Electro-coating and its principle

Electrophoretic coating is a special method of coating film formation, which is only suitable for electrophoretic coating (water-based water-soluble or water emulsion) coatings (referred to as electrophoretic coatings) that are different from general coatings .

It is to immerse the coated object with conductivity in the electrophoretic coating tank solution filled with water and have a relatively low concentration as anode (or cathode), and another corresponding cathode (or anode) is set in the tank. A coating method in which a uniform, water-insoluble coating film is deposited on the object to be coated by passing direct current between the two electrodes for a certain period of time . According to the polarity of the object to be coated and the type of electrophoretic coating, the electrophoretic coating method can be divided into two types: one is the anodic electrophoretic coating method, the object to be coated is an anode, and the electrophoretic coating used is anionic (negatively charged) ; The second is the cathodic electrophoretic coating method, the object to be coated is the cathode, and the electrophoretic coating used is cationic (positively charged). Electrophoretic coating is referred to as ED (electro-deposition), AED (anodic electrophoretic coating) and CED (cathode electrophoretic coating) in English .

The electrophoretic coating process is accompanied by the combination of four chemical and physical effects, such as electrophoresis, electrodeposition, electrolysis, and electroosmosis, to form a coating film The principle is as follows.

① Electrophoresis After the anode and cathode in the colloidal solution are electrified, the phenomenon of positively (or negatively) charged colloidal particles swimming toward the cathode (or anode) under the action of an electric field is called electrophoresis. The substances in the colloidal solution are not in the form of molecules and ions, but the solutes dispersed in the liquid. The substances are relatively large (about 10 ^-7 ~10 ^-9 m ), and will not precipitate, but are in a dispersed state.

②Electrodeposition coagulation The phenomenon that the solid is precipitated from the liquid is called coagulation (coagulation, deposition), which is generally caused by cooling or concentrating the solution, while electrophoretic coating is by means of electricity. During cathodic electrophoretic painting, positively charged particles condense on the cathode, and negatively charged particles (ions) gather on the anode. When the positively charged colloidal particles (resin and pigment) reach the cathode (painted) surface area (high Alkaline interface layer), get electrons, and react with hydroxide ions to become water-insoluble, and deposit on the cathode (coated object).

③Electrolysis In the solution with ion conductivity, the anode and cathode are connected with direct current, anions are attracted to the anode, cations are attracted to the cathode, and a chemical reaction occurs. Metal dissolution occurs at the anode, electrolytic oxidation produces oxygen , chlorine, etc., and the anode is an electrode that can produce oxidation reactions. Metal precipitation occurs at the cathode, and 7 8 is electrolytically reduced to hydrogen.

④ Electroosmosis After the two ends (cathode and anode energized, the phenomenon that the low-concentration solvent migrates to the high-concentration side is called electroosmosis. The coating film just deposited on the surface of the object to be coated is a semi-permeable film. Under the continuous action of the electric field, the water contained in the coating film will seep out from the coating film and move to the bath, dehydrating the coating film . Seep. Electroosmosis makes the hydrophilic coating film into a hydrophobic coating film, and dehydration makes the coating film denser. The wet paint after electrophoretic coating with good electroosmosis can be touched without stickiness, and the bath solution attached to the wet paint film can be washed away with water.