Solutions for pinholes, fine lines and haze on the surface of powder coatings

Update:25-03-2021
Summary:

High-gloss powder coatings often appear haze. This is a […]

High-gloss powder coatings often appear haze. This is an optical phenomenon caused by the diffusion of light due to many pinholes or fine textures on the surface of the coating film. Eliminate pinholes or these fine textures, and the haze problem is solved.

How do pinholes and fine textures come about?
Some people say it is caused by the excessive volatilization of powder coatings. There is no denying that the volatile matter of powder coating will form pinholes.

Someone has done such an experiment: The volatile content of all high-gloss powders (including hybrid and pure polyester) produced at a certain stage was measured at 200 degrees Celsius for 4 hours. The results found that these powder coatings, regardless of whether there are needles For pores or fine lines (some powders have a very good surface finish), their volatile content is basically about 0.4%, and the highest is 0.47%. It is concluded that the pinholes of the powder coating caused by volatile matter are very few, it can be said to be very small.
Through observation, statistics and analysis, it is found that most of the pinholes and fine lines are caused by interpenetration and interference with different polyesters after the material is extruded.

In addition, it was found that these problems are more likely to occur when making small samples, especially when the amount of material is small. The greater the difference in gel time, the more severe the interference between powders. When the mixed powder is squeezed out again, this interference phenomenon is very small, and some are even eliminated.

It was also found in the production that using the same manufacturer’s epoxy resin and the same type of polyester, in the process of mass production of a powder coating, pinholes and fine lines on the surface of the coating film will appear from time to time and from time to time. less. Also, the high volume concentration of pigments and fillers is prone to this phenomenon.

Analyze the film forming process of thermosetting powder:
When the powder is heated to a certain temperature, it begins to melt. Due to the heat exchange and the effect of the difference in surface tension, countless small turbulences (Bernard's nest) are generated. As the temperature rises, the powder continues to melt, flow, and level. , And at a certain temperature, the resin begins to undergo cross-linking reaction, and the viscosity of the coating begins to increase. With the increase in temperature and the continuation of time, the viscosity of the powder coating becomes larger and larger, and the rate of turbulence and surface tension balance becomes higher. The slower it comes until it stops, the powder coating begins to gel state, and then with the increase of temperature or the continuation of the holding time, the rate of cross-linking of the resin tends to be infinite-film formation. The workpiece is out of the oven and the powder coating film is cooled.

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