The surface properties of the substrate
In the cathodic electrophoretic coating process, as the substrate of the cathode, such as low-zinc, normal-zinc, or zinc-nickel-containing phosphating steel plates, due to electrolysis, the deposited coating may be defective, affecting the corrosion resistance of the electrodeposition coating. In addition, when the electrophoresis is performed, the alkaline surface of the cathode is enhanced (the pH around the cathode is about 12 when electrodeposition starts), which will cause some of the phosphate film to dissolve and the microstructure of the substrate surface to change, affecting the performance of the overall coating. At present, the solution is to improve the phosphating process so that the surface of the substrate is covered with Zn and Mn, Ca, Fe and other metal complex phosphates [Zn2M(PO4)2?H2O] (where M is Fe, Ni, Mn, and Ca) ) to increase the alkali resistance of the phosphating film. It is worth noting that the thermal properties of the electrodeposited coatings on different substrate surfaces also vary. For example, under the oxygen atmosphere, the epoxy coating deposited on the unphosphorized steel plate begins to be thermally degraded at 450°C, and the thermal degradation residue is about 25%; the surface of the galvanized-nickel alloy steel plate begins to heat at 520°C. Degrade until complete. This is because the catalytic activity of different substrate surfaces on the cathode hydrogen evolution reaction is different, resulting in the degree of hydrogen absorption reaction of the epoxy backbone and the porosity of the coating are different, thereby affecting the thermal degradation rate. In addition, the substrate also has an effect on the curing temperature of the resin. For example, the curing temperature of the epoxy resin on the nickel-plated surface can be reduced by 20 to 30°C.
Electrophoresis
Second, electrophoresis pretreatment
1, degreasing
The purpose of degreasing is to remove oil on the metal surface. At present, degreasing is generally adopted water-soluble alkaline degreaser, the key lies in controlling the degreasing temperature and degreasing time. The degreasing temperature is too high, the hydrolysis rate is accelerated, the surface of the workpiece is yellowish, and the temperature is too low, which is not favorable for the wetting, emulsification, and solubilization of the surfactant in the degreasing fluid, and the degreasing is not clean. Degreasing fluid degreasing ability increases with the increase of pH, but the high pH may cause corrosion of metal workpieces such as aluminum and aluminum alloys. General control of degreasing temperature 60 ~ 80 °C, degreasing time 10 ~ 15min better. In addition, clean immediately after degreasing. Otherwise, the metal surface is covered with a layer of alkaline substances, which will affect the subsequent derusting and phosphating process, and ultimately reduce the corrosion resistance of the electrophoretic coating.
Phosphating
For cathodic electrophoretic coating, the phosphating film must be light-weight (2 to 6 μm in film thickness). An excessively thick phosphating film results in an increase in resistance, which reduces the electrodeposition efficiency. In addition, the electrophoretic coating requires that the phosphating film be dense and uniform, and a uniform electrodeposited film can be obtained only under the premise that the electrical conductivity and electric field strength of the workpiece are the same. China widely uses zinc or zinc-calcium medium-low temperature, low slag fast phosphating process. The combination of low zinc phosphating and cathodic electrophoresis is good, and can fully take advantage of cathodic electrophoretic coating. Electrodeposition coating of high-grade automobiles in developed countries adopts low zinc phosphating. The phosphating process focuses on the free acid and total acidity of the phosphating solution, the accelerator content, and the phosphating temperature and time. General low-zinc phosphating uses NO3-promoter system (content>15g/L), treatment temperature 50-60°C, immersion time 3-5min, total acidity 20-27 points (when titrating phosphating solution of 10mL to the phenolphthalein end point (milliliters of sodium hydroxide solution), free acid 0.7 to 1.3 points (milliliters of sodium hydroxide solution consumed when titrating 10 mL of phosphating solution to the end of methyl orange). If a certain amount of Ni2+ or Mn2+ (2 to 5 g/L) is added to the zinc-based or zinc-calcium phosphating solution, a dense phosphating film of granular crystals can be formed to enhance the alkali resistance of the phosphating film. In order to improve the corrosion resistance of the electrophoretic coating. In addition, the soluble salts remaining on the phosphating film must be thoroughly washed after phosphating, because the soluble salts easily cause the coating to fall off under the hot and humid conditions, and it is brought into the electrophoresis tank to seriously contaminate the electrophoretic coating.
3, solid points
Cathodic electrophoresis bath solids are generally controlled at 18% to 25% (mass fraction), the level of solids has a greater impact on the amount of electrodeposition. The solid content of the coating is high, the conductivity of the bath is good, and the amount of electrodeposit is also increased, but the solid fraction is too high (> 30%), the amount of electrodeposition is increased too much, the coating becomes too thick, and the flow due to baking Poor flatness and the formation of orange peel on the surface and other ills; when the solids fraction is too low (<10%), the paint's throwing power is low, the hiding power of the coating film is poor, and the electrolysis reaction is also increased, and the coating film is easy to generate needles. Holes, bath stability worsened. In the actual coating process, due to the decrease of the solid content of the coating, it is necessary to periodically detect the decrease in the solid content, and add a fresh electrodeposition coating to the bath solution through calculation.
4, Yan Chi Bi
For cathodic electrophoretic paints with pigments as coloring substances, the imbalance of the pigment-based ratio may cause the appearance and corrosion resistance of the coating film to deteriorate. The Yankee ratio is too high, the coating film is rough and dull, and even the pigment precipitates; the pigment base ratio is too low, the coating film is prone to pinholes, and the corrosion resistance is reduced. In the cathodic electrophoresis process, the pigment base ratio is generally maintained at 0.24 to 0.3 by adding a high pigment ratio pigment slurry to the bath.
5, auxiliary solvent
The co-solvent is an important part of the cathodic electrophoretic coating. On the one hand, it is beneficial to maintain the stability of the coating, and on the other hand affects the quality of the coating film. The content of cosolvent is too low, which reduces the water solubility of the resin, resulting in a decrease in the amount of electrodepositing and the throwing power; the content of the auxiliary solvent is too high, the coating film becomes thick, and at the same time, the penetration force and the breakdown voltage of the coating film decrease. Bath fluid is not easy to control. Usually, the content of cosolvent in the cathodic electrodeposition coating is 20% to 40%; after neutralization and dilution with water, the organic solvent in the bath is generally controlled at 2% to 5%. If the selected cosolvent is a low-boiling alcohol solvent, it is also necessary to pay attention to periodically make up for the loss.
6, the pH of the bath
During electrophoresis, the pH of the bath is an important factor in controlling the stability of the electrophoretic coating. In general, cathodic electrophoretic coating requires strict control of the pH between 5.90 and 6.15. The pH of the bath solution is too high, the electrophoretic coating becomes unstable, and the precipitate will precipitate in severe cases; the pH of the bath should not be too low, although the electrophoretic current will increase when the pH value decreases, and the amount of electrodeposition will increase, which is beneficial to the coating. Film formation, but the degree of re-dissolution of the paint film will also increase. During continuous electrophoresis, due to the continuous deposition of the resin and the continuous accumulation of neutralizing agents, the pH of the bath gradually decreased and the electrical conductivity increased, resulting in a decrease in the throwing power. What is more serious is that the tendency of redeposition of the varnish deposited on the workpiece increases, thinning the deposited film, losing light and even revealing the bottom. In general, the pH value is adjusted by means of a polar mask method and a low-neutralization paint replacement ultrafiltrate solution to stabilize it within the specified range.
7, bath conductivity
Cathodic electrophoretic coating bath conductivity is usually 1200 ~ 1600μS/cm, maintaining a certain conductivity of the bath liquid to ensure the quality of the coating. During the electrophoresis process, the electrical conductivity gradually increases due to the mixing of impurity ions and the concentration of free neutralizing agent. The high electrical conductivity not only increases the power consumption, decreases the throwing power, but also causes the bath temperature to rise too fast, the coating film gloss decreases, the pigment particles precipitate, and the paint film corrosion resistance decreases.
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