Strategy for Coating of Aerogels Using Spouted Bed Technology

In this work a strategy for the coating of open-pore structured, nanoporous materials with low density is developed. As model material organic aerogel particles are used, which are applied as matrix material for active compounds in food or pharmaceutical products due to their structural properties. 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 the influence of light, oxygen or water penetration during storage or to mask taste and odor. 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. Therefore, the aerogel particles need to be protected against environmental influence with a secondary coating layer. The functionalization of light-weighting aerogels in µm- 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.

For the coating of aerogels two different types of materials are used: solutions and melts. Both can affect the porous structure of the particles during coating differently and require various process conditions. In case of using solutions, penetration of the pores due to the low-viscosity solvent can take place before its evaporation and thus, lead to collapse of the network. Additionally, evaporation of the solvent and consequently lower concentration of the coating material require longer processing times to get the same film thickness compared to melts. The application of melts requires higher process temperatures, which can destroy the particles due to the thermal stress, but too low temperature causes too fast solidification and formation of solid bridges and agglomerates. Nevertheless, the influence of process parameters like viscosity, bed temperature, droplet size or spray rate on the quality of the aerogel particles and uniformity of the coating are investigated. Therefore, the coated particles are cross-sectioned using focused ion beam to evaluate the interface between particle and coating layer. Based on these investigations, the question, which kind of material at which process conditions is qualified for coating of aerogels to protect the porous network and form shell-core structure, is answered.

During experimental investigations viscosity, contact angle, strength of the solidified films and water vapour permeability of the solidified films made of melts and coating solutions 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. The uniformity of the layer is indicated by measurement of specific surface area of treated and untreated particles.

Additionally, coupled CFD (Computational Fluid Dynamics) - DEM (Discrete Element Method) simulations are performed to investigate the stability of the spouting process and optimize the coating parameters. These include determination of the residence time of the particles in the spray zone and its influence on coating layer thickness.

We gratefully acknowledge for the financial support: German Research Foundation (DFG). Project number HE 4526/24-1.