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Characterization and mathematical modelling of single fluidised particle coating

Karlsson, Stina, Rasmuson, Anders, Björn, Ingela Niklasson, Schantz, Staffan
Powder technology 2011 v.207 no.1-3 pp. 245-256
air, atomic force microscopy, coatings, dispersions, droplet size, drying, glass transition, latex, mathematical models, scanning electron microscopy, temperature, water content
A study was made of coating single particles with water-based dispersions under realistic fluid dynamic and well-defined operating conditions. The surfaces of the coated particles were observed with atomic force microscopy (AFM) and scanning electron microscopy (SEM). AFM was used to study the latex particle packing and the colloid particle coalescence at the nanoscale, while SEM was used to study the film at the droplet size level. The influence of temperature, moisture content and spray rate were investigated. The experiments showed a coating layer built up of rings of colloid particles for all cases studied except for high spray rate. A variation in the degree of coalescence between colloid particles with different glass transition temperatures, Tg, was shown in AFM. Cracks in the coating layer were observed when the temperature was lower than Tg. Mechanism evaluation using dimensionless numbers showed that a droplet will spread to the equilibrium angle without splashing; the colloid particles accumulate at the interface between the liquid and the air for all cases studied except air with 90% RH and 20°C and a wet-bulb temperature in the coating layer. The evaluation indicated that no skin forms in any of the cases. A model of the drying of a single droplet was developed to describe the experimental results with rings of colloid particles. The simulation of the shape and height of the dried droplet agrees well with the experimental results.