Defects of Paint Film in Electrophoretic Coating and Their Prevention

With the development of automobile industry in our country. The automobile painting industry has attracted more and more attention. More and more automobile painting technology has been introduced into the automobile industry. Electrophoretic coating technology has been widely used in the automobile painting industry in China. Electrophoretic paint is characterized by electrodeposition paint, although the types of coatings used, construction conditions are different. The results will vary, but generally speaking. Compared with impregnation coating, it has the following characteristics:

(1) The quality of the film obtained by electrodeposition coating is approximately proportional to the electric quantity, so the amount of film deposition can be adjusted by increasing or decreasing the electric quantity;

(2) Electrodeposition coating can make the sharp edges or corners of the complex coatings, spot welds and other crevices, and the inner and outer surfaces of the box body can obtain relatively uniform paint film. The anticorrosive performance was improved significantly;

(3) The moisture content of Electrodeposited paint film before drying was very low, it was insoluble in water and did not flow, and it was not easy to produce droplets, flow marks, stagnation marks and other defects of paint film; it also did not produce solvent gas washing phenomena which often occurred during drying process, such as immersion coating film (inside box or tube); it could also significantly shorten the evaporation of water before drying. Pre-drying time;

(4) The water resistance of Electrodeposited paint film is very good due to the directional deposition of negatively charged macromolecule particles under the action of electric field; the adhesion is also higher than that of other construction methods;

(5) The paint solution used for electrodeposited paint is low in concentration and low in viscosity, so the amount of paint adhering to the coating is less due to impregnation. Especially after the application of ultrafiltration technology in electrodeposited paint, the utilization rate of paint is even close to 100%,

(6) is the same as that of general waterborne paint, and electrodeposited paint overcomes the problems of fire and benzene poisoning.

Although electrophoretic coating has obvious advantages over other coating methods, it is precisely because of the unique characteristics of electrophoretic coating methods that the defects of paint film are the same as those of common paint film, but the causes and prevention methods are different. Some defects are unique to electrophoretic coating. In this paper, the common defects of electrophoretic coating film and their causes and prevention methods are introduced as follows.

1. Particles are easy to appear in the painting process, and there are rough (or naked eye visible) and hard particles on the surface of electrophoretic paint film after drying. They are called granules. The causes are as follows:

(1) There are sediments, coagulants or other foreign bodies in the electrophoretic bath, and the filtration of the bath is poor. Baseline determination of sewage in tank fluid: 1 kg of tank fluid is taken and filtered with 500 mesh filter, the amount of residue should be less than 10 mg.

(2) The concentration of lacquer in the flushing fluid was too high after electrophoresis.

(3) The coated workpiece entering the electrophoresis cell is not clean. Phosphating water is not clean.

(4) The environment around the painting production area is dirty and there are more dirt in the drying room during the drying process.

In view of several reasons for the formation of particles, we should take the following measures to prevent them.

(1) Reduce the number of dust carriers and strengthen the filtration of electrophoretic bath solution. All circulating lacquer should be filtered through the filter device. It is recommended to use a filter bag with 25 u m precision to filter, strengthen stirring to prevent precipitation, eliminate "dead corner" and bare metal in the tank, strictly control pH value and alkaline substances, and prevent resin from precipitation and condensation.

(2) Enhance the cleanliness of the flushing water after electrophoresis, and keep the solid content of the flushing water as low as possible, so as to keep the back groove overflowing to the front groove. The cleaning fluid should be filtered to reduce foam.

(3) Clean the drying room, clean the air filter, check the balance system and air leakage.

(4) Strengthen the washing after phosphating and wash the phosphating residue floating on the surface of the workpiece. Check whether the filter of the deionized water circulating washing tank is blocked to prevent secondary contamination of the surface of the coating.

(5) The environment of the painting production area should be kept clean. Phosphating to the electrophoresis tank and draining after electrophoresis (before entering the drying room), check and eliminate the dust source of the air.

2. Shrinkage (pit) is caused by dust, oil stain on the surface of the coated material, phosphating film or electrophoretic wet paint film, or by particles that are not compatible with electrophoretic paint, which become shrinkage center and cause crater-like pits with a diameter of 0.5-3.0 mm at the beginning of drying. They are called shrinkage holes and pits that do not expose the bottom. A large part of the shrinkage of is caused by oil pollution. The specific reasons are as follows:

(1) There are foreign substances (oil and dust) mixed in the tank fluid, and oil floats on the surface of the electrophoretic tank fluid or emulsifies in the tank fluid.

(2) The coated workpiece is contaminated by foreign bodies (such as dust, lubricating oil falling off the transport chain, oily iron powder, paint dust, and oil stain in compressed air for drying).

(3) poor degreasing and oil stain on phosphating film in pretreatment.

(4) When washing after electrophoresis, foreign bodies (oil and dust) were mixed into the cleaning solution, and the purity of pure water was poor.

(5) The oil content in the drying furnace or the circulating air is not clear.

(6) The ratio of pigments to radicals in the bath fluid was out of balance.

(7) Insoluble particles of paint or resin were supplied.

In view of the causes of shrinkage, attention should be paid to

(1) Oil removal filter bags should be set up in tank liquid circulation system to remove dirt.

(2) Maintain painting