High-gloss powder coatings often exhibit a hazy finish. This is an optical phenomenon caused by the scattering of light due to numerous pinholes or fine textures on the coating surface. Eliminating these pinholes or fine textures resolves the haze problem.

1. How are pinholes and fine textures formed?

Some say it's caused by excessive volatile matter in the powder coating. It's undeniable that high volatile matter in powder coatings can lead to pinholes. An experiment was conducted where the volatile matter content of all high-gloss powders produced at a certain stage (including blended and pure polyester types) was measured at 200 degrees Celsius for 4 hours.

The results showed that regardless of whether these powder coatings had pinholes or fine lines (some powders had very good surface finish), their volatile matter content was generally around 0.4%, with a maximum of 0.47%. Therefore, it was concluded that pinholes in powder coatings caused by volatile matter are very rare, practically negligible. Observation, statistics, and analysis revealed that most pinholes and fine lines were caused by the interpenetration and interference of different polyesters after material extrusion. Furthermore, it was found that these problems were more likely to occur during small-scale sampling, especially with small feed amounts. The greater the difference in gelation time between powders, the more severe the interference. However, re-extruding the mixed powders significantly reduced this interference, sometimes even eliminating it entirely. During production, it was also observed that even when using the same manufacturer's epoxy resin and the same type of polyester in mass production of a powder coating, the presence and frequency of pinholes and fine lines on the coating surface varied. Additionally, higher pigment and filler volume concentrations were more prone to this phenomenon. Furthermore, during sample spraying, sometimes high-gloss panels were very smooth immediately after being removed from the oven, but as the panels gradually cooled, some panels developed pinholes or fine lines.

2. How can these phenomena be explained?

First, let's analyze the film-forming process of thermosetting powders: When the powder is heated to a certain temperature, it begins to melt. Due to heat exchange and the effect of surface tension differences, countless tiny turbulent flows (Bernard's holes) are generated.

As the temperature rises, the powder continues to melt, turbulentize, and level out. At a certain temperature, the resin begins to undergo a cross-linking reaction, and the viscosity of the coating begins to increase. With further increases in temperature and time, the viscosity of the powder coating becomes increasingly higher.

The rate of turbulence and surface tension equilibrium also slows down until it stops, and the powder coating begins to gel. With further increases in temperature or longer holding time, the rate of resin cross-linking approaches infinity—film formation. The workpiece is removed from the furnace, and the powder coating film cools down.

Therefore, we can analyze that:

2.1 When different powders are mixed, if the curing rates of the two (or more) powders differ significantly, one part will gel and solidify first, while the other part will remain in turbulent flow, thus forming an interface. After the coating is fully cured, the interface is fixed, resulting in unevenness or wrinkles. These create shadows when exposed to light. If the interface is very small, the unevenness or wrinkles are not visible; at larger sizes, they appear as pinholes; and at sufficiently large sizes, they appear as fine lines. Extruding again disperses and reduces the size of these interfaces.

2.2 If the extruder's kneading and mixing effect is poor, especially when the volume concentration of powder pigments and fillers is high, the dispersion of pigments and fillers is very uneven. After powdering, some particles have low pigment and filler content, while others have high content. Those with low content easily form turbulence during baking, while those with high content do not easily or cannot form turbulence, thus forming an interface, causing unevenness or wrinkles, and resulting in pinholes or fine lines.

2.3 The reactive group content of the purchased resin varies significantly between different batches, often leading to inaccurate formulations. This results in the coating containing varying degrees of unreacted thermoplastic resin after curing.

Alternatively, resin from the cleaning machine may be included in the grinding process during sample production, causing an excess of a certain resin in the powder coating. In the process of temperature changes, thermoplastic resins shrink more than thermosetting resins. Therefore, after cooling, this difference in shrinkage creates unevenness or wrinkles in the coating, resulting in pinholes or fine lines.

This explains why high-gloss panels are initially very smooth when removed from the oven, but as they cool, some panels develop pinholes or fine lines. This problem is quite common in powder coating production.

To reduce or eliminate pinholes or fine lines, it is essential to thoroughly clean the machine, focusing on the area after the extrusion rollers, especially the grinding chamber, particularly when changing to a different product type. Secondly, careful selection of raw materials is crucial, especially pigments and fillers. It is best to choose products from reputable manufacturers that are stable, temperature-resistant, and easily dispersible. The formulation should be reasonable, with the amount of pigments and fillers determined based on the extruder's performance and the process. Thirdly, one should learn to interpret samples carefully, considering the materials, process, formulation, and environmental conditions. This will allow for accurate identification of problems, and with practice, one will develop a keen eye for detail.