Coating particles using liquids and foams based on viscous formulations with industrial mixers: Batch operation

Particle or powder coating with viscous liquids has been essential in industry for surface modification purposes to induce and enhance specific functionalities. This paper evaluates the performance of using foams (of different bubble diameters) versus liquids as a means of coating powder beds based on viscous liquid formulations. Coating with viscous liquids present numerous industrial challenges and therefore preparing foam equivalents can render the liquid component weak enough (through pre shearing to form the foam), to allow it to break up and coat particles under the shear forces exerted in a mixing device. In this study, two shear mixers are used; the first type consists of paddles in different configurations attached to a single rotating shaft, whilst the second type is a commercial twin screw mixer (TSM). The quality of coating achieved on the bulk powder bed using liquids and foams (stained with a dye) is assessed by image analysis to determine the homogeneity of the color distribution. In addition, scanning electron microscopy provides a tool to further investigate the coating quality of individual particles from the bulk product. The results show that large bubble (centimeter size) foams are much more effective at distributing within the fluidized powder bed compared to the starting viscous liquid and small bubble foams (sub-millimeter size). Furthermore, there is a maximum ratio of foam to powder beyond which agglomeration occurs and is insufficient to fully coat the particles. Coating of individual particles is achieved in the case of the TSM, whereas SEM proves that the single shaft paddle mixer crushes the particles and subsequently granulates them together to form granules of a size comparable to the size of coated particles seen after coating with the TSM.