Introduction to electrophoretic coating process characteristics and applications of aluminum
Source: China electroplating net
China aluminum industry after 20 years of development, has formed a complete industrial system, and become one of the pillar industry of national economy. Aluminium production has reached more than 2.5 million tons, accounting for a third of the world, with aluminum processing enterprises, more than 1000, production capacity has reached 3.5 million tons/year, aluminum production has reached 1 million tons/year. Now has entered a high-end development period, the surface of aluminum processing technology rapid development, has developed from a single bronze material to spray with powder coating, fluorocarbon coating, organic and inorganic dyeing, electrophoresis coating, mechanical and chemical polishing process, a new generation of wood processing technology, three sections of electrolytic multicolor coloring and micro-arc oxidation coloring technology is also in trial production, the appearance of the aluminium profile with titanium, gold, champagne, imitation stainless steel and various colors, and shows a mirror, sand, yaguang, crafts and other special effects. In numerous surface treatment technology, electrophoresis painting development direction is the most suitable technology. Of the reentry after aluminum anodizing and electrolytic coloring electrophoresis coating, double protection. High transparent, high metal texture, high corrosion resistance and high weatherability, more decorative than paint spraying, powder spraying, loved by the user, has been widely into the family is decorated, great potential market. Second, the process is easy to implement, just add a few slots in the original production line and aluminum oxidation coloring form assembly line production, water-soluble electrophoresis coating of safety, can achieve closed-circuit circulation, beneficial to environmental protection. Third, more importantly, electrophoresis profiles more attractive, is a model of high-end, luxury profile. With electrophoresis coating line become aluminum factory is an important symbol of comprehensive technical level.
2.4.2 electrophoresis paint of aluminium profile
The characteristics of the 2.4.2.1 electrophoretic painting
Since the 1930 s, electrophoretic coating on the ford motor company wheel coating line use, its application has been rapid development. Transparent, weatherability of acrylic and polyurethane electrophoretic coating since the 70 s has been successfully used for metal surface decoration and protection, trademark protection, such as copper word and electroplating products of stainless steel decoration and protection, as the final protection adornment layer is the most representative buildings of electrophoresis paint of aluminium profile.
Aluminum profiles of traditional surface treatment process is oxidation, coloring and sealing treatment, this kind of aluminum conversion coating in outdoor use for a long time, especially in the acid rain or Marine environment, easy to corrosion, durability is poor, so since the 70 s, people tried to use the method of paint coating to improve the decorative performance and service life of architectural aluminum, at present, has been successful use of industrialization of electrophoretic coating, powder coating and the fluorine carbon spraying. Because of the electrophoretic paint is water-based coating, aluminum premenstrual processing water can be directly after electrophoresis, high transparency, electrophoresis coating with high ornamental and can highlight the metallic luster of the aluminum itself, so has been increasingly used in architectural aluminum electrophoresis coating.
Electrophoretic coating method and compared to other coating method has the following advantages:
(1) easy to realize automatic production. Because of the electrophoretic painting in aqueous electrophoresis tank, like anodic oxidation, electrolytic coloring process, short processing time, easy to implement the entire process of assembly line work.
2 uniform coating. Force due to the high swimming electrophoresis coating, can make the complex shape of profile and uniform coating, at the same time by adjusting the power to control the film thickness.
(3) coating of high utilization rate. Bring out less, because of the low viscosity, workpiece and electrophoresis artifacts available water to wash, the use of recycling equipment make the utilization rate of electrophoresis coating of paint is as high as 95% above.
(4) safety and environmental protection. As a result of the electrophoretic coating water dilution, low solids, less solvent content is environmental friendly coatings, and remove the danger of fire, also guaranteed workers health.
5] coating quality is good. Acrylic resin with amino resin curing, to ensure the high decorative and high corrosion resistance of coating, at the same time due to the highly transparent resin, effectively highlight the metallic simple sense, according to the need can also get inferior smooth, the sand surface, such as pearl adornment effect.
[6] and compared to usually electrolytic coloring hole sealing process, save time, save manpower, the characteristics of electrophoresis coating without hole sealing, to avoid the crack caused by the bad hole sealing, etc.
Once freely control the thickness of the coating, usually control at home and abroad 7 mu mu m and 12 m two notches.
Being supplied by electrophoresis transparent light, so aluminum ingot casting quality is high, the extrusion of material surface smooth, less mechanical defects, strict management to oxidation coloring process requirement, because any minor defects and contamination is exposed under the transparent paint.
'levies on electrophoresis process management demand is high, in order to improve the yield.
The principle of 2.4.2.2 electrophoretic painting
Anode electrophoresis, for example, figure illustrates the process of anodic electrophoresis, 2-4-1 artifacts for the aluminum anode at this time.
Coating state (emulsion) : acrylic resin containing carboxyl, neutralizer for potassium hydroxide, or organic amine, curing agent for melamine formaldehyde resin.
PH value: 7.8 ~ 8.5.
Electroplating: precipitation when the pH is falling
Electrode reaction: the anode (aluminum)
2 h2o - 4 h + + e - 4 + O2 write
R - COO - + H + - R - COOH
Water soluble water insoluble (film)
Me - > Men++ ne -
RCOO - Men + + - > (R - COO) nMe (precipitation)
The cathode (plate)
2 h2o + 2 e - 2 oh - + H2 write
Electrophoresis in workpiece surface coating deposition, anode with oxygen to produce, can promote the film, but the oxygen producing too much will cause the coating appearance of brightness, so in the process of electrodeposition, should control the conductivity, in addition to control liquid pH value, it is closely related to the coating deposition, anode electrophoresis with metal ions, dissolution, it involved in film, aluminum electrophoresis of dissolution and steel pieces of different metal ions, the impact on the color effect.
2.4.2.3 aluminum extrusions with characteristics of the anodic electrophoretic coating
2.4.2.3.1 anodic electrophoretic coating
Because the acrylic resin has excellent weather resistance, wear resistance; Resin colorless, transparent, can be adjusted by formula coating performance, excellent decorative performance and adjustable, and this kind of resin main chain is composed of carbon - carbon bonds, high chemical resistance, with plenty of water into alkaline electrophoresis paint won't decompose, has a good stability, so the acrylic type of anodic electrophoretic coating occupies the mainstream of the aluminium electrodeposition coatings.
There is a kind of for non-ferrous polyurethane electrophoretic paint, good performance, simple process, because the colored polyurethane electrophoretic paint, can cancel the anodic oxidation, coloring and sealing process. To save costs. In addition to meet the different needs of various products, the function of antibacterial, insulation, self-lubricating electrophoresis coatings, and decorative inferior smooth, extinction, pearl electrophoretic coating has been applied.
2.4.2.3.2 electrophoresis paint manufacturing
Refers to the anodic electrophoretic coating aluminum workpiece as the anode, so the anodic electrophoretic coating is anionic resin, is the most commonly used water can be diluted with carboxyl acrylic resin, with three get together ammoniac amine formaldehyde resin as a crosslinking agent curing, introduction of acrylic monomers, the more the water soluble resin, the better. Such as the formula:
The MMA as hard monomers of methyl methacrylate (MMA), the body of the polymer; BA is butyl acrylate and soft monomer, used to adjust the glass transition temperature, provide flexible chain; HEMA is beta hydroxy ethyl methacrylate, it provides free hydroxyl, crosslinking reaction with amino resin, but also enhance the hydrophilicity of resin, AA is acrylic with carboxyl, base content is higher, the more hydrophilic, general electrophoresis coating with 5% ~ 10%, the aggregation in cosolvent propylene glycol butyl ether, concentration of 70%, the initiator available methyl benzoyl peroxide (BPO), but accidentally oxygen two different nitrile as well. Reaction temperature is 100 ℃ or so, after completion of polymerization, using dimethyl ethanolamine neutralization, then diluted, generally about 60% solid content, pH value of 7.5 ~ 7.5, will use the solvent in the coating, sometimes called cosolvent, mostly hydrophilic alcohols, such as propylene glycol, butanol, ethylene glycol or propylene glycol monoether, based primarily on volatile properties, toxicity, the price is to select, neutralizer effect lies in the neutralization of acrylic resin chain carboxyl, protection of water soluble resin, mostly organic amine or sodium hydroxide, with organic amine, such as triethylamine, dimethyl ethanolamine, especially tertiary amine are beneficial to the stability of resin.
2.4.3 electrophoresis technology and equipment
2.4.3.1 electrophoresis process
In the case of anode electrophoresis, usually according to the following process
Skim - water - the water - and - water - anodic oxidation - water - electrolytic coloring - hot water washing to washing to electrophoresis, washing, drying
2.4.3.2 electrophoresis process conditions
The improvement of the conditions of the electrophoresis bath
Solids: 7% ~ 9%
Temperature: 20 ~ 25 ℃
PH: 8.0 ~ 8.8
Resistivity/Ω cm (20 ℃) : 1500 ~ 2500
2 electrophoresis process parameters
Voltage: 80 ~ 250 v (DC)
The electrophoresis time: 1 ~ 3 min
Current density: 15 to 50 a / ㎡
Specific process specification in accordance with the requirements of the product.
Coating thickness: 7 ~ 12 microns
2.4.3.3 aluminum electro-coating process required equipment
(1) the electrophoresis tank. Aluminum electrophoresis tank design for rectangular rectangular groove, inside the base size depends on the size of the hang hang, tub with 4 ~ 6 mm, steel lining with polypropylene or epoxy glass fiber reinforced plastic, need to set the overflow tank.
2 power supply. 0 ~ 250 a arbitrary adjustable dc requirements, according to the current size 50 a / ㎡, about 2000 a/slot. The power supply ripple factor must be less than 6%, its value is larger, electrophoretic coating, the greater danger of the pore.
(3) electrodes. When the anode electrophoresis, aluminum as anode, cathode plate is made of stainless steel plate or aluminum plate, cathode 1 times the area of total area of the workpiece. Plate diaphragm using polypropylene fiber cloth.
(4) heat exchange system. Can guarantee the stability of bath temperature.
5] dock planner tank and automatically add system. Dock planner can complete the preparation of the electrophoresis tank liquid trough, to ensure normal parameters; Automatically adds the system, ensures complement each tank aluminum consumption by electrophoresis paint, guarantee the electrophoresis tank solids in the range of technology, to ensure the uniformity of film thickness.
[6] ion exchange process (IR) system. Anode electrophoresis, cathode pH value will continue to increase, using ion exchange resin system can remove the impurity in the bath, anion and cation make bath pH stability, if the device combined with reverse osmosis (RO) closed loop system can be realized.
Table 2-4-2 RO through liquid composition
Because after RO water nearly pure water, can be directly used to clean, so it can realize water closed system, is conducive to environmental protection.
2.4.4 summary
In our country there are several aluminum factory with electrophoresis production line, in order to better development of electrophoretic coating technology, occupy more market share, we believe that the efforts from several aspects: (1) the development of new applications, such as machinery, aviation, automobile components, home appliances, bathroom, kitchen utensils and appliances, etc.; Improve the aluminum processing technology, improve the qualified rate of electrophoresis profiles; (3) improve the quality of electrophoresis coatings, expand the electrophoresis paint variety, improve localization rate; (4) improve the management level of electrophoresis coating.
To sum up, aluminum electrophoresis coating technology in China is entering a new period of development, due to the high quality, electrophoresis profiles will have a more rapid development in our country.
HuaWei - cleaning, oxidation, electrophoresis, spraying, powder spraying coating line and automation equipment professional manufacturers
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