Coating is often used as a process technique in pharmaceutical industry, by which the outer layer of the material is applied to the surface of the dosage form as a coat. This improves the characteristics of the final product, enabling, for example, easier ingestion of tablets, improved stability of the product, modified release properties of the active substance. Different processes of coating are distinguished based on the coating application. In the presented work spraying is taken into consideration. To ensure a stable process and reproducible properties of the final product, in this case the coating, it is necessary to evaluate the impact of numerous process parameters such as the method of spraying, the spray speed, the temperature of the sprayed agent and the coated core, the spray nozzle parameters etc. This work is focused on the development and application of analytical method for measuring the droplet sizes distribution, as formed in the spraying process. The spray formation is studied on the Huettlin three-channel nozzle, that is used for the pellets coating process on laboratory and industrial level. The size of the droplets in the formed spray is estimated as a parameter, which has a major impact on the formation and properties of final coating. Measurements of the droplet size distribution were performed using an optimized laser diffraction method.
Furthermore, we used statistical tools to optimize the number of analyzes needed to compare and evaluate the results. The experimental design (DoE) approach allows you to study multiple parameters while delivering better results compared to changing only one separate factor at a time (COST). The parameters of the method and the instrument settings were optimized to enable reliable measurements using combinations of different nozzles and liquid dispersions. As part of this work, we conducted experiments with nine liquids of different rheological properties. Certain parameters were fixed during the entire set of experiments (sample to laser distance, sample to detector distance, horizontal nozzle position, percentage of talc added). Among the investigated parameters the following were included: dispersion flow rate, atomization pressure, microclimate pressure, polymer type and the amount of polymer added to the dispersion.
Typical parameters for pellet coating were used to perform the experiments. The presented study includes polymers of hydroxypropylmethylcellulose (HPMC), Eudragit® S and Ethylcellulose (EC N10) in different mass fractions. In most cases, increasing the dispersion flow results in an increase of droplet size. The trend of decreasing droplet size distribution is observed when increasing air flow rates of atomization pressure (Pa) and microclimate pressure (Pm). All the conclusions can be further investigated with the experimental set upgrade and used as useful input for process optimization and process transfer from laboratory to production scale.
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