Powder coatings are one of the most environmentally friendly coatings systems due to their complete absence of organic solvents and low VOC emissions during curing. The so-called coating leveling, that is, coating coating after the construction of the formation of smooth state, leveling surface should not exist orange peel, brush marks, ripples, shrinkage cavity and other irregular form.
Generally we use the naked eye direct observation method is to sample and standard samples for parallel comparison to judge the advantages and disadvantages of film leveling, this method varies from person to person, strong subjectivity. The wavelength scanning method used in the automobile industry to characterize the surface state of the coating film has a semi-quantitative effect. Long wave (10 ~ 0.6mm) and short wave (0.6-0.1mm) are used for scanning. The measured value is between 0 and 100.
Factors influencing the powder coating of the flowing property mainly includes three aspects, one is the flow ping agent, in the formulation of powder coatings add the appropriate flow ping agent, such as the south sea company GL588, to company’s PV88 H98, worlee – chemie, etc., after the powder melting, these additives can quickly reduce the surface tension of the coating, promote the fast flow coating before curing film, eliminate or reduce the orange peel, brush marks, corrugated, surface defects such as shrinkage cavity; The second is the melt viscosity of powder coating. For thermosetting powder coating, in the process of melting flow, with the crosslinking 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 property is restricted.
Therefore, the melt viscosity of powder coating cannot accurately reflect the final leveling property of the coating, and the influence of baking process, heating rate and cross-linking curing temperature on the leveling property of the coating must be considered at the same time. The third is baking process. There is a heating process in the baking of powder coating, and the speed of heating 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 crosslinking reaction. More importantly, the faster the heating rate is, the smaller the minimum value of viscosity is, and the higher the corresponding temperature is, the better the curing and leveling property of the coating is.
The leveling property of powder coatings is restricted by the structure and curing mechanism of leveling AIDS and film forming resin. The coating with excellent leveling performance can be obtained by adopting appropriate leveling assistant, choosing resin system with low melt viscosity and matching curing process (UV curing).