(461a) Characterization of the Flow and Fluidization Properties of Ultrafine Powders

Fine powders have many potential applications in various industries. However, due to their cohesive nature, handling of fine powder is a long-standing problem in many manufacturing processes. In the automobiles industries, for example, powdered resin materials are used for powder coating many components. Powder coating is an environmentally friendly technology to spray fluidized powders directly onto a substrate without using any solvent. Then, the substrate is cured until the coating finishing is formed. Powder coating uses no toxic solvent and can also recover and recycle the overspray, so that it is more cost effective and environmentally-friendly. In comparison, traditional liquid coating emits millions of tons of solvent globally and does not allow recycle. However, the current powder coating technology has only been applied to automobile's underhood parts, because of the low cosmetic coating quality, which is caused by powders of big size. Ultrafine powder coating, adopting ultrafine powders (less than 20 microns) instead, could solve the cosmetic problem, and make it possible to apply to high end products including automobile exterior surface. However, ultrafine powder coating is able to achieve as good coating quality as liquid coating only if the good fluidization quality of ultrafine powders is insured.

The poor fluidization comes from the cohesiveness of ultrafine powders. According to Geldart powder classification, ultrafine powders, which belong to group C powder, is very difficult to fluidize due to the relatively large interparticle forces. In our research, nanoparticles were employed to modify the surface of ultrafine paint powder. Hundreds of formulae formed by several kinds of nanoparticles were tested and the optimum ones were determined in the aspect of improvement of flow and fluidization qualities.

The flow and fluidization qualities of modified ultrafine powders were determined by several widely used flow indics, including angle of repose, shear stress test and fluidized bed test. However, because of their different particular preferences, the flow quality results thus determined are also contradictory to each other. In this reported work, a new equipment has been designed to measure the normal stress between two layers of particles, which can then be translated into cohesiveness of the particles. The initial results showed that it was able to predict the flowability of powders, especially ultrafine powders, accurately.