How to Improve the Leveling Property of Powder Coatings

Powder coatings have become one of the most environmentally friendly coatings systems because they do not contain organic solvents and have low VOC emission during curing. The so-called leveling of coatings refers to the smooth and smooth state of the coatings after construction. The surface with good leveling should not have irregular shape such as orange peel, brush marks, ripple and shrinkage hole. In general, the method of direct observation with naked eyes is to compare the sample with the standard sample in parallel to judge the quality of film leveling. This method varies from person to person and has strong subjectivity. The wavelength scanning method used in the automobile industry to characterize the surface state of the coatings has a semi-quantitative effect. Long wave (10-0.6 mm) and short wave (0.6-0.1 mm) are used to scan the coatings. The measured values are between 0-100. The lower the value, the smoother the surface of the coatings is, the better the leveling performance is. The factors affecting the leveling property of powder coatings mainly include three aspects.

First, leveling aids are added to the formulation of powder coatings, such as GL588 of Nanhai Company, H98 of Laisse Company and PV88 of worlee-chemie Company. When powder coatings melt, these aids can rapidly reduce the surface tension of coatings, promote the rapid flow of coatings before curing and eliminate it. In addition to or reduce surface defects such as orange peel, brush marks, ripple, shrinkage, etc.

The second is the melt viscosity of powder coatings. For thermosetting powder coatings, in the process of melt flow, accompanied by cross-linking curing reaction, the higher the temperature, the faster the curing reaction, the faster the system viscosity rise, the shorter the flow time, and the leveling is restricted. Therefore, the melt viscosity of powder coatings can not accurately reflect the final leveling of coatings, and the effects of baking process, heating rate, cross-linking curing temperature on the leveling of coatings must be taken into account at the same time.

Third, baking process. When powder coatings are baked, there is a heating process, and the speed of heating has a considerable influence on the leveling of coatings. The trend of dynamic viscosity with temperature is basically the same, that is, at the beginning, the viscosity decreases with the increase of temperature 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 is, the smaller the minimum viscosity is. The higher the corresponding temperature is, the better the curing and leveling of the film are. The leveling of powder coatings is restricted by leveling aids, structure of film-forming resins and curing mechanism. With proper leveling aids, low melting viscosity resin system and matching curing process (UV curing), excellent leveling performance coatings can be obtained.