Water-like paint coating problems and solutions for rail vehicles.

In recent years, with the development of economy, the demand for urban rail vehicles has also been growing rapidly. According to market department statistics, the company in the past 3 years the number of urban rail vehicle production more than 5,000 vehicles, coating capacity continues to increase the pressure on environmental protection is increasing. To this end, in order to cope with the serious situation of ecological environment protection, China's medium-speed vehicles to fight the fight against pollution, requiring the production process of urban rail vehicles must improve the environmental protection of coating, strictly control voC emissions

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water-based paint with water as a thinner, benzene-free derivatives and other organic solvents, with good stretch, breathability, aging resistance, not easy to crack off, very friendly to the environment. Its use on rail vehicles can reduce the solvent content of coatings from the front end of coating production, is an important means of VOC emission reduction, is conducive to the promotion of environmental protection of urban rail vehicles. In the coating production of water-based paint to replace the current common use of solvent-based coatings, can reduce the waste of petroleum energy resources, reduce resource consumption and environmental pollution, to solve the negative impact of harm to industrial workers and the surrounding people's health problems, in order to enhance the company's core competitiveness, is a performance of the company's manufacturing capacity. This paper introduces in detail the construction process, coating structure and paint film performance requirements of water-based paint for rail vehicles, and systematically analyzes the characteristics of water-based paint, the problems encountered in the production process and the measures to solve them.
01 City rail vehicle water-based paint introduction
1.1water-based paint coating structure
city rail vehicle coating coating is divided into water-based epoxy primer, water-based paint matching polyester putty, water-based polyurethane paint and water-based polyurethane paint, the vehicle body paint color is 3 kinds;of the coating of urban rail vehicles in Figure 1
the process of coating urban rail vehicles is shown in Figure 2.figure 2 city rail vehicle coating process diagram
1.1.1 water-based epoxy primer
water-based epoxy primer as the first layer of the coating system, directly connected to the substrate, its adhesion and corrosion resistance on the performance and quality of the entire coating has a greater impact. Therefore, water-dependent epoxy primers need to have excellent adhesion, excellent chemical resistance, corrosion resistance, good polishing and so on. Water-based epoxy primer construction according to the quality ratio of 5:1 (base material: curing agent) preparation, mixing evenly, add deionized water, adjust the viscosity to 30 to 50 s (T-4 cups), take wet touch wet spraying process spray two, layer interval 10 to 20 in. After spraying, it will run flat at room temperature by 0.5 and bake for 2 h at 60 degrees C. After the paint film is dried, control the thickness of the primer dry film 40 to 80 m.
1.1.2 Water-based polyester putty
after polishing the primer layer, according to the quality ratio of 100:2 (base material: curing agent) preparation of water-based polyester putty, scraped 3 to 4 times. After the putty is dried, the surface of the polished body should be free of knife marks, pinholes and other defects, and the control putty film thickness should not exceed 3 mm.
1.3 The main role of painting in
water-based polyurethane is to improve the aesthetic appearance of the car body, repair the defects of the putty layer and improve the weather resistance of the coating. Water-based polyurethane paint construction with 180 sandpaper first polished putty layer, and then according to the quality ratio of 6:1 (base material: curing agent) preparation, mixed evenly, add deionized water, adjust the viscosity to 20 to 30 s (T-4 cups), take wet spray process spray two, layer interval 10 to 20 min. After spraying, level 0.5 h at room temperature and bake 2 h at 60 degrees C. After the paint film is dried, the control coating film is 80 to 3160 m thick, and the gloss degree (60 degrees) is 50% to 60%.
1.1.4 Water-based polyurethane finishes
According to the characteristics of urban rail vehicles, water-based polyurethane finishes have low-temperature curing, high appearance, high hardness and good aging and solvent resistance. When spraying water-based polyurethane paint, first with 240 objective sandpaper polishing in the paint layer, and then according to the mass ratio of 5:2 (base material: curing agent) preparation, mixing evenly, add deionized water, adjust the viscosity to 18 to 25 s (T-4 cups), take wet spray process spray two, layer interval 10 to 20 min. After spraying, level 0.5 h at room temperature and bake 2 h at 60 degrees C. After the paint film is dried, the control coating film is 120 to 3240 m thick, the gloss degree (20 degrees) ≥80%;Figure 3 city rail vehicle water-based paint appearance
1.2 water-based paint coating performance requirements
based on the application experience of city rail vehicles sprayed solvent-based paint, combined with water-based paint on urban rail vehicles use, urban rail vehicles water-based paint performance requirements see Table 1.
Table 1 City rail vehicle water-based paint performance requirements and inspection methods
1.3 coating assembly line using water-based paint modification item point

1.3.1 Transport and storage of water-based paint
Water-based paint storage cycle is much shorter than solvent-based paint, usually only 6 months, in the process of use to do a good job of "first-in, first-out", and water-based paint freezing point than the past use of solvent-based paint is much higher, freezing stability is also poor, once frozen, water-based paint will fail, and the process is irreversible. The coating line adds heating equipment during the transportation and storage of water-coated paint to ensure that the ambient temperature ≥ 15 degrees Celsius.
1.3.2 Coating assembly line equipment
(1) water is a corrosive medium, coating assembly line equipment containers and conveyor lines need to be modified into stainless steel material;
(2) Because the construction of water-based paint is greatly affected by temperature and humidity, water-based paint drying needs to be coated with assembly line spray, paint room with mandatory drying function and constant temperature and humidity function to ensure the quality of water-based paint coating and paint film appearance
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02 Necessity of water-quality paint for urban rail vehicles
The requirements of environmental assessment by environmental protection departments at all levels of the country have been increasing, with the issuance of the Special Action Plan of "Two Minus Six And Three Promotions" by the Jiangsu Provincial Committee of the Communist Party of China and the Jiangsu Provincial People's Government in early 2017 .20 16) No. 47), which clearly proposes to control volatile organic pollution, requiring the transportation manufacturing industry to fully use water-based, high solids, solvent-free and other low VOCs content of coatings to replace the original solvent-based coatings from January 1, 2018.
in order to meet the relevant environmental requirements, urban rail vehicles are coated with water-like paint production. After the use of water-based paint, the effect of reducing VOCs is obvious, and the VOC content in the construction state is reduced from 550 g/L of solvent-based paint to 150 g/L, which greatly reduces the emission of VOC during the construction of coating lines.
2.1 . Water-based paint performance is equally superior
water-based paint with reference to solvent-based paint standards testing, with solvent-based paint consistent with chemical stability, corrosion resistance to aging, decorative and adhesion, bending and other comprehensive properties.
2.2 . Water-like paint is more environmentally
the use of water-like paint is conducive to coating assembly line emissions in line with increasingly stringent environmental regulations.
2.3 . Water-based paint is safer
water-based paint with water as a solvent, compared to solvent-based paint, not only reduces energy consumption, but also reduces the fire risk, improves the occupational health level of workers, and ensures the health of construction workers.
2.4 . Water-based paint cleaning
after the city rail vehicles are coated, spraying equipment cleaning is a troublesome and essential work. Solvent-based coating spraying equipment needs to be cleaned with a large number of organic solvents, higher cost and greater pollution, water-based paint spraying equipment can be washed directly with water, low cost, more environmentally friendly.
2.5 . Body surface aesthetics
Water-based paint on aluminum alloy material city rail vehicle body surface adaptability, strong coating adhesion, car body welds, corner parts can be coated with a certain thickness of paint film, improve the body's anti-corrosion performance and paint film uniform, flat, smooth, paint film overall appearance is better than solvent-based paint.
03 city rail vehicle water-based paint coating problems and solutions
At present, water-based paint in the rail vehicle coating production has been used for a period of time, due to its low volatilization rate, surface pressure, poor wetting, evaporation heat, heat content value of large materialization characteristics, in the production process still appeared wetting bad, poor atomization, flow level, flow hanging and other defects, affecting the construction and quality of urban rail vehicle coating.
in addition to improving the quality of water-based paint products, but also in the rail vehicle coating production and construction process, strict water-based paint construction parameters.
3.1 . Effect of pre-treatment on urban rail vehicle coating
water-based paint with water as a solvent, its surface can be 72 mN/m, solvent-based paint solvent surface can be 25 mN/m, so water-based paint for substrate wetting effect is much worse than solvent-based paint. Therefore, water-based paint in the city rail vehicle smooth aluminum alloy surface is difficult to play a good "wetting" effect, and after the pre-treatment of the body surface if adhered to oil and other impurities, the coating may appear paint film shrink hole phenomenon. Therefore, the body of the city rail vehicle must be sanding treatment, so that the coating and the surface of the body form a good adhesion. Select 40 purpose brown coman jade as a blast abrasive, remove all impurities on the substrate surface, achieve roughness 6.3≤Ra≤12.5, substrate surface cleanliness Sa2.5.
3.2 . The influence of water-colored paint pairing city rail vehicle coating
water-like paint is highly denatured and is highly susceptible to shear force. Water-like paint in the deployment process stirring speed over the General Assembly affects its performance, and even denaturation causes the city rail vehicle coating failure. Therefore, it is necessary to strictly control the speed of the pneumatic mixer stirring, while adding curing agent stirring, to be stirred evenly and then add deionized water dilution. Control the amount of water added according to the product verification prior to the start of the city rail vehicle project, and refer to the viscosity cup (T-4) for quantification, according to the quality of the city rail vehicle coating constantly adjust the addition of water to ensure that the water-based paint coating performance will not decay.
3.3 . Effect of water-based paint on the coating of urban rail vehicles
After the two components of water-based paint substrate and curing agent are configured, in addition to the reaction of polyols and isocyanates in the system, it is inevitable that water and isocyanate in the system will also react at the same time. With the extension of time, the greater the amount of isocyanate root being consumed invalidly, the lower the cross-linking degree of the coating and the worse the performance of the paint film. However, there is no obvious change in the appearance of water-based paint, if the water-based paint is still used beyond the applicable period, the coating will appear dry, poor adhesion and solvent resistance performance is not good, such as the performance of the coating instability, resulting in material waste. Therefore, it is necessary to strictly control the application time after mixing water-based paint substation and curing agent and make a good record, generally not more than 3 h.
3.4 . Effect of water-based paint on urban rail vehicle coating
Water-based paint with deionized water as solvent, due to the water's own surface pressure is large, wetting is poor, resulting in water-based paint prone to hanging, shrinking holes, "tweezers" phenomenon
Large (organic solvent: 350 to 450 kJ/kg; deionized water: 2258 kJ/kg), under the same conditions, the evaporation of water takes longer, so the water-based paint use environment temperature and humidity on the drying of water-based paint has a great impact. Temperature a certain time, if the relative humidity is too large, moisture volatility is slow, increasing the chance of the coating flow hanging, shrink holes, "tweezers" phenomenon, affecting the quality of the city rail vehicle coating. Therefore, water-based paint spraying construction needs to strictly control environmental conditions, temperature 15 to 30 degrees C, relative humidity 30% to 80%, paint indoor wind speed <0.5 m/s;
3.5 . The effect of water-coated coating on urban rail vehicle coating
In order to ensure the quality of urban rail vehicle coating, local repair of the body is needed after the coating work is completed, and the effect of repair directly affects the quality of urban rail vehicle coating. Because the solubility of water-based lacquer water is poor, the repair must use weak organic solvent as the interface water, its solubility should not be too strong, otherwise there will be paint colorless, color difference and other problems; At the same time to provide professional training to improve the level of water-quality paint repair technology and operating staff skills.
3.6 . The influence of water-
the cost of water-like paint has a great influence on the promotion and application of water-like paint in urban rail vehicles. According to the water-based paint system used in the coating assembly line, the price of the material increased by about 30% to 40% compared to solvent-based paint, and the mandatory drying equipment and thermostat and humidity equipment were also added during the construction process, while energy consumption was also increased, and the treatment of wastewater after construction also needed to increase costs to avoid water pollution. Therefore, when urban rail vehicles are painted with water-based paint, it is necessary to reduce material consumption and reduce the cost pressure caused by strengthening management, optimizing the process and equipping it with water-based electrostitive equipment.
04 Conclusion
Finally, compared with traditional solvent-based paint, water-based paint has the advantages of environmental protection, safety, good appearance performance and occupational health, which is suitable for the development trend of urban rail vehicle coating, but the problem of water-based paint is still the problem in the process of urban rail vehicle coating production. More, need to improve the quality of water-based paint raw materials and our coating quality management level, production process level and coating equipment capacity can be solved one by one, so that the appearance of urban rail vehicle coating aesthetics can be guaranteed;
(slightly) for
details, see Coatings and Protection, No. 10, 2019
Author/Dai.