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Prevent steel pipe anti-corrosion coating from “whitening”

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The formation mechanism of "whitening" defects in steel pipe anti-corrosion coatings is due to the condensation of moisture mixed into the interior of the coating to emulsify and turn into a translucent white film. There are many reasons for it, such as air humidity in the construction environment, excessive coating dispersion, temperature drop during the dispersion process, local temperature of the steel pipe anti-corrosion coating surface, water content in the coating, too low surface temperature of the workpiece, improper selection of solvent boiling point or ratio, fog The air contains water, etc.

For steel pipe coating, the first three are the main reasons. These are represented as a formation process under certain conditions. As an electrostatic coating, the main reason why the steel pipe anti-corrosion coating film is "whitened" can be described as follows: under the conditions of relatively high air humidity and low temperature in the environment, after the paint comes out of the slot-type electrostatic atomizer, it is dispersed and then accumulates on the surface of the steel pipe. During this process, the paint is over-dispersed into an extremely large number of extremely fine paint droplets. Its specific surface area is large and the area exposed to moist air is large. It can attract a large number of water molecules to its surface and mix into the paint liquid. The flow eventually enters the inside of the coating, causing the "whitening" defect of the steel pipe anti-corrosion coating.

It can be seen from the above discussion that the ambient air humidity is only a condition for the occurrence, and the real cause is excessive dispersion of the paint. Prevent the "whitening" defect of the steel pipe anti-corrosion coating, can be achieved by controlling excessive dispersion under the condition that the ambient air humidity remains unchanged. The use of heated airless spraying is considered to be the best way to control the above two reasons. The so-called heated airless spraying, as the name suggests, heating is to heat the paint to a certain temperature and then spray it. Generally, the temperature is chosen between 40~80℃. Airless spraying refers to atomizing paint without compressed air. The atomization principle is: after the liquid paint is pressurized to a certain pressure with a pressurized pump, it is sent to the spray gun through the pipeline, and a high-speed liquid film is formed after being sprayed out from the nozzle; the high-speed liquid film becomes unstable and disperses into small particles after friction with the air. droplets and atomization. Its atomization effect is moderate without excessive dispersion, making it suitable for pipeline coating. Airless spraying can reduce the amount of solvent used, spray paint with high viscosity, save paint than air spraying, and use slightly more paint than electrostatic spraying. But this weakness can be compensated for by recycling devices. Because heated airless spraying has advantages in temperature and atomization, it is the best choice for steel pipe coating. The effect of this selection process is obvious. As a result of use by multiple manufacturers and multiple coating lines, no "whitening" phenomenon has been found.




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