In the electrophoretic coating process, it is necessary to ensure that the pH value and conductivity parameters of the electrophoresis tank are stable to ensure that a stable and high-quality electrophoretic coating film is obtained. During electrophoresis, when an external DC electric field is applied, the electrolytic reaction of water occurs in the cathode and anode regions to produce OH- and hydrogen, H+ and oxygen, respectively.
Electrophoretic coatings are water-insoluble electrophoretic coating resins that become water-soluble positively or negatively charged ions by adding acids or bases. This water-soluble charged ion reacts withOH- or H+ generated by electrolysis near the workpiece, and evenly precipitates and deposits on the surface of the workpiece to form an electrolytic paint film that is insoluble in water.
Water electrolysis reaction in the cathode region:2H2O+ 4e-→2OH-+H2↑↑
Water electrolysis reaction in the anode region:2H2O- 4e-→4H++ O2↑
For example, in cathodic electrophoresis coating (the workpiece is connected to the negative electrode as the cathode, and the anode is connected to the positive electrode), the cathodic electrophoresis coating (R-N)neutralizes the water-soluble R-NH+ and CH3 COO- by CH3 COOH acetate (or other organic acids such as lactic acid). After the DC power supply is applied to the applied electric field, the OH is generated near the workpiece due to the electrolysis of water- forming a highly alkaline interface, each charged ion moves in a directional manner, and the water-soluble R-NH+ moves to the cathode region and the OH- reaction is generated to form an insoluble electrophoretic coating film, which is deposited on the surface of the body. Its reaction formula in the cathode region: RNH++ OH-→RN (insoluble deposited paint film) + H2O.
At the same time,CH3COOis constantly produced in the electrophoresis tankfluid -when itaccumulates too much, it will directly lead to a decrease in pH and an increase in conductivity. In order to remove the CH3COO-constantly generated by the electrolyticprocess, and the H+ generated in the anode region, an anode system needs to be addedto transfer theCH3COO- from the electrophoresis tank to the anode fluid, The reaction with the H+ in the anode region generates CH3COOH through the anode liquid circulation discharge, which can maintain the pH and conductivity of the electrophoresis tank liquid, so as to obtain a stable and high-quality electrophoretic paint film. Its reactive formula in the anode region: CH3COO-+H+→CH3COOH.
The role and composition, principle and design of the anode system
The anode system mainly provides a stable electric field in electrophoretic coating and excludesch3COO that is continuously generated during electrophoresis- and H+ to maintain the stability of the pH and conductivity of the electrophoresis tank 。 Its main components include: anode liquid tank, polar liquid circulation pump, polar liquid flow meter, valve and anode (the anode is equipped with anode tube and anion exchange membrane) and other components, and its core unit is the anode (see Figure 2). The core components of the anode are the anode tube and the ion exchange membrane.
In cathodic electrophoresis coatings, the anode uses an anion exchange membrane, which has selective permeability, allowing excess CH3COO- and other negative ions in the electrophoresis tank to penetrate, while the useful coating cation RNH+ and some additives do not penetrate the membrane. Ion exchange membranes are generally quaternary ammonium-type polymer compounds with a stereonetular structure [structural formula R-N+(CH3)3OH-],having anionexchangegroup, and the porosity of its ion exchange membrane and the action of the ion exchange group ensure that it has selective permeability. During cathodic electrophoresis, the alkaline groups on the anion exchange membrane dissociate, dissociateOH-,and the positively charged group R-N+(CH3) 3is left on themembrane, forming a strong electric field, under the action of applying a DC current, Due to special-shaped charge attraction, the CATHO regionCH3COO- is attracted to transfer through the micropores into the other side of the membrane to the anode fluid, while the resin cation R-NH+ is repulsed, while the H + produced in the anode fluid in the anode region is repelled, Can only stay in the anode zone.
The electrode tube is made of seamless stainless steel tube 316L, which has a good protection againstH+ corrosion, dissolved rust and pressure conductivity.
The design requirements of the anode system:1, the ratio of the area of the cathode and the anode is 4-6:1. 2, per square meter effective area of 6-10L/ min polar liquid circulating water. 3. The anion exchange membrane should have high strength: compression strength up to 14kg/cm2;high transmittance: selective transmittance of more than 98%; low resistance: less than 8 ohms/m.