Introduction to automotive coatings and various application processes and formulations

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According to China Paint Network, automotive coatings refer to the paints applied to the bodies and components of various types of vehicles such as sedans, jeeps, buses, and trucks. Sometimes they also include paints for agricultural machinery such as tractors, harvesters, and motorcycles. Generally, they denote the paints and auxiliary materials used in manufacturing new vehicles, as well as the paints used for repairing vehicles. As an outdoor mode of transportation, cars have their specific operating conditions. Moreover, with the rapid development of the automobile industry in recent years, the production volume of cars has increased significantly, which has led to the painting processes for cars being transformed entirely toward high-speed and modern assembly-line methods. Based on these characteristics, automobile paint is required to have the following properties. (1) Beautiful appearance. The paint film is required to be full, with a gorgeous and soft luster, good color contrast, and a variety of colors that are in line with current trends. Today, metal flash coatings and coatings containing mica pearlescent pigments are commonly used in cars to give them a more attractive appearance and aesthetic appeal. (2) Excellent weather and corrosion resistance, required to be suitable for various temperatures, exposure to sunlight, and wind and rain erosion, maintaining its appearance without loss of luster, discoloration, bubbling, cracking, peeling, powdering, or rusting under all climate conditions. The service life of the paint film is required to be no less than that of the vehicle itself, typically exceeding 10 years. (3) Excellent constructability and compatibility. Automobile paint is usually applied in multiple layers, as a single layer generally does not yield good performance; therefore, it is necessary for there to be good adhesion between the various layers, with no defects. It also requires that the properties of the paint itself be suitable for the modern painting lines in the automotive industry. (4) Excellent mechanical properties. To withstand the high speeds, frequent vibrations, and stresses associated with vehicles, the paint film needs to have excellent adhesion, be hard yet flexible, and possess properties such as impact resistance, bend resistance, scratch resistance, and friction resistance. (5) Excellent scrub resistance and stain resistance. It must resist cleaning with brushes, soap, and detergents, and leave no residue after contact with other common stains. (6) Good repairability. 1. Main types of car paint (1) Based on the surface to which it is applied, car paint can be classified as: ① Original paint for new cars ； ②Car repair paint. (2) Classified from bottom to top according to the coatings applied on vehicles: ① Primers for vehicles, which are now mostly electrophoretic paints ； ②Intermediate coating for automobiles, that is, the intermediate coat ； ③Automotive base coats (including solid-color base coats and metallic flash base coats) ； ④Car topcoats generally refer to solid-color topcoats that do not require a clear coat ； ⑤Automotive topcoat varnish ； ⑥Car repair paint. (3) Classification by coating application method: ① Electrophoretic paint for automobiles ； ②Liquid paint for cars ； ③Powder coatings for automobiles ； ④Special coatings for automobiles, such as PVC sealing coatings ； ⑤Post-coating treatment materials (anti-rust wax, protective wax, etc.). (4) Classification by application location on the vehicle: ① Coatings for vehicle bodies ； ②Truck bed coating ； ③Corrosion-resistant coatings for components such as wheels and frames ； ④Coatings for engine components ； ⑤Chassis coating ； ⑥Paints for interior vehicle decoration. 2. Characteristics and common types of primers for automobiles: Primers for automobiles are the first coat of paint applied directly to the surface of a vehicle body or component that has undergone surface treatment; they represent the beginning of the entire coating process. Depending on the areas of the vehicle where the automotive primer is used, it is required to have good adhesion to the substrate, good compatibility with the topcoat or intermediate coat applied afterward. It must also possess excellent corrosion resistance, rust resistance, oil resistance, chemical resistance, and water resistance. Of course, the paint film formed by the automotive primer should also possess adequate mechanical properties such as hardness, gloss, flexibility, and stone-chip resistance. With the rapid development of the automotive industry, the requirements for automotive primers are also increasing. In the 1950s, cars were coated with nitrocellulose primers or epoxy primers; thereafter, this evolved gradually to solvent-based dip primers, water-based dip primers, anodic electrophoretic primers, and cathodic electrophoretic primers. Currently, higher-end vehicles, especially sedans, generally use cathodic electrophoretic primers. Over the past 20 years, and through the adoption of advanced technologies and processes, cathodic electrophoretic primers have now been able to meet all the mechanical requirements associated with primers, as well as the compatibility requirements with other coatings – particularly in modern assembly-line painting processes. Today, almost all primers used for sedans are of the cathodic electrophoretic type. Solvent-based primers for automobiles are primarily made using nitro resins, epoxy resins, alkyd resins, amino resins, phenolic resins, etc. as the base materials. Pigments commonly used include iron oxide red, titanium dioxide, carbon black, as well as other pigments and fillers. The coating methods available are spraying and dipping. Electrophoretic paint was developed on the basis of water-based dip-coated primers; it can dissociate in water into charged, water-soluble film-forming polymers, which then move toward the opposite electrode (the surface to be coated) under the influence of a direct current field, without depositing or precipitating on that surface. The electrophoretic coating method requires that the object to be coated must be an electrical conductor. Depending on the electrophoretic coating method used, electrophoretic primers can be divided into anodic electrophoretic primers and cathodic electrophoretic primers. The film-forming polymers used in electrophoretic primers are anionic and cationic resins, while the additives are inorganic bases, organic amines, or organic acids; pigments commonly used include titanium dioxide and carbon black. 3. Characteristics and common types of intermediate coatings for automobiles. Intermediate coatings used in automobiles, also known as the second coat, are coatings applied between the automotive primer and the topcoat or base color paint. It is required to adhere firmly to the primer surface while also allowing for easy bonding with the topcoat applied on top of it, playing an important role as a link between the two layers. In addition to requiring good adhesion and bonding with the coatings above and below it, the intermediate coat must also have filling properties in order to fill holes and imperfections on the surface of the substrate, thereby creating a smooth surface. This allows for a smooth and even topcoat layer to be applied, enhancing the gloss and fullness of the entire paint film and thus improving the decorative qualities of the coating as a whole ； It should also have good sandability, so that a smooth and even surface can be achieved after sanding. Putty, the second primer, and the topcoat are all intermediate layers in a coating system, that is, the intermediate coats. Putty is used to fill in the uneven areas of the surfaces being worked on; it is usually in a thick paste form, contains a high amount of pigment, and has poor mechanical strength as a coating, making it prone to peeling off. As a result, putty is no longer used in most newly manufactured cars produced on assembly lines today, and it is only occasionally used for repairing cars. Sealant is the final intermediate coating applied before the topcoat; it gives the finish a glossy or semi-glossy appearance. It is generally used only in coatings where high decorative standards are required (such as car repairs), as these coatings need a sealant applied before the topcoat to fill in any imperfections left after sanding the underlying layer, thereby achieving a smooth surface.