Talking about the Characteristics and Application of Electrophoretic Coating Process of Aluminum Pro

After 20 years of development, China's aluminum profile industry has formed a complete industrial system and has become one of the pillar industries of the national economy. The output of primary aluminum has reached more than 2.5 million tons, accounting for the third place in the world. It has more than 1,000 aluminum processing enterprises, the production capacity has reached 3.5 million tons/year, and the output of aluminum profiles has reached 1 million tons/year. At present, it has entered a period of high-grade development. The surface treatment technology of aluminum profiles has developed rapidly, from a single bronze material to a process of powder coating, spray fluorocarbon coating, organic and inorganic dyeing, electrophoretic coating, mechanical and chemical polishing. A new generation of wood grain treatment technology, three-stage electro-color multi-color coloring and micro-arc oxidation coloring technology are also in trial production. The appearance of aluminum profiles is titanium gold, golden, champagne, imitation stainless steel and various colors, as well as display. Special effects such as mirror, sand, matt, pearl and so on. Among the many surface treatment technologies, electrophoretic coating is the most suitable technology for development. The aluminum profile is anodized and electrolytically colored and then electrophoretic coated with double protection. It has high transparency, high metal texture, high corrosion resistance and high weather resistance. It is more decorative than spray paint and powder spray. It is deeply loved by users and has been widely used in home decoration. The potential market is huge. Second, the process is easy to implement, as long as a few tanks are added to the original production line to form an assembly line with aluminum oxidation coloring, water-soluble electrophoretic coating is safe, and closed loops can be realized, which is beneficial to environmental protection. Third, more importantly, electrophoretic profiles are more attractive, and they are examples of high-end and luxurious profiles. The possession of electrophoretic coating lines has become an important symbol of the comprehensive technical level of aluminum profile plants.

    2.4.2 Electrophoretic coating of aluminum profiles

    2.4.2.1 Characteristics of electrophoretic coating

    Since the use of electrophoretic coatings on the Ford Motor Company wheel coating line in the 1930s, its application has grown rapidly. Transparent, weather-resistant acrylic and polyurethane electrophoretic coatings have been successfully used for the decoration and protection of metal surfaces since the 1970s, such as copper sign protection, decoration and protection of stainless steel and electroplated products, as the final protective decorative layer. The most representative is the electrophoretic coating of architectural aluminum profiles.

    The traditional surface treatment process of aluminum profiles is oxidation, coloring and sealing. This aluminum conversion film is easy to corrode and has poor durability when used outdoors for a long time, especially in acid rain or marine environment. Therefore, since the 70s Attempts have been made to improve the decorative properties and service life of architectural aluminum profiles by means of paint coating. At present, electrophoretic painting, powder coating and fluorocarbon spraying have been successfully used in industrialization. Since the electrophoretic coating is a water-based coating, the aluminum material can be directly electrophoresed after being pretreated, and the electrophoretic coating has high transparency, which has high decorativeness and can highlight the metallic luster of the aluminum profile itself, and thus has been increasingly used in architectural aluminum profiles. Electrophoretic coating is used.

    Electrophoretic coating has the following advantages over other coating methods:

    (1) Easy to achieve automated production. Since the electrophoretic coating is carried out in an aqueous electrophoresis tank, similar to the anodizing and electrolytic coloring processes, the processing time is short, and it is easy to realize the assembly line operation of the entire process.

    (2) The coating film is uniform and dense. Due to the high throwing power of the electrophoretic coating, a uniform coating can be obtained for the profile of the complex shape, and the film thickness can be controlled by adjusting the electric quantity.

    (3) The utilization rate of the paint is high. Since the viscosity is low, the workpiece is taken out less, and the electrophoresis workpiece can be washed with water, and the utilization of the recovery device makes the utilization rate of the electrophoretic coating up to 95% or more.

    (4) Safety and environmental protection. Due to the water dilution of the electrophoretic coating, the low solid content and low solvent content are environmentally friendly coatings, and the fire hazard is eliminated, and the worker's health is also guaranteed.

    (5) The film quality is good. The acrylic resin is cured with an amino resin to ensure high decoration and high corrosion resistance of the coating film. At the same time, since the resin is highly transparent, the metal texture is effectively highlighted, and decorative effects such as matt, sand surface and pearl light can be obtained as needed.

     (6) Compared with the usual electrolytic color sealing and sealing process, it has the characteristics of saving time and saving manpower, and the electrophoretic coating film does not need to be sealed, thereby avoiding cracks caused by poor sealing.

    (7) The thickness of the coating film can be freely controlled, and the two stages of 7 μm and 12 μm are usually controlled at home and abroad.

    (8) Since the electrophoretic layer is transparent and bright, the aluminum ingot is required to have high quality, the surface of the extruded material is smooth, and the mechanical defects are few, and the oxidation coloring process is required to be strictly managed, because any minute defects and stains are exposed under the transparent paint.

    (9) High requirements for electrophoresis process management to improve yield.

    2.4.2.2 Principle of electrophoretic coating

    Taking anodic electrophoresis as an example, Figure 2-4-1 illustrates the process of anodic electrophoresis, in which case the workpiece is an aluminum profile as the anode.

    Coating state (emulsified state): a carboxyl group-containing acrylic resin, a neutralizing agent is potassium hydroxide, or an organic amine, and a curing agent is a melamine formaldehyde resin.

    pH value: between 7.8 and 8.5.

    Electrodeposition: precipitation when the pH drops

    Electrode reaction: anode (aluminum profile)

    2H2O→4H++4e-+O2↑

    R-COO-+H+→R-COOH

    Water soluble water insoluble (coating film)

    Me→Men++ne-

    RCOO-+Men+→(R-COO)nMe (precipitation)

    Cathode

    2H2O+2e→2OH-+H2↑

    When electrophoresis, the coating film is deposited on the surface of the workpiece, and oxygen is generated at the anode, which can promote film formation. However, excessive oxygen generation may cause poor brightness of the coating film. Therefore, the conductivity should be controlled during electrophoresis, and the pH of the liquid should be controlled. The value is closely related to the deposition of the coating film. Metal ions are eluted during anodic electrophoresis, which participates in film formation. The metal ion elution in the aluminum electrophoresis is different from that of the steel, and has little effect on the color.

    2.4.2.3 Characteristics of anodic electrophoretic coatings for aluminum profiles

    2.4.2.3.1 Variety of anodic electrophoretic coatings

    Acrylic resin has excellent weather resistance and abrasion resistance; resin is colorless and transparent, can adjust coating properties through formula, has excellent decorative property and performance adjustability, and the resin main chain is composed of carbon-carbon bonds. The chemically high, alkaline aqueous electrophoretic paint does not decompose and has good stability. Therefore, the acrylic type anodic electrophoretic coating occupies the mainstream of the aluminum profile electrophoretic coating.

    There is also a class of colored polyurethane electrophoretic paints with good performance and simple process. Because of the use of colored polyurethane electrophoretic paints, the anodizing, coloring and sealing processes can be eliminated. Conducive to cost savings. In addition, in order to meet the different needs of various products, functional antibacterial, insulating, self-lubricating electrophoretic coatings, and decorative matt, matting, pearlescent electrophoretic coatings have also been applied.

    2.4.2.3.2 Manufacture of electrophoretic coatings

    The anodic electrophoretic coating refers to the aluminum workpiece as the anode, so the anodic electrophoretic coating is an anionic resin. The most commonly used is a water-dilutable acrylic resin with a carboxyl group, which is cured with a triammine formaldehyde resin as a crosslinking agent, and acrylic acid is introduced. The more monomers, the better the water solubility of the resin. Such as formula:

MMA is methyl methacrylate is a hard monomer, the main body of the polymer; BA is butyl acrylate, is a soft monomer, used to adjust the glass transition temperature, providing flexibility of the chain; HEMA is β-hydroxyl methacrylate Ester, which provides a free hydroxyl group, cross-links with the amino resin, and also enhances the hydrophilicity of the resin. AA is a carboxyl group provided by acrylic acid. The higher the base content, the better the hydrophilicity. Generally, 5% is used in electrophoretic coatings. 10%, the polymerization is carried out in a cosolvent propylene glycol butyl ether at a concentration of about 70%, and the initiator may be benzoic acid benzoate (BPO), but oxydiisobutyronitrile is preferred. The reaction temperature is about 100 ° C. After the polymerization is completed, it is neutralized with dimethylethanolamine and then diluted with water. The solid content is generally about 60%, and the pH is 7.5 to 8.5. Solvents are used in aqueous coatings. Solvents, mostly hydrophilic alcohols, such as monoethers of propanol, butanol, ethylene glycol (or propylene glycol), are mainly selected according to volatility, toxicity, and price. The neutralizing agent acts to neutralize the acrylic resin chain. The carboxyl group, the protective resin is water-soluble, mostly organic amine or sodium hydroxide, and the organic amine such as triethylamine, dimethylethanolamine, especially tertiary amine is beneficial to the stability of the resin.

    2.4.3 Electrophoresis process and equipment

    2.4.3.1 Electrophoresis process

    Taking anodic electrophoresis as an example, it is usually carried out as follows.

    Degreasing→water washing→water washing→neutralization→water washing→anodizing→water washing→electrolytic coloring→hot water washing→water washing→electrophoresis→water washing→drying

    2.4.3.2 Electrophoresis process conditions

    (1) Conditions of electrophoresis bath

    Solids: 7% to 9%

    Temperature: 20 ~ 25 ° C

    pH: 8.0 ~ 8.8

    Resistivity / Ω · cm (20 ° C): 1500 ~ 2500

    (2) Process parameters of electrophoresis

    Voltage: 80 ~ 250V (DC)

    Electrophoresis time: 1~3min

    Current density: 15 ~ 50A / m2

    The specific process specifications are based on the requirements of the product.

    Coating thickness: 7 ~ 12μm

    2.4.3.3 Equipment required for aluminum electrophoresis process

    (1) Electrophoresis tank. The aluminum electrophoresis tank is designed as a rectangular rectangular groove. The size of the base depends on the size of the hanging crane. The tank body is 4 to 6 mm, and the steel plate is lined with polypropylene or epoxy glass reinforced plastic. An overflow tank is required.

    (2) Power supply. The DC requirement is 0 to 250A, and the current is calculated according to 50A/m2, about 2000A/slot. The corrugation factor of the power supply must be less than 6%, and the larger the value, the greater the probability that the electrophoretic coating film will generate voids.

    (3) Electrode. In the anodic electrophoresis, the aluminum profile is used as the anode, the cathode plate is made of stainless steel or aluminum, and the cathode area is 1 times the total area of the workpiece. The plate diaphragm is made of polypropylene fiber cloth.

    (4) Heat exchange system. The bath temperature is guaranteed to be stable.

    (5) Pre-matching tank and automatic adding system. The pre-dispensing tank can complete the preparation of the electrophoresis bath to ensure the normal parameters; the automatic adding system can ensure the amount of electrophoretic paint consumed by each aluminum strip is ensured, and the solid content of the electrophoresis tank is within the process range to ensure the membrane. Thick and uniform.

    (6) Ion exchange treatment (IR) system. When the anode is electrophoresed, the pH value of the cathode will increase continuously. The ion exchange resin system can remove the impurities in the bath, and the anode and the cation can stabilize the pH of the bath. If the reverse osmosis unit (RO) is added, the closed circuit system can be realized.

    Table 2-4-2 Composition of RO permeate

    Since the water after the RO is almost pure water, it can be directly used for cleaning, so that a closed-circuit washing system can be realized, which is beneficial to environmental protection.

    2.4.4 Summary

     At present, many aluminum profile plants in China have electrophoresis production lines. In order to better develop electrophoretic coating technology, we have more market share. We believe that we should work hard in several aspects: (1) develop new application fields, such as machinery, aviation, and automobiles. Components, home appliances, bathrooms, kitchen utensils, etc.; (2) improve aluminum profile processing technology, improve the pass rate of electrophoretic profiles; (3) improve the quality of electrophoretic coatings, expand the variety of electrophoretic coatings, increase the localization rate; (4) improve the management level of electrophoretic coating.

     In summary, China's aluminum profile electrophoretic coating technology is entering a new period of development, due to the high quality of electrophoresis profiles, there will be a more rapid development in China.