Electro-coating is a coating method in which particles such as pigments and resins suspended in the electrophoretic solution are oriented to migrate and deposit on the surface of one of the electrodes by using an external electric field. The principle of electrophoretic coating was invented at the end of the 1930s, but this technology was developed and obtained industrial application after 1963. Electrophoretic coating is the most practical construction process for water-based coatings. Electrophoretic coating has the characteristics of water solubility, non-toxicity, and easy automatic control. Because it is suitable for the surface treatment of conductive workpieces (metal castings, machined parts, forgings, sheet metal parts and welding parts, etc.), the electrophoretic coating process has quickly been widely used in industries such as automobiles, building materials, hardware, and home appliances.

Principles

The resin contained in the cathodic electrophoretic coating has basic groups, which form a salt after acid neutralization and dissolve in water. After the direct current is applied, the acid radical negative ions move to the anode, and the resin ions and the pigment particles wrapped by them move to the cathode with positive charges and are deposited on the cathode. This is the basic principle of electrophoretic coating (commonly known as plating). Electrophoresis coating is a very complex electrochemical reaction, at least four effects of electrophoresis, electrodeposition, electrolysis, and electroosmosis occur simultaneously.

Electrophoresis

After the anode and cathode in the colloidal solution are powered on, the colloidal particles move to the cathode (or anode) side under the action of the electric field, which is called electrophoresis. The substance in the colloidal solution is not in the state of molecules and ions, but the solute dispersed in the liquid. The substance is large and will not precipitate into a dispersed state.

Electrolysis

In a solution with ionic conductivity, the anode and cathode are connected to direct current, anions are attracted to the anode, and cations are attracted to the cathode, and a chemical reaction occurs. The anode produces metal dissolution and electrolytic oxidation to produce oxygen, chlorine, etc. The anode is an electrode that can produce an oxidation reaction. The metal is precipitated at the cathode and the H+ is electrolytically reduced to hydrogen.

Electroosmosis

After the two ends (cathode and anode) of solutions with different concentrations separated by a semipermeable membrane are energized, the phenomenon that the low-concentration solution moves to the high-concentration side is called electroosmosis. The coating film just deposited on the surface of the coated object is a semi-permeable film. Under the continuous action of the electric field, the water contained in the smearing film dialysis out of the film and moves to the bath to dehydrate the film. This is electroosmosis. Electroosmosis turns the hydrophilic coating film into a hydrophobic coating film, and dehydration makes the coating film dense. The wet paint after swimming with good electro-osmosis electrophoretic paint can be touched and not sticky. You can rinse off the bath liquid adhering to the wet paint film with water.

Electrodeposition

The phenomenon of solid precipitation from liquid is called agglomeration (agglomeration, deposition), which is generally produced when cooling or concentrating the solution, and electrophoretic coating relies on electricity. In cathodic electrophoretic coating, positively charged particles aggregate on the cathode, and negatively charged particles (ie ions) aggregate on the anode. When the positively charged colloidal particles (resin and pigment) reach the cathode (substrate) After the surface area (highly alkaline interface layer), electrons are obtained and react with hydroxide ions to become water-insoluble substances, which are deposited on the cathode (painted workpiece).