[China Aluminum Network] After 20 years of development, China's aluminum 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 the period of high-end development. The surface treatment technology of aluminum profiles has developed rapidly. It has developed from a single bronze material to powder coating, spray fluorocarbon coating, organic and inorganic dyeing, electrophoretic coating, mechanical and chemical polishing. A new generation of wood grain processing technology, three-stage electrolytic multicolor coloring and micro-arc oxidation coloring technology are also in trial production. The appearance of aluminum profiles are titanium, gold, champagne, imitation stainless steel and various colors, as well as display. Mirror, sandstone, matt, pearlescent and other special effects. Among the many surface treatment technologies, electrophoretic coating is a more applicable technology. Aluminum anodized and electrolyzed and then electrophoretically coated, with double protection. With high transparency, high metal texture, high corrosion resistance and high weather resistance, it is more decorative than spray paint and powder coating. It is deeply loved by users and has been widely used in home decoration. The potential market is extremely large. Second, the process is easy to implement, as long as the addition of several tanks in the original production line and aluminum oxidation coloring line production, water-soluble electrophoretic coating security, can achieve closed loop, is conducive to environmental protection. Third, the more important is that the electrophoretic profiles are more attractive and are examples of high-grade and luxurious profiles. Having an electrophoretic coating line becomes an important symbol of the comprehensive technical level of the aluminum profile plant.
2.4.2 electrophoretic coating of aluminum profiles
2.4.2.1 Characteristics of electrophoretic coating
Since the use of electrophoretic coatings in Ford Motor Company's 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 protection of copper signs, decoration and protection of stainless steel and electroplated products, as a protective finishing layer The most representative is the electrophoretic coating of architectural aluminum profiles.
The traditional surface treatment process for aluminum profiles is oxidation, coloration, and sealing. The aluminum conversion film is prone to corrosion and has poor durability when used outdoors for extended periods of time, especially in acid rain or marine environments. , People try to use paint coating method to improve the decorative aluminum building materials and the service life, at present, has been successfully used in industrial electrophoretic paint, powder coating and fluorocarbon spray. Since the electrophoretic coating is a water-based coating, the aluminum material can be directly electrophoresed after being washed by the pretreatment. The electrophoretic coating has a high transparency, and both the high decorativeness and the metallic luster of the aluminum profile can be highlighted. Therefore, there are more and more aluminum profiles. Electrophoretic coating is used.
Electrophoretic coating has the following advantages over other coating methods:
(1) Easy to realize automated production. Since the electrophoretic coating is performed in an aqueous electrophoresis tank, similar to the anodizing and electrolysis coloring processes, the processing time is short, and it is easy to realize the assembly line operation of the entire process.
(2) The film is uniform and dense. Due to the high throwing power of the electrophoretic coating, it is possible to obtain a uniform coating film for a profile of a complex shape, and to control the film thickness by adjusting the amount of electricity.
(3) The utilization rate of paint is high. Due to the low viscosity, the workpieces are brought out less, and the electrophoresis workpiece can be washed with water. The utilization of the recovery device makes the utilization rate of electrophoretic coating paint up to 95%.
(4) Safety and environmental protection. Because the electrophoretic coating is diluted with water, the solid content is low, and the solvent content is low, it is an environment-friendly paint, and the risk of fire is eliminated, and the health of the workers is also guaranteed.
(5) The coating quality is good. Acrylic resin is cured with amino resin to ensure the high decoration and high corrosion resistance of the coating film. At the same time, because the resin is highly transparent, it effectively highlights the metal texture, and can also be decorated with matte, sand, pearlescent and other decorative effects.
(6) Compared with the conventional electrolytic coloring and sealing process, it has the advantages of time saving and labor saving, and the electrophoretic coating film does not need to be sealed, and cracks and the like caused by bad sealing holes are avoided.
(7) The thickness of the coating film can be freely controlled, and both the 7 μm and 12 μm are generally controlled at home and abroad.
(8) Since the electrophoretic layer is transparent and bright, aluminum ingots are required to have a high quality, a smooth surface of the extruded material, fewer mechanical defects, and strict management of the oxidation coloring process, because any minute defects and stains will be exposed in the transparent paint.
(9) High requirements for electrophoresis process management to increase the yield.
2.4.2.2 Electrophoretic Coating Principle
Take anode electrophoresis as an example. Figure 2-4-1 shows the process of anode electrophoresis. At this time, the workpiece is an aluminum profile as an anode.
Coating state (emulsified state): carboxyl-containing acrylic resin, neutralizing agent is potassium hydroxide, or organic amine, curing agent is melamine formaldehyde resin.
pH: between 7.8 and 8.5.
Electrodeposition: Precipitates when the pH drops
Electrode Reaction: Anode (Aluminum Profile)
2H2O→4H++4e-+O2↑
R-COO-+H+→R-COOH
Water-soluble water insoluble (coating)
Me→Men++ne-
RCOO-+Men+→(R-COO)nMe(precipitation)
Cathode (plate)
2H2O+2e→2OH-+H2↑
Electrophoretic coating deposition on the surface of the workpiece, the anode oxygen, can promote film formation, but excessive oxygen will cause the appearance of coating film brightness is poor, so the electrophoresis process to control the conductivity, in addition to control the pH of the liquid The value, which is closely related to the deposition of the coating film. When the anodic electrophoresis elutes metal ions, it participates in film formation. The dissolution of metal ions during the electrophoresis of aluminum profiles is different from that of steel parts, and has little effect on the color.
2.4.2.3 Properties of Anodic Electrophoretic Coatings for Aluminum Profiles
2.4.2.3.1 Varieties of Anodic Electrophoretic Coatings
Acrylic resin has excellent weathering resistance and abrasion resistance; resin is colorless and transparent; it can adjust the properties of coatings through formulas; it has excellent decorative properties and adjustable properties; and the main chain of these resins consists of carbon-carbon bonds. With high chemical properties, a large amount of aqueous electrophoretic paint formulated with alkaline will not decompose and have good stability. Therefore, acrylic type electrophoretic coating occupies the mainstream of aluminum electrophoretic coatings.
At present, there is a type of colored polyurethane electrophoretic paint, which has good performance and simple process. Because the colored polyurethane electrophoretic paint is used, the anodizing, coloring and sealing processes can be eliminated. Conducive to cost savings. In addition, to meet the different needs of various products, functional antibacterial, insulating, self-lubricating electrophoretic coatings, and decorative matt, matte, pearlescent electrophoretic coatings have also been applied.
2.4.2.3.2 Electrophoretic Coating Manufacturing
Anodic electrophoretic coating refers to the aluminum profile workpiece as the anode, so the anode electrophoretic coating is an anionic resin, the more commonly used is a water-reducible carboxyl-containing acrylic resin, and a triamine-amine formaldehyde resin is used as a cross-linking agent to cure acrylic acid. The more monomer, the better the water solubility of the resin. Such as formula:
MMA:BA:HEMA:AA=45:38:10:7
Among them, MMA is methyl methacrylate which is the hard monomer and the main body of the polymer; BA is butyl acrylate, which is a soft monomer, used to adjust the glass transition temperature and provide the flexibility of the chain; HEMA is methacrylic acid beta-hydroxylamine. Ester, it provides free hydroxyl, cross-linking reaction with amino resin, and also enhance the hydrophilicity of the resin, AA is acrylic acid provides carboxyl, the higher the content of the base, the better the hydrophilicity, the general use of electrophoretic coatings 5% ~ At 10%, the polymerization was carried out in the co-solvent propylene glycol butyl ether at a concentration of about 70%. The initiator may be benzoyl peroxide (BPO), but azoxybisisobutyronitrile 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. The solvent is used in water-based paints. Solvents, mostly hydrophilic alcohols, such as propanol, butanol, ethylene glycol (or propylene glycol) monoethers, are mainly selected based on volatility, toxicity, and price. Neutralizers neutralize acrylic resin chains. The carboxyl group protects the water-soluble resin, mostly organic amines or sodium hydroxide. The use of organic amines such as triethylamine, dimethylethanolamine, especially tertiary amines is beneficial to the stability of the resin.
2.4.3 electrophoresis process and equipment
2.4.3.1 electrophoresis process
Take anodic electrophoresis as an example, usually follow the following process
Degreasing → Washing → Washing → Neutralization → Washing → Anodizing → Washing → Electrolytic Coloring → Hot Washing → Washing → Electrophoresis → Washing → Drying
2.4.3.2 Electrophoresis process conditions
(1) Condition of electrophoretic bath
Solids: 7% to 9%
Temperature: 20～25°C
pH: 8.0 to 8.8
Resistivity/Ω·cm (20°C): 1500-2500
(2) Electrophoresis process parameters
Voltage: 80 to 250V (DC)
Electrophoresis time: 1～3min
Current density: 15 to 50 A/?
Specific process specifications are based on product requirements.
Coating thickness: 7～12μm
2.4.3.3 Equipment required for electrophoresis of aluminum profiles
(1) Electrophoresis tank. Aluminum electrophoresis tanks are designed as rectangular rectangular slots. The internal size of the base is determined by the size of the hanging hanger. The slot body is 4 to 6 mm. Polypropylene or epoxy glass fiber reinforced plastic is used for the steel plate lining. An overflow tank is required.
(2) Power supply. DC requirements 0 ~ 250A adjustable, the current size is calculated according to 50A/?, about 2000A/slot. The corrugation factor of the power supply must be less than 6%. The larger the value is, the greater the probability of electrophoretic coating producing voids.
(3) Electrodes. In the anode electrophoresis, the aluminum profile is used as the anode, and the cathode plate is made of stainless steel or aluminum plate. The area of ​​the cathode is 1 time of the total area of ​​the workpiece. Polypropylene fiber cloth is used for the panel diaphragm.
(4) Heat exchange system. It can ensure that the bath temperature is stable.
(5) Pre-aligned tanks and automatic addition system. The preparation tank can complete the preparation of the electrophoresis bath to ensure that the normal parameters are achieved; the system can be automatically added to ensure that the amount of electrophoretic paint consumed by each aluminum section is covered, and the electrophoresis tank solids are kept within the process range to ensure that the membrane Thick uniform.
(6) Ion exchange processing (IR) system. When the anode is electrophoresed, the pH of the cathode will increase continuously. The ion exchange resin system can remove the impurities in the bath. The cathode and cation can stabilize the pH of the bath. If a reverse osmosis device (RO) is added, a closed circuit system can be realized.
Table 2-2-4 Composition of RO permeate
Since the water after RO is nearly pure water, it can be directly used for cleaning, so a closed-circuit washing system can be realized, which is beneficial to environmental protection.
2.4.4 Summary
At present, there are many aluminum profile factories in China that have electrophoresis production lines. In order to better develop electrophoretic coating technology and occupy more market shares, we believe that we should strive in several ways: (1) Develop new application areas such as machinery, aviation, and automobiles. Components, household appliances, bathrooms, kitchen utensils, etc.; (2) Improve the processing technology of aluminum profiles and increase the qualification rate of electrophoretic profiles; (3) Improve the quality of electrophoretic coatings, expand the variety of electrophoretic coatings, and increase the localization rate; (4) Improve the management level of electrophoretic coating.
In summary, China's aluminum 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 our country.
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