Tablets are often film-coated. Perforated coating drums are commonly used for tablet film coating. To apply coating effectively and produce uniformly coated tablets of high quality, the knowledge of the impact of critical process parameters on the coating process is of utmost importance. Both the coating uniformity and the coating process yield can be improved by optimizing coating time, dispersion spraying, tablet movement and drying speed.
To evaluate the impact of the selected process parameters on the inter-tablet coating uniformity and on the coating process yield, we carried out a set of experiments in the lab scale coater with 0.8 L perforated drum. We verified the coating uniformity with the relative standard deviation (RSD) of the tablets individual coating masses. Tablets were coated at two different drum fillings, three peripheral speed of the drum and two numbers of mixing baffles and with different process end times to apply three different coating thicknesses. The inter-tablet coating mass uniformity and the coating process yield were evaluated by the spectrofotometric and gravimetric methods. When assessing impacts of the selected process parameters on the coating mass RSD, a conformity check of the experimental results with a prediction model of the coating mass RSD was carried out. When evaluating impacts of the process parameters on the process yield, key causes for the tablet coating process losses were sought after.
The spectrophotometric method has proved to be more appropriate than gravimetric, when analysing the tablet coating yield. First method has on average given by 2.27 % higher process yiled results. Experimental values of the coating mass RSD was found to be inversely proportional to the square root of the total time of coating. The coating mass RSD is reduced by increasing the drum rotational speed, however only at lower drum filling. The drum filling and the number of mixing baffles will impact the of the coating mass RSD only if the efficiency of the tablets mixing is impacted. 0.8L perforated drum has in comparison with the previously evaluated 1.6L drum demonstrated higher efficiency in assuring inter-tablet coating mass uniformity. The impact of the drum rotational speed, drum filling and of the number of mixing baffles on the coating process yield will mainly be reflected as an impact on forming a polymer film on the drum’s surface. The amount of the film on the drum’s surface will increase leading to lower process yield at lower drum filling and increased drum rotational speed. The coating process time will impact the coating yield only until the steady-state temperature in the coating drum is reached.
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