Under the action of a direct current electric field, the phenomenon that the charged colloidal particles dispersed in a polar medium (such as water) move to the direction of the oppositely charged electrode is called electrophoresis.
Electrophoretic coating is a coating method that uses the basic principle of colloid to immerse the object (workpiece) in water-soluble paint, and under the action of electric field force, rely on the physical and chemical effects of the electric field to deposit the paint on the workpiece.
According to the variety of coatings, it can be divided into electrophoretic primer, bottom-surface combined electrophoresis, electrophoretic topcoat and secondary electrophoresis. According to the type of charge carried by the coating resin and the power source used in electrophoresis, it can be divided into anodic electrophoresis, cathodic electrophoresis, pulse electrophoresis, jet electrophoresis and AC electrophoresis.
In the process of anodic electrophoresis and cathodic electrophoresis, the object to be coated is used as anode and cathode respectively, and ordinary DC power is used. Pulse electrophoresis uses pulse current for electrophoretic coating, which makes the electrophoretic coating film more uniform and compact, reduces or avoids the occurrence of coating defects such as pinholes in the process of direct current electrophoresis, and improves the current efficiency. This method is applicable to any anode or cathode electrophoresis Electrodeposition of coatings.
According to actual needs, pulse electrophoretic coating can be used in the following three ways:
a. Pulse current consists of continuous and intermittent current;
b. Alternate continuous or intermittent current;
c. Use a small continuous current during the entire coating time and superimpose a larger intermittent current or pulse current on the continuous current.
When performing pulse electrophoretic coating, only one pulse generator is needed as a power source, and the rest of the equipment is the same as ordinary electrophoretic coating.
In jet electrophoresis, the workpiece to be coated is used as one pole, and the spray device is used as the other pole. When the coating liquid is sprayed from the nozzle, a potential difference is applied between the workpiece to be coated and the nozzle. on the workpiece to be coated. This process is mainly used for large objects, long objects (strip steel, section steel, pipes), buoyant objects (oil filling, etc.) Large, a lot of electrophoretic coatings are needed for one-time tank matching, which is unreasonable economically, and it is difficult to keep the coating liquid stable.
Although the principle of electrophoresis is the same as that of ordinary electrophoresis, there are many construction parameters that affect the effect of electrophoresis, such as the distance between the nozzle and the workpiece to be coated, the speed at which the bath liquid is sprayed from the nozzle, the spraying time, the size of the nozzle, the current density and the voltage, etc. This method is not suitable for workpieces with complex surface structures.
AC electrophoresis is powered by alternating current with a sinusoidal change at a frequency of 50 Hz, and an alternating current is applied between the workpiece and the plate.
In the positive half cycle of the electric field, the charged particles in the coating liquid move to the workpiece, lose electrons, and deposit a film on the workpiece; in the negative half cycle, the particles are repelled and cannot deposit a film on the workpiece. Compared with DC electrophoresis, AC electrophoresis has simple equipment, does not need to strictly control the pH value of the coating solution, and has stricter requirements for the curing of electrophoretic coatings. It can be used for electrophoresis of iron and aluminum parts. However, it is not suitable for large-scale continuous production because of its large power consumption, high voltage and low efficiency of electrophoretic deposition.
Electrophoretic coating is a professional construction new technology, compared with general spraying or dipping, it has the following characteristics
a. It is conducive to the realization of continuous and automatic coating, greatly improving labor productivity and reducing labor intensity.
b. Since the electrophoretic coating uses water as a solvent, there is no danger of organic solvent poisoning and fire, which fundamentally improves the working conditions.
c. Regardless of workpieces with complex or simple shapes, a coating with good adhesion and uniform thickness can be obtained.
d. The paint is fully utilized, and the utilization rate can reach 90%~95%, which greatly reduces the cost.
e. The electrophoretic equipment is complicated, the investment cost is large, the stability of the electrophoretic coating is high, and the bath management technology is relatively complicated.
