Narrowing the particle size distribution of powder coatings via a reformed classifying cyclone: Improved flowability and film qualities

In this study, a new design of classifying cyclone separator with secondary air inlet and air guider (SAG type) was proposed to narrow the particle size distribution (PSD). CFD simulation reveals that only adding a secondary air inlet (SA type) at the bottom of the cyclone will create a strong updraft. After adding an air guider, the rotating air enhances the strength of the outer vortex, which is conducive to the particle collection. The experiment results also verify these findings. SA cyclone separator effectively narrows the PSD, but the particle collection efficiency is greatly reduced. In contrast, SAG cyclone separator can decrease the PSD while maintaining a high particle collection efficiency. In addition, it is also found that the collection efficiency of particles decreases with the increase of the opening size of the secondary air inlet, but the SAG cyclone separator is less sensitive to this, making it more controllable in practical applications. To further assess the impact of cyclone modification on coating powder and film properties, the powder coatings with wide PSD and narrow PSD were prepared by original cyclone and SAG cyclone, respectively. It was found that reducing the span of the powder by modifying the cyclone can effectively improve the flowability (angle of repose (AOR), avalanche angle (AVA) and rotating bed expansion rate (RBER)) and film properties (distinctness of image (DOI) and surface flatness) of the particles. This study provides guidance for narrowing the PSD and improving the flowability and film properties of powder coatings.