How to improve the leveling of powder coatings?
Powder coating is a new type of solvent-free 100% solid powder coating. It has two major categories: thermoplastic powder coatings and thermosetting powder coatings. The coatings are made of resin, color filler, curing agent and other additives, mixed in a certain ratio, and then prepared by the process of hot extrusion and crushing through the sieve. They are at room temperature, storage stability, by electrostatic spraying or fluidized bed dip coating, and then heated and baked melt curing, so that the formation of a flat and bright permanent coating film, to achieve the purpose of decoration and corrosion prevention.
The so-called leveling of the coating, that is, the coating construction after the film presents a flat and smooth state, leveling good surface should not exist orange peel, brush marks, ripples, shrinkage and other irregularities form. Usually people use the naked eye to observe directly by comparing the specimen with the standard sample in parallel to judge the superiority of the coating film leveling, which varies from person to person and is more subjective. The wavelength scanning method applied in the automotive industry is used to characterize the surface state of the coating film with a semi-quantitative effect, using long wave (10-0.6 mm) and short wave (0.6-0.1 mm) scanning, the measured value is between 0-100, the lower the value, the flatter the surface of the coating film, the better the leveling.
Factors affecting the leveling of powder coatings mainly include 3 aspects.
One, leveling additives
In the powder coating formula to add appropriate leveling additives, such as Lais H98, Nanhai 588, etc., when the powder coating melt, these additives can quickly reduce the surface tension of the coating, promote the coating in the curing of the film before the rapid flow, eliminate or reduce the orange peel, brush marks, ripples, shrinkage and other surface defects.
Second, 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 viscosity of the system rises, the flow time is shortened, and the leveling is restricted. Therefore, the melt viscosity of powder coating cannot accurately reflect the final leveling of the coating film, and the influence of baking process, temperature rise rate and cross-linking curing temperature on the leveling of the coating film must be considered.
There is a heating process when powder coating is coated and baked, the speed of heating rate has an important influence on the leveling of the coating; under different heating rates, the trend of dynamic viscosity changes with temperature is basically the same, that is, the viscosity becomes smaller with the increase of temperature at the beginning and reaches the minimum value, and then the viscosity becomes larger with the increase of temperature, which is caused by the cross-linking reaction. More importantly, the faster the temperature rise rate, the smaller the minimum value of viscosity, and the higher the corresponding temperature is, the better the curing of the coating film and the better the leveling of the coating film.
The leveling of powder coatings is governed by the leveling additives, the structure of the film-forming resin and the curing mechanism. The use of appropriate leveling additives, the use of low melt viscosity resin system and complementary curing process, can obtain excellent leveling performance of the coating film.