Do you know what is electrophoresis? Detailed explanation of fastener electrophoresis process

The principle of electrophoresis is similar to that of electroplating

In the electrolyte composed of conductive water-soluble or water-emulsified paint, the workpiece and the other electrode in the electrolyte are respectively connected to both ends of the DC power supply to form an electrolytic circuit. The cations dissociated in the electrolyte are affected by the electric field force. Under the action, they move to the cathode and the anions move to the anode. These charged resin ions, together with the adsorbed pigment particles, are electrophoresed to the surface of the workpiece, and lose their charge to form a wet coating. Electrophoresis classification The current electrophoretic coating is divided into two types: anode electrophoresis and cathodic electrophoresis.

The water-soluble resin used in anodic electrophoresis is an anionic compound. In water, the water-soluble resin (carboxylic acid amine salt) dissolves into ionic form. If a direct current electric field is applied, the potential difference generated between the two poles will cause the ions to move toward the two poles.

Anions move to the anode and deposit on the surface of the anode, releasing electrons; cations move to the cathode and are reduced to amine (or ammonia) at the cathode to obtain electrons. The water-soluble resin used in cathodic electrophoresis is a cationic compound. After neutralization with organic acid, it dissolves into ionic form in water. After passing through a direct current electric field, the ions move directionally, and the cations move to the cathode, releasing electrons on the surface of the cathode, and are oxidized to acid. The electrophoretic development process so far, the coatings used in electrophoretic coating have gone through 6 generations, among which the first and second generations of electrophoretic coatings are anodic electrophoretic coatings.

Introduction of various generations of coatings (Guide: The advantages and disadvantages of fixing the stadium fence by expansion screws)

The first generation is an anodic electrophoretic coating with low voltage and low throwing power. It is mainly used for interior painting of automobile bodies, and auxiliary cathodes must be installed. Its salt spray resistance is poor, within 100h, represented by maleic anhydride oil, phenolic, epoxy ester, etc.

The second generation is an anodic electrophoretic coating with high voltage and high throwing power. When painting the car body, the auxiliary cathode can no longer be provided. The salt spray resistance is greatly improved, which can reach more than 240h (electrophoretic coating on the phosphating plate), represented by polybutadiene resin.

The third generation is a cathodic electrophoretic coating with low voltage, low throwing power and low pH value. The pH value of the electrophoresis bath is 3~5, and the acid is strong, and the tank body is corroded quickly, but the corrosion resistance of the car body coating after electrophoresis is improved, which can reach 360~500h.

The 4th generation is a cathodic electrophoretic coating with high voltage, high pH value and high throwing power. The pH value of the electrophoresis bath is about 6.0, and the salt spray resistance test on the phosphating plate can reach more than 720h. It is still the mainstream of cathodic electrophoresis in various countries.

The 5th generation is a thick film cathodic electrophoretic coating. Mainly to improve the corrosion resistance of the sharp edges of the painted workpiece and simplify the coating process. The film thickness is 30-35μm, and the salt spray resistance can reach about 1000h.

The 6th generation is a high pH, u200bu200bhigh throwing power, lead-free, environmentally friendly cathodic electrophoretic paint. The remarkable feature of this generation of paint in terms of environmental protection is that it reduces the curing temperature and saves energy and resources. The specific performance comparison is shown in Table 3-20.

Characteristics of electrophoresis

The advantages of electrophoretic coating are:

The working environment is good. The solvent in the electrophoretic coating electrolyte is water, which has no flammable and explosive problems, and does not pollute the air.

High production efficiency. Compared with other coating methods, electrophoretic coating has the highest production efficiency. The workpiece can be immersed in the electrolyte and the electrophoresis can be completed within a few minutes. It is suitable for mass production and easy to realize production automation.

Save raw materials. The material utilization rate of electrophoretic coating is generally above 85%, which is 40% less than spray paint.

The coating quality is good. The electrophoretic coating has a uniform surface, good adhesion to the workpiece, tight paint film, no flow marks, blistering and other defects.

But electrophoresis also has some disadvantages:

The equipment is complex and the investment is large. In addition to the electrophoresis tank, auxiliary equipment, ultrafiltration devices and pure water equipment, special DC power supplies, drying equipment, waste water treatment equipment, etc. are also required.

There are few paint varieties. At present, electrophoretic coatings are limited to water-soluble paints and water-emulsified paints; the colors are limited to dark-colored primers or single-layer dual-use paints. The reason is that the iron ions and resin anions ionized in the electrophoresis process (such as anodic electrophoresis deposition) neutralize and deposit on the workpiece to become yellowish brown.

Electrophoretic coating needs to be baked at 150℃ for 1h, which consumes a lot of energy.

Electrophoresis process

Electrophoretic coating is a very complex electrochemical process, which mainly includes four simultaneous processes of electrophoresis, electrolysis, electrodeposition and electroosmosis.

Electrophoresis. Under the action of an external electric field, the charged particles (colloid resin particles) in the solution move to the oppositely charged electrode plate, and the uncharged pigments are adsorbed on the charged colloidal resin particles with electrophoresis.

Electrodeposition. Under the action of an external electric field, the charged resin particles reach the anode (or cathode) by electrophoresis, release (or get) electrons and deposit on the surface of the anode (or cathode), forming a water-insoluble coating.

Electroosmosis. Electroosmosis is the reverse process of electrophoresis. Its main function is to dehydrate the electrodeposited coating. When the colloidal resin particles are deposited on the anode surface, the water and other media originally adsorbed on the anode plate pass through the coating and enter the solution under the action of infiltration force.

Electrolysis. Under the action of an external electric field, a current flows through the electrolyte solution, which electrolyzes the water, releasing hydrogen gas at the cathode and oxygen gas at the anode. Therefore, in the electrophoretic coating process, the voltage should be appropriately reduced to eliminate the influence of the hydrogen and oxygen generated by the electrolyzed water on the quality of the coating.

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