Investigation of coating layer morphology by micro-computed X-ray tomography

• Measurement of coating layer thickness and porosity using micro-computed X-ray tomography.
• Determination of layer thickness distribution on single particles.
• Alternative measurement method for the investigation of coating layer thicknesses.
• New protocol allows fast evaluation of thickness distribution in particle population.

Spray fluidized bed coating is a common process in food, chemical and pharmaceutical industry. Morphology of the formed layer is essential for product quality. Therefore, layer morphology was investigated in detail by X-ray micro-computed tomography (μ-CT) for porous γ-alumina particles coated with a solution of sodium benzoate with some hydroxypropyl-methylcellulose in water. Investigated properties are: layer thickness distribution on individual particles, porosity of the layer, and average layer thickness. The determination of layer thickness was performed in two different ways: (1) using two-dimensional cross-sections of volume images obtained from full μ-CT measurements, and (2) using single X-ray irradiation images without sample rotation. The porosity of the coating layer was evaluated from the volume images. Further studies were performed to investigate the influence of operating parameter, e.g. exposure time, on image quality and the obtained results. Special attention was put on ways to reduce the necessary measurement time while preserving the quality of the results. Finally, a new measurement protocol is presented which allows to measure coating thickness distributions in a particle population.

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