Dry coating method using magnetically assisted impaction in a randomly turbulent fluidized bed

Surface modified particulates have many potential industrial applications ranging from new technologies such as rechargeable batteries, flat-panel displays, etc. to a wide range of unit operation processes such as dispersion, transport and handling, and separation of particulate systems. Due to environmental constraints, there has been a strong interest in the development of dry methods (chemical and/or water free) for particulate coatings. In this paper, we report the feasibility of novel dry method based on a magnetically-assisted impaction coating (MAIC) process for synthesis of composite particulates. In the MAIC process, magnetic particles are accelerated in the chamber using an alternating electromagnetic field. The magnetic particles in turn collide with the core and with submicron sized fine particles (secondary particles) to form composite particles. The adhesion of the secondary particles on the core particles was found to be dependent on several factors including particle size, particle hardness, etc. and a number of processing parameters. Experiments were conducted on a wide variety of particles systems such TiO2/PMMA, Alumina/PMMA, Ag/PMMA, TiO₂/ Al₂O₃, etc. to understand the effect of these parameters. PMMA were preferred to use as a core (primary) particle because of its smooth surface which minimizes surface roughness effects. The composite particles were characterized using standard materials techniques such as scanning electron microscopy, energy dispersive X-ray microanalysis, etc. The results show that MAIC process significantly improved the surface coverage compared to the coverage obtained from standard blending methods. High frequency ultra-sonication of composite particles was also conducted so that weakly adherent particles could be removed from the surface. The efficiency of the coating process was found to be decreased with increasing secondary particle size. High surface coverage was achieved for composite particle with soft cores (e.g. PMMA). To understand the effect of particle hardness on the adhesion process, atomic force microscopy (AFM) studies were conducted as a function of the particle hardness. Based on experimental observations, a model for the particle coating process has been developed.