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Yi Hongbin, Department of Mechanical Engineering, Hunan Industry Polytechnic, Changsha, Hunan 410007. Abstract: This paper mainly introduces the composition, characteristics, performance requirements of automotive powder coatings, as well as the basic principles of electrostatic powder spraying. It can be seen that automotive powder electrostatic spraying is a new type of environmentally friendly coating technology in the new century. Keywords: environmental protection ； Automotive powder coatings ； Electrostatic spraying 1 Introduction The 21st century is known as a new era focused on the environment; environmental protection has received widespread attention worldwide and has become one of the most urgent research topics for humanity. In European and American countries with strict environmental regulations, the development of low-VOC (volatile organic compounds) coatings in aqueous or powder form, as well as of painting equipment for such coatings, began at an early stage; these technologies have been successfully applied in a large number of production lines. With the significant increase in automobile production, there are growing demands for added values such as the appearance and functionality of coating films, as well as for measures to reduce environmental impact, which has forced changes in coating equipment and coating application systems. This article provides an introduction only to automotive powder coatings and electrostatic spraying technologies in the new century. 2 Composition and Characteristics of Automotive Powder Coatings 2.1 Composition of Automotive Powder Coatings Powder coatings are composed of resins, curing agents (in thermosetting powder coatings), pigments, fillers, and additives (including levelers, stabilizers, etc.). They are similar to ordinary coatings, except that they contain no volatile solvents; therefore, they are an environmentally friendly type of coating. 2.2 Characteristics of automotive powder coatings (1) Powder coatings do not contain organic solvents, thereby avoiding the risks of fires, poisoning, and safety issues associated with transportation caused by organic solvents. Although there is a risk of dust explosions, explosions can be completely avoided as long as the dust concentration in the system is kept under control. (2) There is no atmospheric pollution caused by organic solvents, meeting environmental protection requirements. (3) Powder coatings are 100% solid systems, and a closed-loop system can be used; the spilled powder coating can be recovered and reused, resulting in a coating utilization rate of over 95%. (4) The molecular weight of the resins used in powder coatings is higher than that of solvent-based coatings; therefore, the performance and durability of the coating films are significantly improved compared to solvent-based coatings. (5) When applying powder coatings, the thickness of the coating film can be controlled; the thickness achieved in a single application can range from 30 to 50 um, which is equivalent to the thickness obtained after applying several to a dozen layers of solvent-based coatings. This reduces the number of application steps, which not only helps save energy but also improves production efficiency. (6) During construction, there is no need to adjust the viscosity according to seasonal changes ； It is easy to use in construction; no highly skilled operating techniques are required, and defects such as sagging in the coating do not occur even when applying a thick layer ； It is easy to implement automated assembly line production. (7) It is easy to maintain cleanliness in the construction area; the powder that adheres to the skin can be blown off with compressed air or washed away with warm water or soapy water, without the need for irritating cleaning agents. (8) Powder coatings do not require solvents, making them an effective energy-saving measure. Since the primary raw material for most solvents is crude oil, reducing the amount of solvents used is equivalent to directly saving crude oil consumption. 3 Requirements of powder coatings for electrostatic spraying (1) Considering the charging of the powder coating, the spraying process, adhesion, and the quality of the coating film, an appropriate particle size range is between 10-80 um, with a narrower distribution being preferable. (2) Considering the charging of powder coatings and their melting and leveling during heating, particles that are nearly spherical yield better results. (3) The volume resistivity of powder coatings should be appropriate; if it is too low, the powder does not charge easily, and if it is too high, the powder does not adhere well to the workpiece. (4) The surface resistance of powder coatings should be high; if it is low, charge can easily leak from the edges and corners of the workpiece, causing the powder coating to fall off. The rate at which this charge disappears increases rapidly, bounded by a certain surface resistance. 4 Powder electrostatic spraying for automobiles 4.1 Equipment for powder electrostatic spraying of automobiles The equipment for powder electrostatic spraying consists mainly of four components: a charged powder gun, a powder supply device, an air supply system, and a recovery device. 4.2 Automotive Powder Electrostatic Spraying Process 4.2.1 Basic Principles of Automotive Powder Electrostatic Spraying The basic principles of automotive powder electrostatic spraying are the same as those of electrostatic coating using liquid coatings. High-voltage direct current (30-90 kv) is applied between the grounded metal workpiece and the gun electrode; compressed air is used to spray the powder coating from the gun into the air. These powder particles collide with air particles that have been ionized by the corona discharge generated at the gun electrode, resulting in charged powder particles, which then adhere to the surface of the grounded metal workpiece. Due to the high volume resistivity of powder coatings (1010–1016 Ω·cm), the powder particles adsorbed on the workpiece release their charge slowly; as a result, the charged powder particles can continue to adhere to the surface of the workpiece until the insulation is broken down, at which point the thickness of the coating layer no longer increases. However, care should be taken during spraying to avoid pinholes in the coating film. 4.2.2 Automotive Powder Electrostatic Spraying Process The automotive powder electrostatic spraying process consists of four steps: (1) Pre-treatment of the workpiece – pickling and phosphating: The workpiece, after pre-treatment, should be free of oil and rust, with a smooth surface; any weld slag must be completely removed from the welded areas. The process flow is: electrochemical degreasing --> hot water washing --> cold water washing --> acid washing --> cold water washing --> hot water washing --> phosphating --> cold water washing --> air drying --> drying. (2) Powder electrostatic spraying: An automatic multi-gun system is used to spray from various directions such as top, bottom, left, and right, ensuring even powder distribution with a consistent thickness. The spray gun is placed 150-250 mm away from the workpiece, and the electrostatic powder coating process is shown in the figure below. (3) Baking and curing: An infrared tunnel oven or an infrared oven can be used. The temperature of the box furnace should be maintained between 180±5°C, with a baking time of 25±5 minutes. The temperature inside the tunnel oven is 170–180°C, and the baking time should be maintained at 20–25°C. The components to be baked should be 80-100 mm away from the bottom and side walls of the oven. (4) Cooling inspection: The components taken out of the oven after baking are at a high temperature; they should be immediately quenched in water and inspected to ensure the quality of the coating. 4.3 Characteristics of the automotive electrostatic powder coating method (1) The coating thickness is uniform and easy to control. (2) Coatings have strong adaptability; most powder coatings can be applied using electrostatic powder spraying. (3) It has strong adaptability to the objects to be coated; it can be used for spraying on parts of various sizes, and is also suitable for coating parts of different shapes. (4) The construction process is simple, resulting in a low rate of defective coated parts. (5) It contains no solvents, so there are no safety or hygiene issues associated with solvents during use. (6) It can be used for mixed coating with solvent-based coatings or electrophoretic deposition coatings. 5 Conclusion As a representative of solvent-free coatings, powder coatings, just like water-based coatings, have attracted worldwide attention in the automotive coating industry, experiencing sustained rapid growth over the past 10 years. Powder coatings come second only to water-based coatings, accounting for about 20% of total production, and are increasingly replacing solvent-based coatings. Powder electrostatic spraying will be an environmentally friendly new spraying technology for sustainable development in the automotive industry of the new century. After 20 years of development, China’s aluminum profile industry has established a complete industrial system and has become one of the pillar industries of the national economy. The production of primary aluminum has exceeded 2.5 million tons, ranking it third in the world. There are over 1,000 aluminum processing enterprises, with a production capacity of 3.5 million tons per year; the output of aluminum profiles has reached 1 million tons per year. It has now entered an era of advanced development, with rapid progress in surface treatment technologies for aluminum profiles. These technologies have evolved from the simple use of bronze-colored coatings to include processes such as powder coating, fluorocarbon spraying, organic and inorganic coloring, electrophoretic coating, as well as mechanical and chemical polishing. New generation wood-grain simulation techniques, three-stage electrolytic multi-color coloring, and micro-arc oxidation coloring methods are also in the trial production stage. Aluminum profiles can come in various colors such as titanium gold, golden yellow, champagne, stainless steel imitation, among others; they can also exhibit special effects like a mirror finish, matte finish, or pearlescent finish. Among the various surface treatment techniques, electrophoretic coating is the most promising and applicable technology. Aluminum profiles undergo anodization and electrolytic coloring before being electrophoretically coated, providing a double layer of protection. It features high transparency, a strong metallic appearance, excellent corrosion resistance and weather resistance. It is more decorative than painted or powder-coated surfaces, is highly favored by users, and has been widely used in home decoration, offering a huge potential market. Secondly, the process is easy to implement; simply adding a few tanks to the existing production line and establishing a pipeline for aluminum oxidation coloring will suffice. Water-based electrophoretic coating is safe, allows for closed-loop recycling, and is beneficial to the environment. Thirdly, and more importantly, electrophoretic profiles are more attractive and serve as a model for high-end, luxurious profiles. Having an electrophoretic coating line is an important indicator of the overall technical level of an aluminum profile factory. 2.4.2 Electrophoretic coating of aluminum profiles 2.4.2.1 Characteristics of electrophoretic coating Since its use in Ford Motor Company’s wheel coating lines in the 1930s, electrophoretic coatings have seen rapid development in terms of application. Transparent, weather-resistant acrylic and polyurethane electrophoretic coatings have been successfully used since the 1970s for the decoration and protection of metal surfaces, such as the protection of copper signs, as well as for the decoration and protection of stainless steel and electroplated products. The most representative use of these coatings as a final protective coating is in the electrophoretic coating of architectural aluminum profiles. The traditional surface treatment processes for aluminum profiles include oxidation, coloring, and sealing. The conversion film formed on such aluminum is prone to corrosion when used outdoors over extended periods, especially in environments with acid rain or in marine settings, resulting in poor durability. Therefore, since the 1970s, efforts have been made to use coating methods to improve the decorative qualities and service life of aluminum profiles used in construction. Currently, electrocoating, powder coating, and fluorocarbon spraying are among the methods that have been successfully implemented on an industrial scale. Since electrophoretic coatings are water-based, aluminum can be directly electrophoretically coated after pretreatment and washing. The electrophoretic coating has a high degree of transparency; it not only provides good decorative properties but also enhances the natural metallic luster of the aluminum profiles. As a result, electrophoretic coating is being used increasingly on architectural aluminum profiles. Compared with other coating methods, electrophoretic coating has the following advantages: (1) It facilitates automated production. Since electrophoretic coating is carried out in an aqueous electrophoretic tank, similar to anodizing and electrolytic coloring processes, the processing time is short, making it easy to implement a pipeline operation for the entire process. ⑵The coating film is uniform and dense. Thanks to the high penetration ability of electrophoretic coatings, even profiles with complex shapes can achieve a uniform coating layer, and the film thickness can be controlled by adjusting the charge amount. ⑶The utilization rate of the coating is high. Due to its low viscosity, less material is carried away from the workpiece, and electrophoretic-coated parts can be washed with water. The use of recovery devices enables the utilization rate of paint in electrophoretic coating to exceed 95%. ⑷Safety and environmental protection. Due to the water-based formulation of electrophoretic coatings, they have a low solid content and low solvent levels, making them environmentally friendly coatings; they also eliminate the risk of fires and ensure the health of workers. ⑸The coating quality is good. Acrylic resins are cured with amino resins, ensuring high decorative qualities and excellent corrosion resistance for the coating film. Moreover, due to the high transparency of these resins, the metallic texture is effectively highlighted; depending on requirements, matte, textured, or pearlescent decorative effects can also be achieved. ⑹Compared with the conventional electrolytic coloring and sealing process, it is time-saving and labor-efficient; the electrophoretic coating does not require sealing, thus avoiding problems such as cracks caused by inadequate sealing. ⑺The thickness of the coating film can be freely controlled; domestically and internationally, it is usually set at 7μm and 12μm. ⑻Since the electrophoretic layer is transparent and shiny, high quality is required of the aluminum ingots, as well as a smooth surface and few mechanical defects in the extruded materials. Strict control is necessary over the oxidation coloring process, because any minor defect or contamination will become visible beneath the transparent coating. ⑼High requirements are placed on the management of the electrophoresis process to improve the yield of finished products.