide, 1-5 parts by weight of triethylamine, 0.1-3 parts by weight of 2-amino-4-hydroxy-6-methylpyrimidine and 40-100 parts by weight of deionized water;

[0008] Wherein, the chain-extended modified polyurethane prepolymer is prepared by the following method:

[0009] S1. 2,2-dimethylolpropionic acid and polytetramethylene ether glycol were uniformly dispersed under nitrogen protection;

[0010] S2. Slowly add isophorone diisocyanate at 60-90°C, stir evenly, then add dibutyltin dilaurate in portions, after addition, keep warm for 10-50 minutes, and then stop heating;

[0011] S3. When the temperature of the reaction system drops by 10 to 20 ° C, 2,2'-diaminodiphenyl disulfide and p-menthane-1,8-diol are added, and the reaction is kept warm for 1 to 5 hours, and then the heating is stopped;

[0012] S4. After the temperature of the reaction system drops by 5 to 10 ° C, polydimethylsiloxane is added, and the reaction is kept warm for 0.5 to 1.5 hours, and then triethylamine is added to neutralize, and the result is obtained;

[0013] The molar ratio of 2,2-dihydroxymethyl propionic acid to polytetramethylene ether glycol is 1:1-3.

[0014] The molar ratio of isophorone diisocyanate to polytetramethylene ether glycol is 2 to 18:1.

[0015] The molar ratio of 2,2'-diaminodiphenyl disulfide to p-menthane-1,8-diol is 1:0.8-1.5,

[0016] The molar ratio of 2,2'-diaminodiphenyl disulfide to isophorone diisocyanate is 0.5 to 2:8.

[0017] The molar ratio of polydimethylsiloxane to isophorone diisocyanate is 0.5 to 2:4.

[0018] The weight of dibutyltin dilaurate is 6-8% of the total weight of 2,2-dihydroxymethylpropionic acid, polytetramethylene ether glycol, isophorone diisocyanate, 2,2'-diaminodiphenyl disulfide and p-menthane-1,8-diol.

[0019] The molar ratio of triethylamine to 2,2-dihydroxymethylpropionic acid is 0.8-1.2:1.

[0020] Preferably, in step S1, the molar ratio of 2,2-dihydroxymethylpropionic acid to polytetramethylene ether glycol is 1:2.

[0021] Preferably, in step S2, the molar ratio of isophorone diisocyanate to polytetramethylene ether glycol is 6 to 12:1.

[0022] Preferably, in step S2, the reaction temperature is 80°C.

[0023] Also preferably, in step S2, the reaction time is 30 min.

[0024] Preferably, in step S3, the molar ratio of 2,2'-diaminodiphenyl disulfide to p-menthane-1,8-diol is 1:1.2.

[0025] Preferably, in step S3, the reaction temperature is 60°C.

[0026] More preferably, in step S3, the reaction time is 3 hours.

[0027] Preferably, in step S4, the molar ratio of triethylamine to 2,2-dihydroxymethylpropionic acid is 1:1.

[0028] Also preferably, in step S4, the reaction temperature is 50°C.

[0029] More preferably, in step S4, the reaction time is 1 h.

[0030] Preferably, in step S4, the neutralization reaction time of triethylamine is 30 min.

[0031] In the preparation method of the chain-extended modified polyurethane prepolymer provided by the present invention, the isocyanate monomer used isophorone diisocyanate, which has lower toxicity, better yellowing resistance and relatively easy reaction control compared to aromatic isocyanate monomers.

[0032] Polydimethylsiloxane has the characteristics of high flexibility, stability, hydrophobicity and low surface energy due to the existence of ether bonds in the chain structure. Polydimethylsiloxane is copolymerized with -NCO in isophorone diisocyanate to introduce soft segments, making the prepared polyurethane have good flexibility, excellent low temperature performance and strong hydrolysis resistance.

[0033] When preparing the chain-extended modified polyurethane prepolymer, the present invention firstly disperses polytetramethylene ether glycol and 2,2-dihydroxymethyl propionic acid uniformly, then slowly adds isophorone diisocyanate, and finally adds dibutyltin dilaurate for catalysis, so that the reaction speed of polytetramethylene ether glycol and isophorone diisocyanate is slowed down, and the relative viscosity of the product is improved; at the same time, the reaction heat release is slowed down, so that the polymer chain of the polyurethane molecule becomes soft and neat, and the tensile resistance of the coating is enhanced.

[0034] The present invention realizes the self-healing property of waterborne polyurethane by introducing the reversible disulfide bond of 2,2'-diaminodiphenyl disulfide. The molar ratio of 2,2'-diaminodiphenyl disulfide to isophorone diisocyanate is regulated so that enough -NCO can be connected to -NH2, and the chain length of polyurethane can be increased; and the exchange rate of the disulfide bond of 2,2'-diaminodiphenyl disulfide is faster than that of aliphatic disulfide bonds and the healing ability is outstanding. However, 2,2'-diaminodiphenyl disulfide has a negative effect on the hardness of waterborne polyurethane. The inventor unexpectedly found that adding a certain amount of para-menthane-1,8-diol with a rigid structure can not only help improve the hardness, but also its asymmetric structure accelerates the exchange of disulfide bonds, further promoting the self-healing ability of polyurethane.

[0035] As a preferred embodiment, the aqueous coating composition comprises the following raw materials in parts by weight: 30 to 50 parts by weight of chain-extended modified polyurethane prepolymer, 2 to 6 parts by weight of L-cystine dimethyl ester hydrochloride, 6 to 8 parts by weight of N,N'-dimethylformamide, 2 to 5 parts by weight of triethylamine, 1 to 3 parts by weight of 2-amino-4-hydroxy-6-methylpyrimidine and 60 to 80 parts by weight of deionized water.

[0036] Preferably, the molar ratio of L-cystine dimethyl ester hydrochloride to 2-amino-4-hydroxy-6-methylpyrimidine is 1-2:1.

[0037] The present invention also provides a method for preparing the water-based coating composition, comprising the following steps:

[0038] (5) preparing the components according to the weight parts;

[0039] (6) Dissolve L-cystine dimethyl ester hydrochloride in N,N ’- dimethylformamide, add triethylamine, keep warm for 10 to 60 minutes, filter, and the filtrate is a modified cystine dimethyl ester solution for standby use;

[0040] (7) cooling the chain-extended modified polyurethane prepolymer to 0-10° C., slowly adding the modified L-cystine dimethyl ester solution obtained in step (1), then slowly warming to room temperature and reacting for 1-4 hours;

[0041] (8) The reaction solution obtained in step (3) is heated to 50-90° C., 2-amino-4-hydroxy-6-methylpyrimidine is added, and the reaction is kept warm for 2-4 hours, then the temperature is lowered to 30-40° C., deionized water is added in portions, and the mixture is stirred and dispersed for 20-40 minutes to obtain the product.

[0042] Preferably, in step (2), the reaction temperature is 40°C.

[0043] More preferably, in step (2), the reaction time is 30 min.

[0044] More preferably, in step (3), the reaction time is 3 h.

[0045] Preferably, in step (4), the reaction solution obtained in step (3) is heated to 70° C., and 2-amino-4-hydroxy-6-methylpyrimidine is added.

[0046] More preferably, in step (4), the temperature is lowered to 35° C. after the reaction is completed.

[0047] Also preferably, in the step (4), after adding deionized water, the stirring and dispersing time is 30 minutes.

[0048] Therefore, the present invention provides a water-based coating, which is prepared by the above preparation method.

[0049] Another object of the present invention is to provide the use of the above-mentioned water-based coating composition or the water-based coating prepared by the above-mentioned method as a topcoat.

[0050] Preferably, the above water-based coating composition or the water-based coating prepared by the above method is used as a topcoat on the outer surface of a rail vehicle.

[0051] Therefore, the present invention provides a process for spraying topcoat on the outer surface of a rail vehicle, comprising using the water-based coating composition of the present invention or the water-based coating prepared by the above method, spraying twice, and the dry film thickness is 80 to 120 μm.

[0052] In the water-based coating composition of the present invention, cyano and L-cystine dimethyl ester hydrochloride and 2-amino-4-hydroxy-6-methylpyrimidine in the polyurethane prepolymer are polymerized under the catalysis of dibutyltin dilaurate to obtain a water-based polyurethane coating with a certain hardness, not easy to creep, and self-healing ability. In the water-based coating composition, the positively charged end of the N,N-dimethylformamide molecule is surrounded by methyl groups, forming a spatial hindrance, so that negative ions cannot approach and can only associate with positive ions. Therefore, N,N-dimethylformamide as a solvent makes the reaction of L-cystine dimethyl ester hydrochloride and 2-amino-4-hydroxy-6-methylpyrimidine easier to carry out than in other solvents, and is particularly suitable for the synthesis of sterically hindered esters, thereby enhancing the self-healing ability of the topcoat.

[0053] The present invention further realizes the self-healing waterborne polyurethane with the synergistic effect of disulfide bonds and quadruple hydrogen bonds by regulating the molar ratio of 2-amino-4-hydroxy-6-methylpyrimidine and L-cystine dimethyl ester hydrochloride in the main chain of the waterborne polyurethane. The polyurethane cross-linked only by hydrogen bonds in 2-amino-4-hydroxy-6-methylpyrimidine has high dynamics, so it is easy to creep and has low stability. After the introduction of covalent disulfide bonds, the synergistic effect with non-covalent quadruple hydrogen bonds makes the polyurethane have stronger healing ability and better hardness. At the same time, the 2-amino-4-hydroxy-6-methylpyrimidine used contains urea groups, so that the thermal decomposition temperature of the polyurethane is higher, and the reaction between isocyanate and hydroxyl can introduce 2-amino-4-hydroxy-6-methylpyrimidine into the end group of the polyurethane, which enhances the elasticity and enables the polyurethane coating to adapt to various adverse conditions encountered by the EMU.

[0054] In summary, the topcoat layer formed by the water-based coating composition of the present invention after spraying has the characteristics of high hardness, suitable tensile properties, good wear resistance, excellent self-healing ability and long service life, and can adapt to various adverse environments encountered by EMU trains. DETAILED DESCRIPTION

[0055] The present invention is described below with reference to specific examples. It will be appreciated by those skilled in the art that these examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention in any way.

[0056] The experimental methods in the following examples are conventional methods unless otherwise specified. The raw materials, reagents, etc. used in the following examples are commercially available products unless otherwise specified. The purchase of some raw materials and reagents is as follows:

[0057] Polytetramethylene ether glycol was purchased from Guangzhou Haoyi New Materials Technology Co., Ltd. with a molecular weight of 2000.

[0058] Polydimethylsiloxane was purchased from Shandong Longhui Chemical Co., Ltd., model 201.

[0059] Example 1 Preparation of chain-extended modified polyurethane prepolymer

[0060] 1. Preparation of chain-extended modified polyurethane prepolymer A

[0061] S1. Under nitrogen protection, 1 mol of 2,2-dimethylol propionic acid and 2 mol of polytetramethylene ether glycol are uniformly dispersed at a speed of 300 rpm, and this speed remains unchanged in the subsequent steps;

[0062] S2. Slowly add 12 mol of isophorone diisocyanate at 80 ° C, stir evenly, add 0.99 mol of dibutyltin dilaurate in portions, react for 30 minutes, and then stop heating;

[0063] S3. When the temperature of the reaction system drops to 60°C, 1 mol of 2,2'-diaminodiphenyl disulfide and 1.2 mol of p-menthane-1,8-diol are added and reacted for 3h, and then heating is stopped;

[0064] S4. After the temperature of the reaction system drops to 50°C, 2 mol of polydimethylsiloxane is added to react for 1 hour, and then 1.1 mol of triethylamine is added to neutralize for 30 minutes to obtain a chain-extended modified polyurethane prepolymer A.

[0065] 2. Preparation of chain-extended modified polyurethane prepolymer B:

[0066] The preparation steps are substantially the same as those of the chain-extended modified polyurethane prepolymer A, except that in step S2, 1 mol of isophorone diisocyanate is added.

[0067] 3. Preparation of chain-extended modified polyurethane prepolymer C:

[0068] The preparation steps are basically the same as those of the chain-extended modified polyurethane prepolymer A, except that in step S3, 0.5 mol of p-menthane-1,8-diol is added.

[0069] Example 2 A water-based coating composition and water-based coating prepared therefrom

[0070] The water-based coating composition of this embodiment is composed of the following raw materials in parts by weight:

[0071] 30 parts of chain-extended modified polyurethane prepolymer A prepared in Example 1, 2 parts of L-cystine dimethyl ester hydrochloride, N,N ， - 8 parts of dimethylformamide, 1.7 parts of triethylamine, 1.5 parts of 2-amino-4-hydroxy-6-methylpyrimidine and 100 parts of deionized water; wherein 1 part by weight = 1 kg.

[0072] The above water-based paint is used as a raw material, and the water-based paint is prepared by the following steps:

[0073] (1) dissolving L-cystine dimethyl ester hydrochloride in N,N'-dimethylformamide at 40°C, adding triethylamine, keeping the temperature for 30 minutes, filtering to remove the triethylamine hydrochloride, and the filtrate is a modified L-cystine dimethyl ester solution;

[0074] (2) Cooling the chain-extended modified polyurethane prepolymer A to 5° C., slowly adding it to the modified L-cystine dimethyl ester solution obtained in step (1), then slowly warming it to room temperature and reacting for 2 hours;

[0075] (3) The reaction solution obtained in step (2) was heated to 70° C., 2-amino-4-hydroxy-6-methylpyrimidine was added, and the reaction was carried out for 3 h. The temperature was then lowered to 35° C., and deionized water was slowly injected in batches. After the addition was completed, the mixture was dispersed at a stirring speed of 200 rpm for 30 min.

[0076] Example 3 A water-based coating composition and water-based coating prepared therefrom

[0077] The water-based coating composition of this embodiment is composed of the following raw materials in parts by weight:

[0078] 10 parts of chain-extended modified polyurethane prepolymer A prepared in Example 1, 2 parts of L-cystine dimethyl ester hydrochloride, N,N ， - 10 parts of dimethylformamide, 1.7 parts of triethylamine, 0.7 parts of 2-amino-4-hydroxy-6-methylpyrimidine and 100 parts of deionized water; wherein 1 part by weight = 1 kg.

[0079] The above water-based paint is used as a raw material, and the water-based paint is prepared by the following steps:

[0080] (1) Dissolve L-cystine dimethyl ester hydrochloride in N,N ， - dimethylformamide, add triethylamine, keep warm for 60 minutes, filter to remove triethylamine hydrochloride, and the filtrate is a modified L-cystine dimethyl ester solution;

[0081] (2) cooling the chain-extended modified polyurethane prepolymer A to 1° C., slowly adding the modified L-cystine dimethyl ester solution obtained in step (1), then slowly warming to room temperature and reacting for 1 hour;

[0082] (3) The reaction solution obtained in step (2) was heated to 50° C., 2-amino-4-hydroxy-6-methylpyrimidine was added, the reaction was carried out for 4 h, the temperature was lowered to 30° C., and deionized water was slowly injected in batches. After the addition was completed, the mixture was dispersed at a stirring speed of 200 rpm for 40 min.

[0083] Example 4A water-based coating composition and water-based coating prepared therefrom

[0084] The water-based coating composition of this embodiment is composed of the following raw materials in parts by weight:

[0085] 50 parts of polyurethane prepolymer A prepared in Example 1, 3 parts of L-cystine dimethyl ester hydrochloride, N,N ， - 10 parts of dimethylformamide, 1.7 parts of triethylamine, 1.8 parts of 2-amino-4-hydroxy-6-methylpyrimidine and 100 parts of deionized water; wherein 1 part by weight = 1 kg.

[0086] The above water-based paint is used as a raw material, and the water-based paint is prepared by the following steps:

[0087] (1) Dissolve L-cystine dimethyl ester hydrochloride in N,N ， - dimethylformamide, add triethylamine, keep warm for 10 minutes, filter to remove triethylamine hydrochloride, and the filtrate is a modified L-cystine dimethyl ester solution;

[0088] (2) cooling the chain-extended modified polyurethane prepolymer A to 10° C., slowly adding the modified L-cystine dimethyl ester solution obtained in step (1), then slowly warming to room temperature and reacting for 2 h;

[0089] (3) The reaction solution obtained in step (2) was heated to 90° C., 2-amino-4-hydroxy-6-methylpyrimidine was added, the reaction was continued for 2 h, the temperature was lowered to 40° C., deionized water was slowly injected in batches, and after the addition was completed, the mixture was dispersed at a stirring speed of 200 rpm for 20 min to obtain the product.

[0090] Example 5 A water-based coating composition and water-based coating prepared therefrom

[0091] The water-based coating composition of this embodiment is composed of the following raw materials in parts by weight:

[0092] 30 parts of chain-extended modified polyurethane prepolymer B prepared in Example 1, 2 parts of L-cystine dimethyl ester hydrochloride, N,N ， - 8 parts of dimethylformamide, 1.7 parts of triethylamine, 1.5 parts of 2-amino-4-hydroxy-6-methylpyrimidine and 100 parts of deionized water; wherein 1 part by weight = 1 kg.

[0093] The above water-based coating was used as a raw material and the steps of Example 2 were followed to prepare a water-based coating, except that the chain-extended modified polyurethane prepolymer was the chain-extended modified polyurethane prepolymer B prepared in Example 1.

[0094] Example 6 A water-based coating composition and water-based coating prepared therefrom

[0095] The water-based coating composition of this embodiment is composed of the following raw materials in parts by weight:

[0096] 30 parts of the chain-extended modified polyurethane prepolymer C prepared in Example 1, 2 parts of L-cystine dimethyl ester hydrochloride, N,N ， - 8 parts of dimethylformamide, 1.7 parts of triethylamine, 1.5 parts of 2-amino-4-hydroxy-6-methylpyrimidine and 100 parts of deionized water; wherein 1 part by weight = 1 kg.

[0097] The water-based coating was prepared using the above water-based coating as a raw material according to the steps of Example 2, except that the chain-extended modified polyurethane prepolymer was the chain-extended modified polyurethane prepolymer C prepared in Example 1.

[0098] Example 7 A water-based coating composition and water-based coating prepared therefrom

[0099] The water-based coating composition of this embodiment is composed of the following raw materials in parts by weight:

[0100] 30 parts of chain-extended modified polyurethane prepolymer A prepared in Example 1, 2 parts of L-cystine dimethyl ester hydrochloride, N,N ， - 8 parts of dimethylformamide, 1.7 parts of triethylamine, 0.35 parts of 2-amino-4-hydroxy-6-methylpyrimidine and 100 parts of deionized water; wherein 1 part by weight = 1 kg.

[0101] The water-based coating was prepared using the above water-based coating as a raw material according to the steps of Example 2.

[0102] Example 8 A water-based coating composition and water-based coating prepared therefrom

[0103] The water-based coating composition of this embodiment is composed of the following raw materials in parts by weight:

[0104] 30 parts of chain-extended modified polyurethane prepolymer A prepared in Example 1, 2 parts of L-cystine dimethyl ester hydrochloride, N,N ， - 8 parts of dimethylformamide, 1.7 parts of triethylamine, 2.1 parts of 2-amino-4-hydroxy-6-methylpyrimidine and 100 parts of deionized water; wherein 1 part by weight = 1 kg.

[0105] The water-based coating was prepared using the above water-based coating as a raw material according to the steps of Example 2. Test Case Performance measurement of water-based coatings prepared in Examples 2-8

[0106] 1. Determination of the properties of the water-based coatings prepared in Examples 2-8

[0107] 20 g of the water-based coatings prepared in Examples 2-8 were weighed respectively, poured into 11 cm×11 cm×0.5 cm polytetrafluoroethylene molds, evenly spread and placed in a 30° C. oven for 12 hours to obtain the corresponding water-based polyurethane films. The films were tested for the following items.

[0108] 1.1 Tensile properties test

[0109] At room temperature, the mechanical properties of waterborne polyurethane film samples were tested using a Shanghai Jiezhun XWW-20A universal tensile testing machine: the waterborne polyurethane film samples were cut into dumbbell-shaped specimens (the specimens must be free of gaps and bubbles), the clamp spacing was 5 cm, the tensile rate was set to 100 mm / min, and the same sample was measured 3 times and the average value was taken. The results are shown in Table 1.

[0110] 1.2 Healing performance test

[0111] The film was cut into dumbbell-shaped specimens of 10 mm × 10 mm × 0.4 mm, and a 0.1 mm deep wound was made on the dumbbell-shaped specimens with a scalpel. The surface healing of the sample was observed under an optical microscope (BKPOL, Chongqing, China), and the healing time was recorded. The results are shown in Table 1.

[0112] 1.3 Hardness test

[0113] The hardness of the film was characterized by a pencil hardness tester. During the test, a pressure of 765g was applied, the angle between the pencil lead and the surface to be tested was 45°, and three strokes were made at the position to be tested, each stroke was 10mm long, and then the pencil marks were erased with an eraser. No scratches should appear on the surface, and no visible abrasions or scratches were allowed at other locations. During the test, the pencil hardness grade should be judged from hard to soft. The hardness corresponding to the first pencil that does not leave scratches on the surface of the sample to be tested is the "pencil hardness" of the sample to be tested. The results are shown in Table 1.

[0114] 2. Determination of Adhesion of Water-Based Coatings Prepared in Examples 2-8

[0115] The water-based coating prepared in Example 2-8 was sprayed on the surface of the water-based polyurethane varnish, and the spraying thickness was controlled at 40±5 μm. Adhesion test was performed according to the cross-hatch method of GB / T 31586.2. The results are shown in Table 1.

[0116] Table 1 Performance test results of various embodiments

[0117]

[0118] It can be seen from the data in Table 1 that the tensile strength of the water-based coatings of Examples 2-8 makes the cured films have certain anti-twisting properties, wherein the adjustment of the coating composition formula will make the film too soft or too hard, resulting in easy deformation during application. The polyurethane films formed by the water-based coatings of Examples 2-4, 6 and 8 have good self-healing ability. However, because Example 5 uses chain-extended modified polyurethane prepolymer B, the film loses its self-healing ability. It can be seen from the hardness data in the table that the water-based polyurethane coating of the present invention has a certain wear resistance after curing, but Example 5 uses chain-extended modified polyurethane prepolymer B, and the amount of 2-amino-4-hydroxy-6-methylpyrimidine in Example 7 is relatively small, which has a certain impact on the hardness; the reason may be the reduction of dynamic repair chains.

[0119] The embodiments described above do not impose any form of limitation on the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A water-based coating composition comprising the following raw materials in parts by weight: 10-50 parts by weight of chain-extended modified polyurethane prepolymer, 1-6 parts by weight of L-cystine dimethyl ester hydrochloride, 5-10 parts by weight of N,N'-dimethylformamide, 1-5 parts by weight of triethylamine, 0.1-3 parts by weight of 2-amino-4-hydroxy-6-methylpyrimidine and 40-100 parts by weight of deionized water; in, The chain-extended modified polyurethane prepolymer is prepared by the following method: S1. 2,2-dimethylolpropionic acid and polytetramethylene ether glycol were uniformly dispersed under nitrogen protection; S2. Slowly add isophorone diisocyanate at 60-90°C, stir evenly, then add dibutyltin dilaurate in portions, after addition, keep warm for 10-50 minutes, and then stop heating; S3. When the temperature of the reaction system drops by 10 to 20 ° C, 2,2'-diaminodiphenyl disulfide and p-menthane-1,8-diol are added, and the reaction is kept warm for 1 to 5 hours, and then the heating is stopped; S4. After the temperature of the reaction system drops by 5 to 10 ° C, polydimethylsiloxane is added, and the reaction is kept warm for 0.5 to 1.5 hours, and then triethylamine is added to neutralize, and the result is obtained; The molar ratio of 2,2-dihydroxymethyl propionic acid to polytetramethylene ether glycol is 1:1-3. The molar ratio of isophorone diisocyanate to polytetramethylene ether glycol is 2 to 18:

1. The molar ratio of 2,2'-diaminodiphenyl disulfide to p-menthane-1,8-diol is 1:0.8-1.5, The molar ratio of 2,2'-diaminodiphenyl disulfide to isophorone diisocyanate is 0.5 to 2:

8. The molar ratio of polydimethylsiloxane to isophorone diisocyanate is 0.5 to 2:

4. The weight of dibutyltin dilaurate is 6-8% of the total weight of 2,2-dihydroxymethylpropionic acid, polytetramethylene ether glycol, isophorone diisocyanate, 2,2'-diaminodiphenyl disulfide and p-menthane-1,8-diol. The molar ratio of triethylamine to 2,2-dihydroxymethylpropionic acid is 0.8-1.2:

1.

2. The water-based coating composition according to claim 1, characterized in that: In the step S1, the molar ratio of 2,2-dihydroxymethylpropionic acid to polytetramethylene ether glycol is 1:

2.

3. The aqueous coating composition according to claim 1 or 2, characterized in that: In the step S2, the molar ratio of isophorone diisocyanate to polytetramethylene ether glycol is 6 to 12:1; Preferably, in step S2, the reaction temperature is 80°C; Also preferably, in step S2, the reaction time is 30 min.

4. The aqueous coating composition according to any one of claims 1 to 3, characterized in that In the step S3, the molar ratio of 2,2'-diaminodiphenyl disulfide to p-menthane-1,8-diol is 1:1.2; Preferably, in step S3, the reaction temperature is 60°C; More preferably, in step S3, the reaction time is 3 hours.

5. The aqueous coating composition according to any one of claims 1 to 4, characterized in that In the step S4, the molar ratio of triethylamine to 2,2-dimethylolpropionic acid is 1:1; Preferably, in step S4, the reaction temperature is 50°C; Preferably, in step S4, the reaction time is 1 h; Preferably, in step S4, the neutralization reaction time of triethylamine is 30 min.

6. The aqueous coating composition according to any one of claims 1 to 5, characterized in that The water-based coating composition comprises the following raw materials in parts by weight: 30 to 50 parts by weight of the chain-extended modified polyurethane prepolymer, 2 to 6 parts by weight of L-cystine dimethyl ester hydrochloride, 6 to 8 parts by weight of N,N'-dimethylformamide, 2 to 5 parts by weight of triethylamine, 1 to 3 parts by weight of 2-amino-4-hydroxy-6-methylpyrimidine and 60 to 80 parts by weight of deionized water; Preferably, the molar ratio of L-cystine dimethyl ester hydrochloride to 2-amino-4-hydroxy-6-methylpyrimidine is 1-2:

1.

7. The method for preparing the aqueous coating composition according to any one of claims 1 to 6, comprising the steps of: (1) preparing the components according to the weight parts; (2) Dissolve L-cystine dimethyl ester hydrochloride in N,N ’ - dimethylformamide, add triethylamine, keep warm for 10 to 60 minutes, filter, and the filtrate is a modified cystine dimethyl ester solution for standby use; (3) cooling the chain-extended modified polyurethane prepolymer to 0-10° C., slowly adding the modified L-cystine dimethyl ester solution obtained in step (1), then slowly warming to room temperature and reacting for 1-4 hours; (4) heating the reaction solution obtained in step (3) to 50-90° C., adding 2-amino-4-hydroxy-6-methylpyrimidine, keeping the temperature to react for 2-4 hours, then cooling to 30-40° C., adding deionized water in portions, stirring and dispersing for 20-40 minutes, and obtaining the product; Preferably, in step (2), the reaction temperature is 40°C; Further preferably, in the step (2), the reaction time is 30 min; Further preferably, in step (3), the reaction time is 3 h; Preferably, in step (4), the reaction solution obtained in step (3) is heated to 70° C., and 2-amino-4-hydroxy-6-methylpyrimidine is added; Further preferably, in the step (4), the temperature is lowered to 35° C. after the reaction is completed; Also preferably, in the step (4), after adding deionized water, the stirring and dispersing time is 30 minutes.

8. A water-based coating prepared by the preparation method according to claim 7.

9. Use of the water-based coating composition according to any one of claims 1 to 6 or the water-based coating prepared by the method according to claim 7 as a topcoat; Preferably, the water-based coating composition according to any one of claims 1 to 6 or the water-based coating prepared by the method according to claim 7 is used as a topcoat on the outer surface of a rail vehicle.

10. A process for spraying topcoat on the outer surface of a rail vehicle, comprising using the water-based coating composition according to any one of claims 1 to 6 or the water-based coating prepared by the method according to claim 7, spraying twice, and having a dry film thickness of 80 to 120 μm.

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