Electrophoretic coating is the use of resin components in water-soluble coatings. After being neutralized by acid or alkali, it can form polymer electrolyte charged colloidal particles dissolved or dispersed in water. Electrophoretic methods are used to deposit coatings on workpieces. Surface coating method. It is completely different from the various coating methods described above using physical principles.
The electrochemical changes in the process of electrophoretic painting are very complicated, including various chemical changes such as electrophoresis, electrolysis, electrodeposition, and electroosmosis. The process of electrophoretic painting can be simply described as: under the action of a direct current electric field, the charged paint colloid particles dispersed in a polar medium (water) move electrochemically towards the electrode opposite to its charge (electrophoresis). , and changes in gain and loss of electrons and deposition (electrolysis, electrodeposition, electroosmosis) occur on the surface of the workpiece as an electrode, and finally a coating film is formed on the surface of the workpiece.
According to the direction in which the charged paint colloidal particles move in the electric field, it is divided into two categories: anodic electrophoretic painting and cathodic electrophoretic painting.
Hanging the workpiece on the positive pole (anode) of the battery and the charged particles of the paint are negatively charged anions is called anodic electrophoretic painting, while hanging the workpiece on the negative pole (cathode) of the battery and the charged particles of the paint are positively charged cations is called Paint for cathodic electrocoating.
(1) Advantages and disadvantages of electrophoretic painting
1. Advantages
① High production efficiency. Compared with other coating methods, the production efficiency of electrophoretic coating is very high. Because it is easy to adopt automatic assembly line production, it not only reduces labor intensity, but also greatly improves labor productivity.
②The quality of the coating film is good. The coating film formed by electrophoretic coating has a uniform coating rate on the edge of the workpiece, and the formed coating film is tight and has good adhesion, and will not form defects such as flow marks that often occur in other coating methods. Moreover, it can also deposit and form a coating film on the surface of the inner cavity of the workpiece with complex shapes.
③ High paint utilization rate. Studies have shown that the paint utilization rate of electrophoretic paint is above 95%.
④ Good working conditions are conducive to environmental protection. Since water is used as a solvent and no organic solvent is used, problems such as personnel poisoning, fire hazard, and environmental pollution caused by organic solvents are avoided.
2. Disadvantages
①Large investment and huge equipment. It takes up a large area of the site and requires a lot of auxiliary equipment.
② The types of paints used are limited, only water-soluble or water-emulsified paints can be used, and the coating is limited to dark-colored primers.
③High power consumption. Not only the electrophoretic coating itself consumes electricity, but also electric heating is required for subsequent drying, so the electricity consumption is very large.
④ There are high requirements for the treatment of workpieces before painting, and if the pretreatment of workpieces is not good, the effect of electrophoretic painting will be greatly reduced.
⑤ High water consumption. There is a lot of water for equipment flushing and waste water, so the requirements for water treatment equipment are very high.
(2) Electrophoretic coating equipment and technology
1. Electrophoretic coating equipment
Electrophoretic coating equipment generally consists of electrophoretic tank, electrophoretic paint transfer tank, circulating stirring and filtering equipment, paint liquid replenishment system, ultrafiltration system, temperature-controlled cold and heat exchange system, anode (cathode) circulation system, DC power supply device, closed Ultrafiltration cleaning system and other components.
Electrophoretic coating is mainly done in the electrophoretic tank. According to the operation mode, the electrophoresis tank is divided into two types: the direct immersion type produced by the station and the pass-through type produced automatically. The size of the tank body is determined according to the size of the workpiece, and there is an overflow auxiliary tank for controlling the liquid level of the electrophoresis tank and removing foam on the liquid surface.
2. Main process parameters of electrophoretic coating
In order to obtain a satisfactory coating effect, the following process parameters need to be well controlled.
① The solid content of the bath liquid. The solid content of the bath liquid has a great influence on the stability of the bath liquid, the throwing power, the thickness of the coating film and the appearance quality.
If the solid content of the bath liquid is too high, the thickness of the paint film will increase, the electroosmotic performance will decrease, and the paint film will be rough and orange peel will appear. If the solid content of the bath solution is too low, the stability of the bath solution will be poor, the pigment will settle seriously, the paint film will be thin and rough, and pinholes will occur. Therefore, the solid content of the electrophoresis tank needs to be strictly controlled. Anodic electrophoresis should be controlled at 10%~15%, cathodic electrophoretic paint should be controlled at 20%.
② The pH value of the bath solution. If the pH value is too low (too acidic), the corrosion of metal equipment will be aggravated; if the pH value is too high (the neutralization degree of the bath solution is high), the electrolyte concentration in the bath solution will increase significantly, and the increase in conductivity will make the electrolysis too intense. A large number of air bubbles generated by electrolysis will cause rough paint film. Therefore, the pH of cathodic electrophoresis should be controlled at 5.8~6.7, and the pH of anodic electrophoresis should be controlled at 7.5~8.5.
③Conductivity. The conductivity is an index to measure the conductivity of the electrolyte. The higher the conductivity, the stronger the conductivity of the electrolyte. If the conductivity is too large, the electrolysis will be intensified during electrophoretic painting, resulting in rough and porous paint film. The amount will gradually increase, causing the conductivity to increase, so it needs to be adjusted by periodically discharging the ultrafiltrate. Rinsing and potting water used should also have low conductivity.
④ bath temperature. The higher temperature of the bad liquid is beneficial to the electrophoretic movement of the charged resin particles. However, if the temperature of the bath solution is too high, the electrolysis will be intensified, the coating film will become rough, sagging will occur, and the bath solution will deteriorate rapidly. However, if the temperature of the bath solution is too low and the viscosity is high, the bubbles formed by electrolysis on the surface of the workpiece will not easily escape, and it will also cause the coating film to be rough. Therefore, the temperature of the bath solution should be controlled according to the actual needs, and generally it should not be lower than 20°C and should not be higher than 35°C.
⑤ voltage. During electrophoretic coating, the voltage needs to be higher than a certain value before the paint film can be deposited on the workpiece. This voltage is called the critical voltage. As the voltage increases and the current increases, it is beneficial to obtain a thicker coating film, but if the voltage is too high, it will Cause the wet film to be broken down and produce defects such as roughness and pinholes. In general operation, the low working voltage is applied first, and then raised to the working voltage.
⑥ Electrophoresis time. With the progress of electrophoretic coating, the coating film on the surface of the workpiece gradually thickens, but generally the coating film tends to be saturated in about 2 minutes, and no longer continues to thicken, but some defects may continue to occur on the inner surface of the inner cavity and gap of the workpiece. Therefore, the general electrophoresis can be controlled at about 3 minutes.
⑦ pole distance and pole ratio. The distance between the electrode where the workpiece is hung and the other electrode is called the pole distance, and the ratio of the surface area of the workpiece to the other electrode is called the pole ratio. Since the workpiece has a certain shape, too small pole distance will cause excessive local current, resulting in uneven coating thickness, but too large pole distance, low current intensity, and poor deposition efficiency. Generally, the distance between electrodes of electrophoretic coating is controlled at 150 ~800mm. If the area ratio of the workpiece to the electrode is too large or too small, problems such as uneven current density distribution on the surface of the workpiece, poor throwing power, and abnormal deposition will occur. Usually the pole ratio is controlled at about 1:1.
⑧ Stability of electrophoretic paint. Due to the poor stability of the electrophoresis bath, it is difficult to adjust the process parameters, so it is required to use the original paint with good stability. In order to keep the bath liquid stable during use, the bath liquid should be replenished and updated in time.
