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Steel formwork electrophoretic coating technology

steel formwork, also known as box plate and formwork. Steel formwork is a kind of tool material widely used in the construction industry. Replacing wooden springboard with steel formwork for concrete construction is not only convenient to use, fast in construction, good in quality, but also low in cost and high in benefit

due to the direct contact between the steel formwork and the concrete, the so-called electrochemical corrosion will occur, which will reduce the accuracy of the formwork and affect the project quality

in the past, the anti-corrosion of steel formwork was mostly sprayed or painted, which had many disadvantages. For example, the welding joint of the formwork's negative rib plate is not easy to paint, the appearance of the paint film is poor, its adhesion, impact resistance and other technical indicators are low, and the labor intensity of workers is high, the construction speed is slow, and the paint waste is serious

In 1979, the trial production of electrophoretic coated steel formwork by WISCO metal structure factory was successful. In 1980, the first domestic production line for electrophoretic coating of steel formwork was built. Since the production line was put into operation, not only the product quality has been greatly improved, but also the output has increased year by year. Now the annual output has reached 5000T, and the social and economic benefits are good

II. Basic principle of steel template electrophoretic coating

the so-called electrophoretic coating is to immerse the coated object (steel template) in a water-soluble coating as an anode (anode electrophoresis). In addition, China's first self-developed domestic large aircraft C919 successfully set up a corresponding cathode in the test flight in Shanghai in May, connecting DC between the two poles and relying on the physical and chemical effects produced by the current, A coating technology that makes the coating evenly spread on the coated object

electrophoretic paint must be used for electrophoretic coating. Electrophoretic paint is usually called water-soluble paint. Electrophoretic paint and distilled water must be diluted in a certain proportion before use

It should be noted that pipeline cleaning

electrophoretic coating generally includes four simultaneous processes:

1. Electrophoresis: under the action of DC electric field, positive and negative charged colloidal particles move in the negative and positive directions, also known as swimming

2. Electrolysis: oxidation and reduction reactions are carried out on the electrode respectively, but oxidation and reduction phenomena are formed on the electrode

3. Electrodeposition: due to electrophoresis, charged colloidal particles moved near the anode release electrons on the surface of the template, which are deposited in an insoluble state and precipitated. At this time, the paint film is formed

4. Electroosmosis: under the action of electric field, the solid phase does not move, but the liquid phase moves. Electroosmosis causes the water contained in the paint film to be gradually discharged outside the film, and finally forms a dense paint film that can hardly even pass the current, with extremely low moisture content and high resistance

5. Iron red epoxy electrophoretic paint as an example: the electrophoretic paint is composed of modified epoxy resin, butanol, ethanolamine, talc powder and iron red. After the electrophoretic paint is mixed with distilled water, it is separated into positively charged cations and negatively charged anions under the action of DC electric field, and a series of complex physical and chemical colloidal chemistry and electrochemical change processes are carried out

III. process

1. Steel formwork electrophoretic coating process

oil and rust removal → water washing → drying → electrophoretic kneeling → water washing → drying → trademark spraying → packaging → delivery

2. Description of the process flow

oil and rust removal: the two in one method is adopted, that is, the one-step method of oil and rust removal, and the solution is prepared by our factory

water flushing and washing: the purpose is to remove the acid-base residues on the surface of the template. When washing, it is best to use running water

electrophoretic painting: This is the purpose and core of the process. The current, voltage, solid content, pH value, electrophoresis temperature and speed in the electrophoresis tank and the correct use of additional equipment in the electrophoresis tank are the keys to the success or failure of electrophoresis painting

drying: the steel formwork must be dried after electrophoretic painting. The working temperature of the drying oven is ℃, and the baking time is 30min

main equipment required for electrophoretic coating. Various water tanks, electrophoresis tanks, channel drying boxes, mechanical transmission systems, electrical control systems, paint solutions and paint film equipment, such as conductivity meters, Adhesion Testers, etc. Before using the experimental machine, it is suggested that everyone should carefully read the manual

3, quality detection and control

(1) paint concentration

after the electrophoretic paint is mixed with distilled water, its concentration should reach 10%-15%, the concentration is too high, the film leveling is poor, and orange peel and other defects are easy to appear. If the concentration is too low, the paint film becomes thinner and pinholes are easy to appear

(2) voltage

generally, the electrophoresis voltage should be controlled at about 60V. When the voltage increases, the deposition increases, the paint film becomes rough and forms orange peel. When the voltage is too low, the paint film is very thin, and even cannot be painted

(3) pH value

generally, a satisfactory paint film can be obtained at the pH value of the paint solution. If the pH value is too low, the pulse current density in the electrophoresis tank will be reduced, resulting in no paint on the template, forming a painted face. If the pH value is too high, it will cause the wet film to dissolve again, thinning the paint film, and affecting the coating quality. There are many ways to adjust the pH value of the tank liquid, and we chose the cathode cover method. The cathode cover is to put the cathode plate in a canvas bag, which is supported by a steel wire frame and filled with distilled water. During electrophoresis, the positive ions of amine and the positively charged impurity ions in the paint solution (such as Ca + and Mg + in water) can enter the cathode cover through the canvas and discharge at the cathode. The impurities can be removed by changing the distilled water in the hood

(4) electrophoretic time

the thickness of electrophoretic paint film generally increases with the increase of electrophoretic time. However, when the electrophoretic time reaches a certain point, the thickness of paint film will not increase. The reason is that the paint film produces insulation when it reaches a certain thickness. Therefore, it is not feasible to use long electrophoresis time to thicken the paint film, and it is not economical due to the increased power consumption. The electrophoresis time is generally 3min

(5) conductivity

the conductivity of electrophoretic paint working fluid is usually expressed by conductivity. The conductivity of paint solution is too low, and the paint film is not easy to form; The conductivity is too high, resulting in rough appearance of the paint film. For the same paint, the biggest factor affecting the conductivity of paint solution is the existence of impurity ions in paint solution. Therefore, we must maintain the normal value of conductivity by controlling the impurity content. See Table 1

(Table 1 partial properties of steel template electrophoretic paint film)

inspection item index

(1) the color and appearance of the paint film are red, which meets the color code standard

(2) flexibility, mm 1

(3) adhesion (circle drawing method), grade ≤ 2

(4) impact strength, kg, CM 50

(5) paint film thickness, μ M ≥ 2

(6) film luminosity, degree

(7) salt water resistance (25 ℃ =1 ℃, 24h in 3% sodium chloride solution) no change by visual observation

four breaking elongation - elongation at sample breaking, conclusion

electrophoretic coating and the application of water-based paint, marking a major progress in the coating industry

electrophoretic coating has fast construction speed, can realize mechanized and automated continuous operation, reduce labor intensity, uniform paint film, strong adhesion, and can obtain a uniform, flat and smooth paint film for parts that are not easy to be painted or poorly painted by general coating methods, such as the rib plate welds mentioned above. The utilization rate of the coating is as high as 90%-95%. Because the electrophoretic coating uses water as the solvent, it has the advantages of non combustibility, non toxicity, convenient operation and so on. The paint film dried after electrophoresis has excellent adhesion, and its rust resistance, corrosion resistance, weather resistance and other properties are better than ordinary paint and general construction methods. Wugang brand electrophoretic templates produced by Jinjie factory sell well in more than 20 provinces, cities, autonomous regions and municipalities directly under the central government. From 1991 to October 1993 alone, the output value of steel template electrophoresis was 2.77 million yuan, and the profit and tax was 240000 yuan. (end)

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