Principle and process conditions of anode electrophoretic coating

Ionization: After the electrophoretic paint is diluted with water, it is ionized in the solution to form negatively charged resins and positively charged amines, and makes irregular movements in the electrophoretic paint.

Electrophoresis: After electrification, an electric field is formed between the anode and the cathode, and under the action of the electric field force, the positive and negative ions G, which were originally scattered and irregularly moved, move in a directional manner, which is called electrophoresis.

Electrodeposition : Negatively charged resin ions adsorb uncharged pigments and discharge toward the cathode (workpiece), gradually depositing a water-insoluble paint film on the surface of the workpiece.

Electrodialysis : The paint film deposited on the surface of the workpiece contains a certain amount of water. Under the action of the electric field force, the water seeps out from the paint film and moves to the paint liquid. The shape or adhesion of the paint film is dense. The electrodialysis process can reduce the water content of the paint film by 5% to 15%. The paint film with poor electrodialysis is watery and sticky; the paint film with good electrodialysis is smooth and smooth and does not stick to hands.

Electrolysis: When the current passes through the aqueous electrolyte solution, the water undergoes an electrolytic reaction, and gas is released at the cathode. A part of the oxygen is absorbed by the workpiece and produces micropores on the surface of the workpiece, so the electrolysis should be reduced. Generally, the voltage is lowered to reduce the electrolysis, and at the same time, the anode dissolves during the electrolysis process (Fe→Fe2+ +2e). If the electrophoretic paint is mixed with Fe2+, it is easy to cause loose paint film and reduce corrosion resistance. If the workpiece has a phosphating layer before electrophoresis, it will greatly reduce the dissolution of the anode and improve the quality of the paint film. To prevent the dissolution of the anode, the development of cathodic electrophoresis paint.

pH value : pH value is an important control index in electrophoretic coating. When the pH value is too low during construction, the hydrophilicity of the paint liquid decreases, the water solubility of the resin weakens, the dispersion of the pigment is poor, the paint film is thick, and the adhesion is poor. The paint liquid is easy to agglomerate and deteriorate; when it is too high, the electrolytic effect will be strengthened during the electrodeposition process, the amount of electrodeposition will decrease, the throwing power will be low, and the pulse current will be too large, and the paint film will be prone to defects such as remelting and pinholes.

Voltage : The constant voltage method is often used in actual production. The choice of voltage is related to the type and color of the electrophoretic paint, the material, shape and size of the workpiece to be coated, and the distance between the workpiece and the cathode. Generally, the voltage is high, and the electrophoretic paint film is thick, but the voltage is too high, the electrolysis is intensified, the paint film is loose, rough and has pinholes, etc., and the anode dissolution is accelerated at the same time; the voltage is too low, the electrolytic reaction is slow, the deposition is small, and the paint film is thin and uniform. . Poor penetration. Therefore, it is necessary to select an appropriate voltage in production.

Solid content : Generally, the solid content of the upper liquid during construction is 10%~15%. When the solid content of the paint liquid is low, the paint waste is less, the paint film is thin, and the paint liquid is unstable. When it is too low, the throwing power is low, and the paint film is rough and easy produce pinholes. The solid content is high, the paint film is thick and has good throwing power, and the surface state of the paint film is improved, but if it is too high, the paint film will be rough and loose, not dense, and the loss of paint will be large.

Electrophoresis time: The length of electrophoresis time is related to voltage, solid content, workpiece shape, size and thickness, distance between electrodes, etc. The general rule is that the electrophoresis time is long and the paint film is thick, but if the time is too long, when the paint film reaches a certain thickness , the thickness no longer increases, the paint film is easy to crust and the adhesion is poor. Usually the specified time is 1-3 minutes.

Paint liquid temperature: The temperature of light liquid is generally 15-30 ℃. If the temperature is high, although the film is formed quickly, the paint film is rough, and it is easy to cause bad phenomena such as sagging and piled paint. The humidity is low and the electrophoresis ability is poor. Slow film formation, thin paint film and fine appearance.