Introduction to the operation method to solve the leveling problem of powder coatings

Update:04-03-2021
Summary:

Because powder coatings do not contain organic solvents […]

Because powder coatings do not contain organic solvents at all, they have low VOC (volatile organic compounds) emissions during curing and have become one of the most environmentally friendly coating systems. The so-called leveling of the coating refers to the smooth and smooth state of the coating film after the coating is applied. The surface with good leveling should not have irregular shapes such as orange peel, brush marks, ripples, and shrinkage holes.

Generally, the method of direct observation with the naked eye is to compare the sample and the standard sample in parallel to judge the level of the coating film. This method varies from person to person and is highly subjective. The wavelength scanning method used in the automotive industry to characterize the surface state of the coating has a semi-quantitative effect. The long wave (10 ~ 0.6mm) and short wave (0.6-0.1mm) are used for scanning, and the measured value is between 0 and 100. In the meantime, the lower the value, the smoother the surface of the coating film and the better the leveling.

The factors that affect the leveling of powder coatings mainly include three aspects. One is leveling aids. Add appropriate leveling aids to the formulation of powder coatings, such as GL588 from Nanhai Company, H98 from Laisi Company, and Worlee-Chemie Company. PV88, etc., when the powder coating is melted, these additives can quickly reduce the surface tension of the coating, promote the rapid flow of the coating before curing into a film, and eliminate or reduce surface defects such as orange peel, brush marks, ripples, and shrinkage; the second is powder The melt viscosity of the coating. For thermosetting powder coatings, during the melt flow process, with the cross-linking curing reaction, the higher the temperature, the faster the curing reaction, the faster the system viscosity rises, the shorter the flow time, and the leveling performance is restricted. 

Therefore, the melt viscosity of powder coatings cannot accurately reflect the final leveling of the coating film, and the effects of the baking process, heating rate, and crosslinking curing temperature on the leveling of the coating must be considered at the same time; the third is the baking process. When the powder coating is baked, there is a heating process, and the heating rate has a considerable influence on the leveling of the coating. The trend of dynamic viscosity changes with temperature is basically the same, that is, the viscosity decreases with the increase of temperature at the beginning, and reaches the minimum value, and then the viscosity increases with the increase of temperature, which is caused by the cross-linking reaction. More importantly, the faster the heating rate, the smaller the minimum value of the viscosity, and the higher the corresponding temperature, the better the curing of the coating film and the better the leveling of the coating film.

The leveling of powder coatings is restricted by the leveling aids, the structure of the film-forming resin and the curing mechanism. The use of appropriate leveling additives, selection of low melt viscosity resin system and matching curing process (UV curing) can obtain a coating film with excellent leveling performance.

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