After 20 years of development, China's aluminium profile industry has formed a complete industrial system and become one of the pillar industries of the national economy. The output of raw aluminium has reached more than 2.5 million tons, ranking third in the world, with more than 1,000 aluminium processing enterprises, production capacity has reached 3.5 million tons per year, and the output of aluminium profiles has reached 1 million tons per year. At present, it has entered a period of high-grade development. The surface treatment technology of aluminium profiles has developed rapidly, from a single bronze material to a powder coating, spraying fluorocarbon coatings, organic and inorganic dyeing, electrophoretic coating, mechanical and chemical polishing and other technological means. A new generation of wood grain treatment technology, three-stage electrolytic multi-color coloring and micro-arc oxidation coloring technology are also in trial production. The appearance of titanium, gold, champagne, imitation stainless steel and various colors, as well as display mirror, sand, matt, pearlescent and other special effects. Among many surface treatment technologies, electrophoretic coating is the most promising one. Aluminum profiles are electrophoretic coated after anodizing and electrolytic coloring, which has double protection. With high transparency, high metal texture, high corrosion resistance and weatherability, it is more decorative than spraying paint and powder spraying. It has been widely used in home decoration and has great potential market. Secondly, the process is easy to implement. As long as several slots are added to the original production line to form a pipeline with aluminium oxide coloring, water-soluble electrophoretic coating is safe, closed-circuit circulation can be realized and environmental protection can be facilitated. Thirdly, the more important thing is that electrophoretic profiles are more attractive and a model of high-grade and luxury profiles. The possession of electrophoretic coating line has become an important symbol of the comprehensive technical level of aluminium profile factory.
Electrophoretic Coating of 2.4.2 Aluminum Profile
Characteristics of 2.4.2.1 Electrophoretic Coating
Since the electrophoretic coating was used on the wheel coating line of Ford Motor Company in the 1930s, its application has developed rapidly. Transparent, weatherproof acrylic and polyurethane electrophoretic coatings have been successfully applied to the decoration and protection of metal surfaces since the 1970s, such as the protection of bronze signboards, the decoration and protection of stainless steel and electroplated products. As the final protective decorative layer, the most representative is the electrophoretic coating of building aluminium profiles.
The traditional surface treatment technology of aluminium profile is oxidation, colouring and sealing. The conversion film of aluminium profile is easy to corrode and has poor durability when used outdoors for a long time, especially in acid rain or marine environment. Since the 1970s, people have tried to improve the decorative performance and service life of aluminium profile for building by coating. At present, it has been industrialized successfully. Electrophoretic coating, powder coating and fluorocarbon spraying are used. As electrophoretic coatings are water-borne coatings, aluminium can be directly electrophoretic after pre-treatment and washing. The electrophoretic coatings have high transparency, high decoration and highlight the metallic luster of aluminium profiles themselves. Therefore, electrophoretic coatings have been used more and more in building aluminium profiles.
Compared with other coating methods, electrophoretic coating has the following advantages:
(1) It is easy to realize automatic production. Because electrophoretic coating is carried out in water-borne electrophoresis tank, it is similar to the process of anodic oxidation and electrolytic coloring, and the treatment time is short, so it is easy to realize the pipeline operation of the whole process.
(2) The film is uniform and compact. Because of the high electrophoretic permeability of electrophoretic coatings, the profiles with complex shapes can also be uniformly coated. At the same time, the film thickness can be controlled by adjusting the electric quantity.
(3) High utilization rate of coatings. Because of the low viscosity, less work pieces are brought out, and electrophoretic work pieces can be washed by water. The utilization rate of electrophoretic coatings can be up to 95% by using the recovery device.
(4) Safety and environmental protection. Due to the dilution of water, low solids content and low solvent content of electrophoretic coatings, they are environmentally friendly coatings, and avoid the danger of fire. Workers'health is also guaranteed.
_The quality of the film is good. The curing of acrylic resin with amino resin ensures the high decoration and corrosion resistance of the film. At the same time, because of the high transparency of the resin, the metal texture is effectively highlighted, and the decorative effects such as matt, sand and pearlescent can be obtained as needed.
_Compared with the conventional electrolytic colouring sealing process, it has the characteristics of saving time and manpower. The electrophoretic coating does not need to seal holes, and avoids the cracks caused by poor sealing.
_The thickness of the coating can be controlled freely. Usually, the thickness of the coating can be controlled in 7 and 12 microns at home and abroad.
_Because the electrophoretic layer is transparent and bright, the quality of aluminium ingot is required to be high, the surface of extruded material is smooth, and there are few mechanical defects. The oxidation coloring process is strictly regulated, because any minor defects and contamination will be exposed under the transparent paint.
_The requirement of electrophoresis process management is high to improve the yield.
Principle of 2.4.2.2 Electrophoretic Coating
Taking anode electrophoresis as an example, Figure 2-4-1 illustrates the process of anode electrophoresis, in which the workpiece is an aluminium profile as an anode.
Coating state (emulsification state): acrylic resin containing carboxyl group, neutralizer is potassium hydroxide, or organic amine, curing agent is melamine formaldehyde resin.
PH value: 7.8-8.5.
Electrodeposition: precipitation when pH value decreases
Electrode Reaction: Anode (Aluminum Profile)
2H_2O_4H+4e-+O_2_
R-COO-+H+R-COOH
Water-soluble insoluble (film)
Me_Men+ne
RCOO-+Men+(R-COO)nMe(precipitation)
