Principle and process of electrophoretic coating
The core principle of electrophoretic coating is based on the synergistic effects of colloid chemistry and electrochemistry. An applied electric field is used to induce the directional migration of charged paint particles in water and their deposition on the workpiece surface. The paint particles, in water, are transformed into charged colloidal particles by the action of a dispersant. When the workpiece, acting as an electrode (cathode or anode), is immersed in the electrophoretic bath, the electric field forces the charged particles toward the opposite electrode. For example, in cathodic electrophoresis, the positively charged paint particles migrate toward the workpiece surface, acting as the cathode. Once there, the charge is neutralized and the particles are deposited to form a coating. This process involves not only electrophoretic migration but also multiple electrochemical processes, including electrolysis, electrodeposition, and electroosmosis. The hydrogen and oxygen bubbles generated by electrolysis can affect the coating’s compactness, requiring process control to mitigate these adverse effects.
The electrophoretic coating process can be divided into four key stages: electrolysis, electrophoresis, electrodeposition, and electroosmosis. Electrolysis involves the decomposition of water in the bath under the influence of an electric field, producing hydrogen at the cathode and oxygen at the anode. This process alters the pH of the bath, affecting the stability of the paint particles. Therefore, a buffer is required to maintain the pH balance of the bath. The electrophoretic stage involves the migration of paint particles toward the electrodes under the influence of an electric field. The speed of particle migration is related to the electric field strength, particle charge, and bath viscosity. Generally, higher electric field strengths result in faster migration, but excessively high field strengths can lead to overheating of the bath.
Electrodeposition is the core stage in which paint particles form a coating on a workpiece’s surface. When the charged particles reach the surface, they lose stability due to charge neutralization and aggregate to form an insoluble coating film. During the electrodeposition process, the coating thickness increases over time, but after reaching a certain thickness, it stabilizes due to increased resistance. This is a key reason electrophoretic coating can achieve a uniform coating. Electroosmosis, the process in which moisture within the coating penetrates into the bath under the influence of an electric field, helps reduce moisture in the coating and improve its density and adhesion. Coatings with good electroosmosis are less likely to develop pinholes and bubbles after drying.
The specific operation process of electrophoretic coating begins with the pretreatment of the workpiece. After degreasing, rust removal, phosphating and other steps, the workpiece needs to be thoroughly cleaned to prevent residual impurities from affecting the electrodeposition effect. The workpiece is then hung on a special sling and immersed in the electrophoretic tank liquid, ensuring complete immersion and maintaining an appropriate distance from the electrodes. After the power is turned on, electrophoresis is carried out according to the preset voltage and time. During this period, the tank liquid needs to be kept in circulation and stirred to ensure uniform concentration. After the electrophoresis is completed, the workpiece is slowly lifted out of the tank liquid to reduce the amount of tank liquid carried out. It then enters the post-cleaning process, and the undeposited paint remaining on the surface is rinsed with ultrafiltered water. The recovered cleaning liquid can be returned to the electrophoretic tank for recycling.
Finally, in the drying stage, the workpiece with the wet coating is placed in an oven and baked at a specified temperature (usually 160-180°C). This crosslinks and cures the resin in the coating, forming a hard protective film. The temperature and time of the drying process must be strictly controlled. Too low a temperature will result in incomplete curing, insufficient coating adhesion and hardness, while too high a temperature may cause discoloration and cracking. Throughout the entire process, an online monitoring system monitors the bath parameters and process conditions in real time to ensure stability and control at each stage, ultimately achieving a high-performance electrophoretic coating.
