(107b) Functionalization of Aerogel Particles By Coating in a Spouted Bed (Invited)
Aerogels are groups of highly porous solid materials with high inner surface area, open pore structure and low density. The combination of these extraordinary properties enables their application in different areas of life sciences. For example, due to high porosity and high adsorption capacity, the aerogels are used as matrix material for active compounds in food or pharmaceutical products . These kind of particulate porous solids have to be coated with polymeric films for protective, functional or visual purposes . The coatings can exemplary be used to stabilize the impregnated drug, to protect against influence of light, oxygen or water penetration during storage, to mask taste and odor, to improve the optical appearance or to design a defined drug release profile in the human body [2,3]. Especially in case of porous aerogels, the exposition of the porous network to water during storage or to gastrointestinal tract fluids during ingestion leads to change or destruction of their structure due to high capillary forces. Applying a protective, thin polymeric layer on the aerogelâs surface can avoid this problem and enable the particle to fulfil its primary function .
In this work the functionalization of light-weighting aerogels in Âµm- to mm-range by film coating is performed using a novel in-house built prismatic spouted bed apparatus. This technique is excellent for coating of particles with dense, thin and uniform layers enabling the production of sustainable particles, which provide new options for food and pharmaceutical applications. Spouted bed technology is frequently used for industrial processes like spray granulation, coating or drying of particles, when intensive contacting of gas and solid phases is required . Our experimental plant consists of a prismatic process chamber with two horizontal gas inlets, a conical expansion zone and a cylindrical relaxation zone. The fluidization air enters the process chamber from the bottom through two slots. The velocity of the gas can be adjusted by changing the height of these slots. For the atomization of the coating material a two-fluid-nozzle, placed in the middle of the process chamber, is used. Spraying of coating solutions or melts with pressured air occurs in the direction of the gas flow in so called bottom-spray configuration. Low spray rate was chosen to avoid bed collapse during coating due to a high amount of the liquid in the process chamber and in order to reduce additional drying time of the bed during operation.
For the coating of aerogel particles the optimum process conditions for coating with polymer melts and solutions have been found in order to achieve a core-shell structure while preserving the pore structure of the aerogel particles. In case of using polymer melts, the penetration of the pores during processing can be prevented by relatively high viscosity of the used liquids. The applied coating layer can solidify very fast at optimum bed temperature, but there is a risk of viscous bridges between impacting particles when the bed temperature is too close to the melting point of the used polymer. On the other hand polymeric solutions have lower viscosity. Thus, they can easier penetrate into the pores of the particles compared to the melts. During processing optimum temperature for the evaporation of the solvent should be ensured to avoid agglomeration of the particles .
During experimental investigations viscosity, contact angle, strength of the solidified films and water vapour permeability of the solidified films made of melts (shellac, carnauba wax, beeswax) and coating solutions (shellac-ethanol, carnauba wax acetone) are characterized. The influence of different process parameters during coating, like nozzle air and coating material flows or droplet sizes on the coating layer, is investigated. For this purpose particle growth and coating layer quality are measured. The layer thickness of coated particles is determined by cross-sectioning using focused ion beam (FIB) instrument in case of fine particles (µm-range). Layer thickness of millimeter sized particles is measured using computer micro tomography (µCT). The uniformity of the layer is indicated by measurement of specific surface area of treated and untreated particles. The penetration of the coating into the pores is analyzed using transmission electron microscope (TEM). Furthermore, the fluidization and the entire process chain of spraying - drying - shell formation are modeled and optimized by means of coupled CFD-DEM simulations.
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