What is the future direction of coating technology?

The four major elements of automotive coating are coating materials, coating processes, coating equipment, and coating management, which complement each other and promote the progress and development of coating technology.

The 21st century is known as the new century facing the environment, with environmental protection receiving global attention and becoming one of the most urgent research topics for humanity. The coating of automobiles and their components is one of the processes in automobile manufacturing that consumes the most energy and generates the most waste. Therefore, reducing coating pollution, lowering coating costs, and improving coating quality have always been the themes of coating technology development.

The application of new coating materials is the precursor to the advancement of coating technology. While continuously meeting the performance requirements of coatings, the main development goal is to use materials that can reduce pollution and lower coating costs. De-greasers formulated with biodegradable surfactants, nickel-free phosphating solutions, nitrite-free phosphating solutions, chromium-free passivation agents, low-temperature de-greasers (processing temperature 43°C), low-temperature (35°C) low-residue (10-30% lower than conventional) phosphating solutions, lead-free and tin-free cathodic electrophoretic coatings, and low-temperature curing (160°C for 10 minutes), low heating reduction (below 4), and low VOC emission (0.4-0.8) cathodic electrophoretic coatings have been promoted and applied in Europe, America, and Japan for many years. In North America and Europe, secondary electrophoretic coatings that can replace traditional intermediate coatings have begun to be used. In Europe, some automobile companies have recently built new coating lines that exclusively use water-based coatings, with VOC emissions lower than the regulatory requirement of 35g/m2 (Germany TA-Luft, 1995). Since the 1990s, all newly built coating lines have used electrophoretic primers or powder coatings for primers, water-based coatings or high solid content materials for intermediates, and water-based base colors plus high solid content clear coats for topcoats. Currently, powder clear coats have begun to be used in the body coating of passenger cars. Following the industrial application of powder topcoats, powder metallic base colors have also been commercialized. In North America, powder intermediates have been industrially applied for many years, while water-based topcoat base colors have rapidly gained popularity in recent years, and high solid content intermediates and topcoats are also quite common. The application technology of UV-cured coatings in automotive coatings in Europe and America is nearing maturity. Japan is also actively developing and promoting water-based coatings, high solid content, and ultra-high solid content topcoats.

As the global environment continues to deteriorate, we must significantly improve the acid rain resistance and scratch resistance of automotive coatings. In recent years, high permeability, low pigment content electrophoretic coatings, acid rain-resistant and scratch-resistant topcoats, multi-color intermediates, and coatings using viscosity control technology have been widely applied to reduce the thickness difference of the electrophoretic primer film on the inner and outer surfaces of the vehicle body. Several major automobile companies in China are not far behind the international level in the application of pre-treatment materials and electrophoretic paints, but the need for specialized equipment to apply water-based intermediates and water-based topcoats has increased coating costs. Although foreign-owned and joint-venture enterprises have the capability to produce water-based intermediates and water-based topcoats domestically, traditional medium and low solid content solvent-based coatings are still used for automotive coating intermediates and topcoats, and VOC emissions from coatings are far behind Europe. The progress of coating processes and equipment in the past decade has mainly been reflected in the application of environmentally friendly coating materials, reducing wastewater and waste residue emissions, lowering costs, and optimizing the automobile production process. Due to advancements in coating materials, there have been revolutionary developments in the design of vehicle body coating systems, and several typical new coating systems and technologies have been or will be used in industrial production. Currently, China's coating processes and equipment are generally equivalent to the level of Europe and America ten years ago, with some enterprises adopting some of today's internationally advanced new equipment in new coating lines.

1. Water-saving and waste recycling technology. Pre-treatment and electrophoresis are the stages in automotive coating that consume the most water and generate the most wastewater. With the continuous maturity of membrane technology, it has become possible to achieve true closed-loop cleaning in electrophoresis by using membrane separation technology (UF and RO) to recycle de-greasing liquids, regenerate cleaning water, and treat pre-treatment wastewater. Currently, membrane separation technology has begun to be applied. In recent years, with the industrial application of environmentally friendly coatings in developed countries, some recycling technologies for coatings (such as powder slurry recycling technology, powder base color and clear coat recovery technology, waste paint flocculation dryer technology, ultrafiltration, cooling, and electrostatic adsorption methods for recovering water-based paints, and water-based paint recovery technology for overspray mist, etc.) have also been applied, further improving the utilization rate of coatings in the coating line and minimizing the discharge of waste paint residue. Currently, China does not pay enough attention to water-saving and comprehensive utilization of waste in coatings, and is relatively lagging in the application of new technologies.

2. Several new body coating processes. Reverse process technology: Powder coatings are first sprayed onto the outer surface of the vehicle body, and after thermal melting, electrophoretic coating is performed, followed by drying the powder/electrophoretic film together. This process can reduce the amount of electrophoretic coating by about 60%, replacing the electrophoretic primer and intermediate layer on the outer surface of the vehicle body with a 70μm thick powder coating, eliminating the intermediate coating and drying processes, thus saving material and energy costs and reducing VOC emissions. Secondary electrophoresis process: Two-layer electrophoretic materials are used, with the second layer of electrophoresis (35-40μm) replacing the intermediate layer. The automation of the electrophoresis process is stable and reliable, with a high first-pass yield, high material utilization, and low equipment investment (no need for air conditioning systems), thus saving costs by 48%, reducing maintenance frequency, and decreasing the paint residue and VOC emissions from traditional intermediate coatings. Integrated coating process (three-layer concept): A functional layer (15μm) of the same color as the topcoat replaces the intermediate layer, eliminating the need for drying between the functional layer and the topcoat base color, removing the intermediate coating line, significantly improving production efficiency while greatly reducing VOC emissions.

3. Film application technology as a substitute for plastic cover part coating. Film application technology involves pre-fabricating a paint film suitable for thermal forming, whose performance and appearance after thermal forming are very similar to traditional baked spray coatings. This technology is mainly applied in the production of plastic parts, using clamping mold pressing or internal mold processes to complete the forming of the pre-fabricated composite coating while the plastic part is being cast, resulting in defect-free coated parts. The vehicle body frame is manufactured using traditional stamping and welding processes, and the coating workshop only coats the vehicle body frame, using powder spraying technology for the topcoat. Since the exposed area of the vehicle body frame is relatively small, the topcoat color does not need to match the cover parts, with only one shade of each being sufficient. Large-area cover parts are manufactured using film application technology, with thousands of color options. This greatly simplifies the vehicle body coating process, reducing coating costs while achieving VOC emissions of around 7g/m2, far below the requirements of European emission regulations.

4. Car Body Coating P2ZERO Concept The so-called P2Zero concept refers to a zero-emission paint workshop. Under the premise of meeting stringent environmental protection requirements and user quality requirements, it reduces the cost of handling three wastes, lowers the operating costs of the paint workshop, and simplifies the painting process. The anti-corrosion primer for the car body steel plates is applied before the parts are made, so the car body entering the paint workshop does not need to be primed again; it only requires a layer of powder base color and a layer of powder topcoat. Therefore, it can minimize process waiting time, eliminate traditional paint mixing rooms, make process adjustments more flexible, cancel the rust prevention process from steel plates to painted car bodies, completely eliminate the traditional coating issues of welds and cavity structures, save space in the painting workshop, reduce three waste handling costs, eliminate the need for paint sludge systems and waste paint treatment systems, have no spray booth exhaust, and reduce air pollution and solid waste to nearly zero, with no liquid emissions, a paint manufacturing and usage efficiency greater than 95, and no odor or danger.

5. Coating of Chassis Parts In developed countries, the coating of chassis parts commonly uses phosphating, cathodic electrophoresis, or powder coating processes. Generally, the parts undergo electrophoresis or powder coating first, then assembly, and depending on the needs, a low-temperature or room-temperature curing topcoat is applied to the assembly. Due to the good mechanical properties of cathodic electrophoresis and powder coatings, some mechanical processing of parts can be done after coating, which helps avoid rusting due to prolonged processing times before coating. The raw materials for frames and chassis parts are mostly hot-rolled plates and castings, which are often treated with shot peening or sandblasting before forming or coating, while acid cleaning has gradually been phased out. Currently, the scale of component coating production in our country is generally small and relatively backward. The coating processes and the level of coatings used for passenger cars and other small vehicles are relatively better, while those for cargo vehicles, buses, and agricultural transport vehicles are not high.

6. New Coating Transport Machines Currently, the typical transport machines used for car body coating line pretreatment and electrophoresis in the country include push rod suspended chains, swing rod chains, and programmable hoists. Each has its advantages and disadvantages, but a common drawback is that they cannot effectively solve the exhaust issues within many internal cavity structures of the car body, especially the airbag problem inside the roof. These areas cannot undergo phosphating and electrophoresis treatment. With the advent of new multifunctional shuttles (Vario-Shuttle) and roller transport machines (RoDip), these issues have been resolved while inheriting all the advantages of the aforementioned transport machines. The multifunctional shuttle also has the capability to implement multiple varieties of different processes on a single production line, fully meeting the requirements for automated flexible coating production. These two types of transport machines have already begun to be applied in newly built car body coating lines in the country.

7. Improvements in the Structure and Function of Other Coating Equipment In Europe, the structural materials of coating equipment are mainly stainless steel, and the electrical circuits of the equipment are designed within the equipment structure, eliminating external electrical pipelines. The modular design of the equipment and the three-dimensional layout of the workshop maximize the cleanliness and fire safety requirements of the coating process. The new generation of drying rooms, which do not have external air ducts and enhance internal convection, significantly improves thermal efficiency and the uniformity of drying temperatures. With advancements in robotic technology, automatic car body coating machines are gradually being replaced by multi-degree-of-freedom spraying robots, with spraying and sealing largely performed by robots, making them more suitable for flexible production than coating machines. The mechanical drive chains commonly use non-metallic materials, significantly reducing transmission noise. Coating Management Coating management includes ordering coating materials, controlling material quality, and managing the construction process. Due to the wide variety of materials used in automotive coating, their susceptibility to deterioration during storage and transportation, the long process flow, and the numerous parameters that need to be controlled, its complexity is the highest in automotive production management. In developed countries in the automotive industry, the development of automotive coating materials has promoted advancements in automotive coating technology. To reduce coating costs, the supply method has transitioned from a single material supply to a system supply. The system supply method began in the early 1990s and is now widely adopted in Europe and America. Major automotive manufacturers have always placed great importance on coating management, with paint manufacturers directly responsible for the technical management of coating production, greatly simplifying the coating management of automotive production plants, improving the first-pass qualification rate of coatings, and reducing production costs. Currently, this management approach is still in the trial stage in various automotive companies in the country. In recent years, some capable coating material companies and coating equipment companies have introduced the BOT service model, where the coating workshop of automotive companies is built and managed by coating material or equipment companies, producing qualified coatings according to the automotive company's production plan. Automotive companies only need to set technical standards and supervise the quality of the coating products. This will become a significant trend in coating management. The gap between China's automotive coating technology and international standards is gradually narrowing, but development remains uneven. In terms of coating quality assurance, several major passenger car manufacturers have reached international levels, but there is still about a 10-year gap in comprehensive comparison, mainly reflected in clean production technology. Regarding key equipment technology for automotive coating production, China may rely mainly on imports for a considerable time in the future. It is expected that within the next 10 to 15 years, China's automotive coating level will fully align with international standards, accelerating the application of resource-saving and environmentally friendly technologies.