ã€Chinese aluminum industry network】 Electrophoretic coating (electro-coating) is the use of an external electric field so that suspended in the electrophoretic solution of pigments and resins and other particles of the direction of migration and deposition in one of the electrode substrate surface coating method. The principle of electrophoretic coating was invented at the end of the 1930s, but after this technology was developed and industrial applications were obtained, after 1963, electrophoretic coating was a special method of film formation developed in the past 30 years. It is a water-based coating. More practical construction process. It has the characteristics of water-soluble, non-toxic, easy-to-automate control, etc. It has been widely used in automobiles, building materials, hardware, home appliances and other industries.
Electrophoresis coating is to put the workpiece and the corresponding electrode into the water-soluble paint. After connecting the power supply, rely on the physical and chemical effects generated by the electric field to make the resin and pigment filler in the coating uniform on the surface of the coated electrode. A coating method is deposited to form a water-insoluble paint film. Electrophoretic coating is an extremely complex electrochemical reaction process, which includes at least four processes of electrophoresis, electrodeposition, electroosmosis and electrolysis. Electrophoretic coating according to the deposition performance can be divided into anode electrophoresis (workpieces are anodes, coatings are anionic) and cathodic electrophoresis (workpieces are cathodes, coatings are cationic); according to the power can be divided into DC electrophoresis and ac electrophoresis; according to the process method A constant voltage and constant current method. Currently widely used in the industry is the anode electrophoresis of the DC voltage constant voltage method.
1- Workpieces after surface treatment; 2-power supply; 3-workpiece; 4-water flush; 5-tank filtration; 6-slot bath; 7-circulation pump electrophoretic coating compared to other coating methods, with The following features:
(1) Using water-soluble paint, using water as the dissolution medium, saving a lot of organic solvents, greatly reducing the air pollution and environmental hazards, safety and health, while avoiding the hidden dangers of fire;
(2) high coating efficiency, small paint loss, and coating utilization rate of up to 90% to 95%;
(3) The coating film has uniform thickness, strong adhesion, good coating quality, uniform and smooth paint film can be obtained at various parts of the workpiece such as the inner layer, depressions, welds, etc., and solves other coating methods for complex-shaped workpieces. Painting problems;
(4) High production efficiency, construction can realize automatic continuous production, and greatly improve labor efficiency;
(5) The equipment is complicated, the investment cost is high, the power consumption is large, the temperature required for drying and curing is high, the management of coating and painting is complicated, the construction conditions are strict, and wastewater treatment is required;
(6) Only water-soluble paints can be used. Color cannot be changed during the coating process. It is not easy to control the stability of the paint when it is stored for too long.
1. Electrophoretic Coating Equipment Electrophoretic coating equipment consists of electrophoresis tank, stirring device, coating filtration device, temperature adjustment device, coating management device, DC power supply device, water washing device after electrophoretic coating, ultrafiltration device, and baking. Device, spare tank and other components.
The size and shape of the electrophoresis tank should be determined according to the size, shape and construction process of the workpiece. To ensure a certain distance between poles, smaller ones should be designed as much as possible. The tank is equipped with a filtering device and a temperature regulating device to ensure a certain temperature of the paint and to remove impurities and bubbles in the circulating paint. The stirring device can keep the working lacquer fluid uniform, and the circulator pump is often used. The circulation of the lacquer fluid is generally 4 to 6 times per hour. When the circulation pump is started, the liquid level of the lacquer liquid in the tank should be uniformly turned. The role of the paint management device is to supplement the paint composition, control the pH of the bath, remove the neutralizer with a diaphragm electrode, and remove the low molecular weight components with an ultrafiltration device. The choice of electrophoretic power supply generally uses a DC power supply. Rectifier devices can use silicon rectifiers or thyristors. The size of the current is related to the nature of the paint, temperature, working area, power supply, etc. It is generally 30 to 50 A/m2. The water washing device is used for flushing the workpiece before and after the electrophoretic coating. Deionized water is generally used, but pressurization equipment is required. A washing nozzle with a spiral body is commonly used. The baking device is used to promote the dry film formation of the electrophoretic coating, and resistance furnace, induction heating furnace and infrared baking equipment can be used. The drying room design must have three sections of preheating, heating and post-heating. It should be formulated according to the type of coating and the condition of the workpiece.
Second, the main process parameters affecting the electrophoretic coating 1, the voltage electrophoretic coating uses a constant voltage method, the device is relatively simple and easy to control. The voltage has a great influence on the paint film; the higher the voltage is, the thicker the electrophoretic paint film is, and the paintability can be improved accordingly for the parts that are difficult to coat and the construction time can be shortened. However, if the voltage is too high, the surface of the paint film will be rough, and the “orange peel†phenomenon will easily occur after drying. The voltage is too low, the electrolysis reaction is slow, the paint film is thin and even, and the swimming penetration is poor. The choice of voltage is determined by the type of coating and construction requirements. Under normal circumstances, the voltage is inversely proportional to the solid content of the paint and the paint temperature, which is proportional to the distance between the two poles. The surface of steel is 40 to 70V, the surface of aluminum and aluminum alloy can be 60 to 100V, and the galvanized part is 70 to 85V.
2, the electrophoretic time paint film thickness with the increase of electrophoresis time, but when the film reaches a certain thickness, continue to extend the time, can not increase the thickness, but will exacerbate the side reaction; the other hand, the electrophoresis time is too short, coating over thin. The electrophoresis time should be as short as possible, depending on the voltage used. The average workpiece electrophoresis time is 1 to 3 minutes, and large workpieces are 3 to 4 minutes. If the surface geometry of the object to be coated is complex, the voltage and extension time can be appropriately increased.
3, coating temperature Coating temperature is high, the film formation rate is fast, but the appearance of the paint film is rough, but also cause the paint to deteriorate; low temperature, less electrodeposition, slow film formation, film thin and dense. During the construction process, part of the electrical energy is converted into thermal energy during the electrodeposition, and mechanical friction in the circulating system generates heat, which will cause the temperature of the coating to rise. General paint temperature control in some aspects 15 ~ 30 °C.
4. The solid content of the paint and the pigment base are generally about 50% of the commercially available electrophoretic coating. During the construction, the solid content of the coating should be controlled at 10% to 15% with distilled water. The solid content is too low, the hiding power of the paint film is not good, the pigment is easy to precipitate, and the stability of the paint is poor. If the solid content is too high and the viscosity is increased, the paint film will be loose and the adhesion will be poor. Generally, the base ratio of pigment is about 1 to 2, and the base ratio of high gloss electrophoretic coating can be controlled at 1 to 4. As the actual operation, the amount of paint pigment will gradually decline, you must add paint with high pigment points to adjust.
5, the PH value of the coating electrophoretic coating PH value directly affects the stability of the bath. If the pH is too high, the newly deposited coating film will be redissolved, the paint film will be thin, and the rinse will peel off after electrophoresis. If the PH value is too low, the surface gloss of the workpiece is inconsistent, the stability of the paint solution is not good, the dissolved resin will precipitate, the surface of the paint film will be rough, and the adhesion will be reduced. General requirements During construction, the pH is controlled between 7.5 and 8.5. In the construction process, due to continuous electrophoresis, cationic ammonium compounds accumulate in the paint, resulting in an increase in pH. The stock solution with low PH value can be used to replace cathode cover distilled water, ion exchange resin can be used to remove ammonium ions, and anode cover can be used to reduce the pH. If the pH is too low, ammonium acetate may be added to adjust.
6, coating resistance coating object from the previous process into the electrophoresis tank impurity ions caused by the decline in the value of the paint resistance, resulting in uneven film paint and pinholes and other ills. In the painting process, the paint must be purified. In order to obtain a high-quality coating film, a cathode mask device can be used to remove impurities such as ammonium and calcium, magnesium and other impurities.
7. The distance between the workpiece and the cathode is close and the deposition efficiency is high. However, if the distance is too close, the paint film will be too thick and cause sag, orange peel and other ills. The general distance is not less than 20cm. For large-scale and complex-shaped workpieces, when a thick film has been deposited on the outside and the interior coating is still thin, the auxiliary cathode should be added at a position farther from the cathode.
Third, electrophoretic coating methods and techniques (1) general metal surface electrophoretic coating, the process flow is:
Pre-cleaning→on-line→deoiling→washing→rust removal→washing→neutralization→washing→phosphorization→washing→passivation→electrophoretic coating→tank cleaning→ultrafiltration washing→drying→downline
(2) The substrate and pretreatment of the substrate to be coated have a great influence on the electrophoretic coating film. Castings are generally rusted by sandblasting or shot blasting. The cotton dust is used to remove the dust on the surface of the workpiece. 80#~120# sandpaper is used to remove the residual steel balls and other debris on the surface. The surface of the steel is deoiled and rust-removed. Phosphating and passivating the surface are performed when the surface requirements are too high. Before the anodic electrophoresis, the black metal workpiece must be phosphatized, otherwise the corrosion resistance of the paint film is poor. Phosphate treatment, the general selection of zinc phosphate film, thickness of about 1 ~ 2μm, requires a fine and uniform phosphating film crystal.
(3) In the filtration system, one-stage filtration is generally adopted. The filter is a mesh bag structure with an aperture of 25 to 75 μm. The electrophoretic coating is filtered by a vertical pump to the filter. Considering the comprehensive replacement cycle and the quality of the paint film, the filter bag with a pore size of 50 μm is preferable. It not only satisfies the quality requirements of the paint film, but also solves the plugging problem of the filter bag.
(4) The size of the circulating system of the electrophoretic coating directly affects the stability of the bath and the quality of the paint film. Increasing the amount of circulation will reduce the precipitation and air bubbles of the bath; however, the aging of the bath will increase, the energy consumption will increase, and the stability of the bath will deteriorate. It is ideal to control the circulation times of the bath to 6-8 times per hour, which not only ensures the quality of the paint film, but also ensures the stable operation of the bath.
(5) As the production time increases, the impedance of the anode separator increases, and the effective operating voltage decreases. Therefore, in the production, the operating voltage of the power supply should be gradually increased to compensate for the voltage drop of the anode separator according to the loss of the voltage.
(6) The ultrafiltration system controls the concentration of impurity ions introduced into the workpiece to ensure coating quality. In the operation of this system, it should be noted that the system should be continuously operated after running, and intermittent operation is strictly prohibited to prevent the ultrafiltration membrane from drying out. The dried resin and pigment adhere to the ultrafiltration membrane and cannot be thoroughly cleaned, which will seriously affect the water permeability and service life of the ultrafiltration membrane. The output rate of UF membranes tends to decrease with the running time. The continuous operation of 30-40 days should be cleaned once to ensure the ultrafiltration water required for ultrafiltration and washing.
(7) The electrophoretic coating method is applicable to a large number of production lines. The electrophoresis bath should be refreshed within 3 months. Taking an electrophoresis production line with an annual output of 300,000 steel rings as an example, the scientific management of the bath fluid is extremely important. The various parameters of the bath fluid are regularly tested, and the bath fluid is adjusted and replaced according to the test results. The bath solution parameters are generally measured at the following frequencies:
Electrophoretic liquid, ultrafiltrate and ultrafiltration cleaning solution, yin (positive) polar solution, circulating washing solution, deionized cleaning solution PH value, solid content and electrical conductivity once a day; Yan base ratio, organic solvent content, laboratory small Groove test twice a week.
(8) For the management of paint film quality, the uniformity and film thickness of the paint film should be inspected frequently. The appearance should not be pinholes, sagging, orange peel, wrinkles, etc., and the adhesiveness and corrosion resistance of the paint film should be regularly checked. Physical and chemical indicators. Inspection cycle according to the manufacturer's inspection standards, generally every batch needs testing.
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