Electrophoretic coating process for aluminum profiles

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Article abstract: Focusing on the current development status of electrophoretic coating for aluminum profiles in China, this article introduces in detail the electrophoretic coating process used for such profiles. It also introduces the principle of anodic electrophoretic coating, electrophoretic coatings and coating processes, as well as the equipment for anodic electrophoresis, and analyzes the main factors affecting electrophoretic coating. In addition, the development of aluminum profiles was also discussed. 　Keywords: electrophoresis, coating, paint, resin. 　Focusing on the current status of electrophoretic coating for aluminum profiles in China, this text provides an in-depth introduction to the electrophoretic coating processes used for such profiles. It also explains the principles of anodic electrophoretic coating, as well as the electrophoresis and coating processes and the equipment required for anodic electrophoresis, while analyzing the main factors that affect electrophoretic coating. In addition, some suggestions for the development of electrophoretic coating for aluminum profiles were also put forward. 　　 　Electrocoating; aluminum profiles; process management 　　0 　Introduction 　　After 20 years of development, China’s aluminum profile industry has established a complete industrial system and has become one of the key industries in 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 a stage of development characterized by mid-to-high-end quality. The surface treatment of aluminum profiles has advanced rapidly, evolving from simple bronze plating to various techniques such as powder coating, fluorocarbon spraying, organic and inorganic coloring, electrophoretic coating, mechanical and chemical polishing. New generation wood-grain finishing technologies, three-stage electrolytic multi-color coloring, and micro-arc oxidation coloring methods are also in the trial production phase. Aluminum profiles can come in colors such as titanium gold, golden yellow, champagne, stainless steel imitation, and various other hues; they can also exhibit special effects like a mirror finish, sandstone texture, matte finish, or pearlescent effect. 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 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 combining it with aluminum oxidation coloring allows for assembly-line production. Water-soluble electrophoretic coating is safe, and a closed-loop system can be achieved, which is beneficial for environmental protection. Thirdly, and more importantly, electrophoretic-coated profiles are more attractive and represent the epitome of high-end, luxury profiles. Having an electrophoretic coating line is an important indicator of a aluminum profile factory’s overall technical capabilities. 　　1 Electrocoating of aluminum profiles 1.1 Characteristics of electrocoating Since electrocoating paints were first used in Ford Motor’s wheel coating lines in the 1930s, their application has seen rapid development. 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 methods for aluminum profiles include oxidation, coloring, and sealing. The conversion film formed on such aluminum materials 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 materials can be directly electrophoretically coated after pretreatment and washing. The electrophoretic coating has a high degree of transparency; it not only provides excellent 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 takes place in an aqueous electrophoretic tank, and similar to anodizing and electrolytic coloring processes, the processing time is short, making it easy to implement a pipeline system for the entire process. 　　(2) Uniform and dense coating: Thanks to the high penetration ability of electrophoretic coatings, even profiles with complex shapes can achieve a uniform coating layer, and the coating thickness can be controlled by adjusting the charge amount. 　　(3) High utilization rate of coating: Due to its low viscosity, less coating is carried away with the workpiece, and electrophoretic-coated parts can be washed with water. The use of recovery systems enables the utilization rate of coatings in electrophoretic painting to reach over 95%. 　　(4) Safety and environmental protection: Due to the water-based formulation of electrophoretic coatings, their solid content is low, which makes them environmentally friendly coatings. They also eliminate the risk of fires and ensure the health of workers. 　　(5) Good coating quality: The acrylic resin is cured using amino resin, which ensures high decorative qualities and excellent corrosion resistance for the coating. Additionally, due to the high transparency of the resin, the metallic texture is effectively highlighted; depending on requirements, matte, textured, or pearlescent decorative effects can also be achieved. 　　(6) Compared with the conventional electrolytic coloring and sealing process, it is time-saving and labor-saving; the electrophoretic coating does not require sealing, thus avoiding problems such as cracks caused by poor sealing. 　　(7) The thickness of the coating film can be freely controlled; domestically and internationally, it is usually set at two levels: 7 μm and 12 μm. 　　(8) Since the electrophoretic layer is transparent and shiny, high quality of aluminum ingots is required, as well as a smooth surface of the extruded materials with few mechanical defects. Strict control over the oxidation coloring process is necessary, because any minor defect or contamination will be clearly visible beneath the transparent paint. 　　(9) High requirements are placed on the management of the electrophoresis process to improve the yield of finished products. 　　1.2 Principle of electrophoretic coating Taking anodic electrophoresis as an example, Figure 1 illustrates the process of anodic electrophoresis, in which the workpiece is an aluminum profile serving as the anode. 　　 　　Figure 1 Schematic diagram of anodic electrophoresis Paint state (emulsified state): acrylic resin containing carboxyl groups, with potassium hydroxide or organic amines as neutralizing agents, and melamine formaldehyde resin as a curing agent. 　　pH value: Between 7.8 and 8.5. Electrodeposition: Occurs as the pH value decreases. Electrode reaction: Anode (aluminum profiles). As can be seen from Figure 1, during electrophoresis, a coating is deposited on the surface of the workpiece; oxygen is generated at the anode, which helps promote film formation. However, excessive oxygen production can result in a poor luster of the coating. Therefore, it is necessary to control the conductivity during the electrophoresis process, as well as the pH value of the solution, since it is closely related to coating deposition. During anodic electrophoresis, metal ions are released, and these ions play a role in film formation. The release of metal ions during the electrophoresis of aluminum profiles differs from that in steel components, and it has little impact on color. 　　1.3 Characteristics of Anodic Electrophoretic Coatings for Aluminum Profiles 1.3.1 Types of Anodic Electrophoretic Coatings Due to the excellent weather resistance and wear resistance of acrylic resins, their colorless and transparent nature, and the ability to adjust their properties to achieve good decorative effects as well as tunable performance, coupled with the fact that their main chains are composed of carbon-carbon bonds granting them high chemical resistance, acrylic-based anodic electrophoretic coatings do not decompose even when used in alkaline, water-rich electrophoretic paints, thus exhibiting good stability. As a result, acrylic-type anodic electrophoretic coatings dominate the market for electrophoretic coatings used on aluminum profiles. 　　There is also a category of colored polyurethane electrophoretic coatings, which offer good performance and simple processing procedures. By using such colored polyurethane electrophoretic coatings, the processes of anodizing, coloring, and sealing can be eliminated, which helps to reduce costs. In addition, to meet various different requirements, functional electrophoretic coatings with antibacterial, insulating, and self-lubricating properties, as well as decorative electrophoretic coatings with matte, flat, or pearlescent finishes, have also been put into use. 　　1. 3. 2 Production of electrophoretic coatings Anodine electrophoretic coatings are those in which aluminum profiles are used as the anode; therefore, such coatings consist of anionic resins. The most commonly used type is water-soluble acrylic resins containing carboxyl groups, which are cured using triamine formaldehyde resin as a crosslinking agent. The more acrylic monomers are incorporated, the better the water solubility of the resin. For example, the formula is: MMA:BA:HEMA:AA = 45:38:10:7. Here, MMA is methyl methacrylate, a hard monomer that constitutes the main component of the polymer; BA is butyl acrylate, a soft monomer used to adjust the glass transition temperature and enhance the flexibility of the chains; HEMA is β-hydroxyethyl methacrylate, which provides free hydroxyl groups that react with amino resins to facilitate cross-linking, while also increasing the hydrophilicity of the resin. AA is acrylic acid, which provides carboxyl groups; the higher its concentration, the greater the hydrophilicity. In electrophoretic coatings, a concentration of 5% to 10% is typically used. The polymerization takes place in a co-solvent such as butyl ether, at a concentration of around 70%. As an initiator, benzoyl peroxide (BPO) can be used, but azobisisobutyronitrile is preferred. The reaction temperature is around 100 °C. After polymerization is complete, it is neutralized using dimethylethanolamine, and then diluted with water. The solid content is generally around 60%, while the pH value ranges from 7.5 to 8.5. Solvents are required in water-based coatings; these are sometimes referred to as co-solvents, and they are usually hydrophilic alcohols such as monogly ethers of propanol, butanol, or ethylene glycol (or propylene glycol). The choice of solvent is based primarily on its volatility, toxicity, and cost. The function of the neutralizing agent is to neutralize the carboxyl groups on the acrylic resin chains, thereby preserving the water solubility of the resin. Organic amines or sodium hydroxide are commonly used for this purpose; organic amines such as triethylamine and dimethylethanolamine, especially tertiary amines, contribute to the stability of the resin. 　　2 　Electrophoretic coating process and equipment 　　2.1 　The process of electrophoretic coating – taking anodic electrophoresis as an example – usually follows this sequence: degreasing → washing → washing → neutralization → washing → anodizing → washing → electrolytic coloring → washing with hot water → washing → electrophoresis → washing → drying 　　2.2 　Process conditions for electrophoretic coating 　　(1) 　Conditions of the electrophoretic coating bath 　　Solid content: 7% – 9% 　　Temperature: 20 – 25 °C 　　pH: 8.0 – 8.8 　　Resistivity Ω·cm (at 20 °C): 1500 – 2500 　　(2) 　Process parameters for electrophoretic coating 　　Voltage: 80 – 250 V (DC) 　　Electrophoresis time: 1 – 3 minutes 　　Current density: 15 – 50 A/m² 　　The specific process specifications depend on the requirements of the product. 　　Coating thickness: 7 – 12 μm. 2.3 Equipment required for the electrophoretic coating process of aluminum profiles: (1) Electrophoretic tank – The electrophoretic tanks for aluminum profiles are usually designed as rectangular tanks; their internal dimensions depend on the size of the profiles to be coated. The tank body is made of steel plates with a thickness of 4 – 6 mm, while the lining is made of polypropylene or epoxy fiberglass. An overflow tank must also be provided. 　　(2) Power supply: DC current requirement is adjustable from 0 to 250A; the current value is calculated at 50A/m2, resulting in approximately 2000A per slot. The ripple factor of the power supply must be less than 6%; the higher this value, the greater the likelihood of pores forming in the electrophoretic coating. 　　(3) Electrodes: In anodic electrophoresis, the aluminum profile serves as the anode, while the cathode plate is made of stainless steel or aluminum; the area of the cathode plate is equal to the total area of the workpiece. The plate diaphragm is made of polypropylene fiber cloth. 　　(4) Heat exchange system: Ensures stable tank fluid temperature. 　　(5) Pre-mixing tank and automatic addition system: The pre-mixing tank is used to prepare the electrophoretic coating solution so as to ensure that the parameters are within the normal range; the automatic addition system ensures that the amount of electrophoretic paint consumed by each aluminum profile is replenished, keeping the solid content of the electrophoretic coating solution within the specified range and thus ensuring uniform film thickness. 　　(6) Ion exchange treatment (IR) system: During anodic electrophoresis, the pH value at the cathode continues to increase. An ion exchange resin system can be used to remove impurities from the bath solution, while cations and anions help to stabilize the pH of the bath. If a reverse osmosis unit (RO) is added, a RO closed-loop system can be implemented. 　　The process of the RO closed-loop system is shown in Figure 2. 　　 　　Figure 2 RO closed-loop system 　　(7) 　Baking system: Radiation-convection type is commonly used; the anodic electrophoretic paint film needs to be baked at a temperature of 140～170 ℃. 　　2.4 Process Management for Electrophoresis The production process management for anodic electrophoresis of aluminum profiles is a systematic task that requires strict standards; the items that need to be checked regularly are listed in Table 1. In addition, the solvent amount, electrophoretic properties, and coating properties also need to be monitored regularly. 　　Table 1 List of Common Inspection Items for Anodic Electrophoresis 2.4.1 Several Key Factors Affecting Anodic Electrophoresis and Methods for Their Control In the anodic electrophoresis process, factors such as voltage, charging time, solid content, and increased resistivity contribute to an increase in film thickness; however, the trend for pH value is the opposite – an increase in pH value leads to a decrease in film thickness. To effectively control the electrophoresis process, it is necessary to manage and control these factors. 　　(1) Solid content: As the amount of electrophoretic treatment increases, it must be continuously replenished. The amount of solid content has the most direct impact on film thickness; therefore, to ensure uniform film thickness, the solid content should be kept within a range of ±0.2%. 　　(2) pH value: Anodic electrophoresis of aluminum profiles achieves a stable water-soluble system by neutralization with alkalis or amines to form salts, resulting in a weakly alkaline environment; its pH value ranges from 7.5 to 9. During the electrophoresis process, negatively charged resin deposits on the workpiece, and at the same time amines are continuously generated at the cathode, causing the pH value of the paint solution to increase gradually. Furthermore, if the electrophoresis tank solution is left for too long, the pH value tends to decrease due to the volatilization of amines. Changes in pH value can affect the electrodeposition properties and the characteristics of the paint film. If the pH value is too high, the paint film becomes thinner; after electrophoresis, the film dissolves again, resulting in insufficient thickness and an abnormal loss of luster. If the pH value is too low, the stability of the electrophoresis bath is poor. 　　Therefore, the pH value must be strictly controlled to remain within the specified process parameters. The pH value of anodic electrophoresis for aluminum profiles can be effectively controlled using the ion exchange resin method. 　　(3) Resistivity: The resistivity varies with the amount of amine, temperature, solid content, and the amount of amino resin; it should be kept within the range of 2000 ± 500 Ω·cm. Changes in the amount of amine have a greater impact on resistivity than changes in pH value. By performing regular ion exchange treatments to remove excess amine and impurity ions, the resistivity can be maintained within a normal range. 　　(4) Amine value: The level of this value indicates the amount of amine in the electrophoretic solution. The amine value increases as the amount of electrophoretic treatment increases. The amine value can be reduced through ion exchange treatment. The level of the amine value is reflected in the pH value and resistivity, and it is usually controlled by regulating these two parameters. 　　(5) Impurity ions: The main impurities include anions such as SO42-, CO32-, and Cl-, as well as metal cations like Na+ and Mg+, all of which are harmful to electrophoretic coating. Due to the oxidation coloring process, the surface of aluminum profiles often contains SO2-4; therefore, thorough cleaning is necessary to prevent its entry into the electrophoresis tank. 　　2.4.2 Pretreatment and post-washing processes for electrophoretic coating (1) Tank preparation and replenishment: Typically, the anodic electrophoretic coating solution needs to be diluted and mixed evenly before being poured into the electrophoretic tank; in addition, it must be replenished on a daily basis, and all water used for dilution should be deionized water. 　　(2) Washing before electrophoresis: Two or more washes should be performed, with the final wash using pure water, to ensure that the electrophoresis tank solution is not contaminated. 　　(3) Washing after electrophoresis: After electrophoresis, washing should be carried out two or more times, with pure water being used for the final wash. If an RO system is employed, an RO closed-loop system can be used on a macro scale. After washing, it is advisable to have a device for absorbing or dripping water; compressed air or high-pressure air can be used to blow away the moisture from the surface of the profiles and from the gaps, and an inclined mechanism should be installed to allow any accumulated water to drip off into the grooves. Because when water on the surface dries, water marks are likely to appear, affecting its aesthetic appearance. 　　(4) RO closed-loop system 　　RO stands for reverse osmosis device; it is a process of reverse osmosis driven by pressure. Desalination of seawater and dialysis both make use of the principle of reverse osmosis. (See Figure 3). 　　Function of the RO device:　　　Figure 3 Diagram of the reverse osmosis (RO) process　1) Recycle electrophoretic coatings, thereby increasing their utilization rate from 55% to over 98%. 2) The filtrate can be used as washing water, enabling a closed-loop system. 3) Reducing or eliminating wastewater discharge is beneficial for environmental protection. The composition of the RO permeate is shown in Table 2. Table 2: Composition of RO Permeate. Since the water obtained after RO treatment is almost pure water, it can be used directly for cleaning; thus, a closed-loop washing system can be implemented, which is beneficial for environmental protection. 　　3 　Conclusion 　At present, many aluminum profile manufacturers in our country have electrophoretic coating production lines. To further develop this technology and gain a larger share of the market, we believe it is necessary to make efforts in several areas: (1) Developing new application fields such as machinery, aviation, automotive components, home appliances, bathrooms, and kitchenware; (2) Improving the processing techniques for aluminum profiles in order to increase the quality rate of electrophoretically coated products; (3) Enhancing the quality of electrophoretic coatings, expanding the range of available coatings, and increasing the proportion of domestically produced coatings; (4) Improving the management level of electrophoretic coating processes. 　　In summary, the electrophoretic coating technology for aluminum profiles in our country is entering a new phase of development. Thanks to the high quality of electrophoretically coated profiles, this technology is bound to see even faster growth in our country.