Water-based coatings are used in the rail bus industry

China Coatings Online News Information
Abstract: The differences between water-based coatings and traditional solvent-based coatings are analyzed from the perspectives of rail bus coating functionality and water-based coating design, and water-based coating performance requirements for rail vehicles. At the same time, combined with the problems of testing water-based coatings on a city railway vehicle, the feasibility of applying water-based coatings on rail vehicles is analyzed.
Keywords: water-based coatings; rail buses; coating design; performance requirements
0. Introduction
With the development of high-speed train set technology and urban rail vehicle technology in recent years, China has been one of the world's largest manufacturers of rail vehicles, the annual output of rail buses has exceeded the 10,000 mark. At present, most of China's rail bus coating to solvent-based coatings, of which a large number of VOC emissions of environmental pollution is becoming more and more serious. In addition, solvent-based coatings contain a large number of volatile organic solvents, the organic exhaust gas emitted by breathing directly on the human body, to cause harm to the human body. At the same time, the atomized dust particles formed by solvent-based coatings under the action of air pressure are easily adsorbed to contacts and are potential carcinogens. In order to protect the environment of human co-existence, the development trend of the coating industry today is low pollution, pollution-free, tending to use environmentally friendly materials.(Japan Shinkansen" train With the continuous development of coating technology, coating materials are constantly being updated, water-based coatings are in the development process of the emergence of a new generation of coating products. With the gradual introduction of laws, regulations and standards for the reduction of VOC emissions requirements around the world, water-based coatings are bound to replace or partially replace traditional solvent-based coatings, and become an indispensable part of the coating industry. It is necessary for us to carry out in-depth research and technical accumulation from the aspects of coating design, coating selection, the performance of the coating itself and construction methods.
1. Functional analysis of rail bus coating and water-based coating design
At present, China's rail vehicle body material to aluminum alloy, stainless steel, carbon steel-based. Among them, the rail vehicles of stainless steel body mainly use brushed passivation technology, coating-free, because of the heavy body of carbon steel body, with the large area of new aluminum alloy material use, its use is gradually reduced. This paper focuses on the application of water-based coatings in aluminum alloy vehicles.
from the vehicle's point of view, aluminum alloy body in the natural environment does not have enough corrosion resistance to ensure that the vehicle body life cycle is not damaged, so must be coated with primer corrosion treatment. The flatness of the vehicle requires
2mm/m, while the surface decoration of the vehicle is high, so putty must be applied. Paint coatings are essential for decorative requirements.
From the point of view of water-based coatings, because the solvent-water surface of water-based coatings can be 72mN/m, solvent-based paint solvents can be 25mN/m, so water-based coatings for substrate wetting effect is much worse than solvent-based coatings, easy-forming piles into "oil drops." At present, the most common aluminum alloy pre-treatment methods are sanding, sanding, oxidation and so on. One of the most effective ways to improve water-based coatings is to improve the surface area of the substrate by sanding, improve the mechanical binding force of water-based coatings and substrates. The other 2 treatments are not suitable for water-based coatings.
for the paint coating, there are currently 2 commonly used coatings, one is solid paint, one is the substrate cover paint system. For industrial design with fewer colors and few rules, it is more suitable to use solid color paint. For the complex color division, metal color is more suitable to use the background color cover light paint system. Among them, the water-like background color shield light system also has the characteristics of easy lighting, convenient color division, graffiti resistance, weather resistance and excellent performance. Based on the above theoretical analysis, the water-based coating structure applied to aluminum alloy body is designed, as in Table 1.
2. Water-based coating performance requirements for rail vehicles
Based on the experience of solvent-based coatings and the characteristics of water-based coatings, through a large number of routine performance tests, the performance requirements of aluminum alloy vehicles with water-based coatings are summarized, as shown in Table 2.
3. Problems encountered in the actual construction
Based on the previous coating structure design and laboratory performance test, the effect is more ideal, we apply water-based paint in a city iron low-floor car, which has a lot of details to pay attention to.
3.1 pre-treatment
as the saying goes: "three-point coating
, seven-point coating", and the quality of coating is the key to pre-treatment, choose the appropriate pre-treatment method is very important. Since the surface of water-based coatings can be more than 3 times lower than solvent-based coatings, water-based coatings in the smooth aluminum alloy metal surface is difficult to play a good "wetting" effect, so aluminum alloy substrates must be sanding treatment, Ra≥5 m. If the substrate is thin and cannot be sanded, solvent-based epoxy primers can only be applied through grinding.
3.2 paint
because most of the water-based coatings are water dispersion type, it is necessary to apply shear force to the main agent curing agent and other substances fully mixed. Water-based coating preparation process must be carried out using a special agitator, in compliance with strict mixing methods (mixing while adding curing agents, and then adding deionized water dilution). To ensure that the performance of the coating after filming will not decay.
3.3 paint after mixing available time
water-based paint after mixing available time of 2 to 4h (varies between specific product types), must strictly control the application time after mixing, more than the applicable time of the coating can not be used. There is no obvious change in the appearance of the coating beyond the applicable time, once the misoperation, the use of the coating more than the applicable time, there will be paint shedding, non-drying, powdering and other phenomena, affecting the quality of products.
3.4 Diluent use
The main thinner of water-based coatings is water (there are also a few manufacturers in the thinner to add additives to improve the performance of a certain aspect), water selection is very important, the production is commonly used in deionized water, it removes the yin and yang in the water, to ensure that the coating in a stable liquid medium reaction.
At the same time must control the amount of water added to adjust the viscosity to facilitate construction is appropriate, but it is not appropriate to add too much, the general primer added less than 20% of the original paint, medium coating, paint addition is less than 15% of the original paint. Too much thinner can affect the drying of the paint film and produce defects. At the same time, it will affect the crosslinking of the main agent curing agent and the final performance after filming.
3.5 construction environment
the construction of spraying needs to control the environmental conditions more strictly, the temperature of 18 to 30 degrees C. Relative humidity 30% to 80%, spray indoor wind speed 0.5m/s. Practice has proved that the above conditions can better ensure the realization of coating functions (corrosion, color, gloss). If the above conditions can not be met, there will be many problems in the construction, these problems are in our daily production can be avoided by adjusting the construction parameters.
drying of 3.6 coatings
most sprayed coatings require mandatory drying due to the need to match the continuous production of beats in the workshop. It is important to note that before the mandatory drying we need to flash dry 20 to 30min, and then 60C baking 2h. The above conditions are optimal, once the control is not good, it is easy to shrink holes, tweezers and other defects.
3.7 Water-based coating repair
because of the poor solubility of water, so water-based coating interface water must use weak organic solvents, its solubility should not be too strong, too strong will appear paint coloring, spots and other issues. At the same time, its solubility should not be too weak, solubility is not strong interface insoluble phenomenon, affecting the appearance.
3.8 wastewater treatment
If the use of water-spin paint room, must choose a suitable paint mist clotting agent, to ensure that paint mist can be gathered and suspended in the pool, easy to salvage. In the final step of wastewater treatment, acid and alkali must be leveled to ensure that the excluded sewage meets environmental requirements.
with the development of industry
people pay more and more attention to the production process that is beneficial to the environment. The coating industry is responsible for the development of environmentally friendly coatings, which requires us to carefully study a variety of environmentally beneficial coatings. Water-based coatings just started, we need to do a lot of work, must be in the future work, do not forget the original intention of protecting the environment, to ensure product quality, design needs, comprehensive consideration of a variety of complex factors, water-based coatings development work.
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