Q: What makes one powder fluidize better than another?
A: This is an excellent question. For a powder coating to perform well, many physical and chemical properties must be aligned. Fluidization performance is one of the critical physical properties necessary to successfully apply a powder coating. Fluidization involves incorporating a volume of air into a quantity of powder to prepare it to be transported from the feed hopper to the application gun. This mixture of air and particles not only helps transport the material but also enhances the delivery of electrostatic charge to the individual particles as they exit the spray gun.
Particle size distribution (PSD) is the primary factor that influences fluidization. Powder coatings are comprised of a range of different-sized particles, from about 1.0 micron upwards to around 100 microns. The best fluidization occurs within the heart of this range, typically 20-60 microns.
Fines (< 10 microns) tend to agglomerate and impair fluidization. Coarse particles (> 100 microns) are difficult to fluidize because of their relatively high mass. Hence, the best fluidizing powder coatings possess tight particle size distributions. This means a minimal level of fines and also a minimum of coarse particles.
If you employ a reclaim system to capture and recycle oversprayed powder, this can be a factor in fluidization. The best-suited particles are preferentially deposited on the parts you coat. And as you can surmise, the fines and coarse particles populate the overspray. Reintroducing overspray into your virgin powder feed will affect fluidization. This increase in fines and coarse particles may require adjustments to your fluidization process.
How do you avoid this problem? The best strategy is to use high-quality powder coating from a reputable powder manufacturer. The most reliable suppliers have well-controlled grinding processes that yield relatively narrow PSDs. Narrow PSDs deliver high first-pass transfer efficiency and hence less overspray to contend with.
A secondary factor that contributes to poor fluidization is moisture. “Wet” powder is very difficult to fluidize and transport through a powder application system. High levels of moisture are usually due to poor storage and handling of powder coating inventory. Powder should be stored in an air-conditioned environment, and open bags/containers should be avoided.
In addition, you should be careful to avoid introducing “cool” powder into a warmer, moister environment. The cool powder will act as a desiccant and absorb the ambient moisture. It is wise, as you move powder from storage to the application area, to allow it to acclimate to the ambient application environment before opening the package.