Identification and characterization of factors controlling tablet coating uniformity in a Wurster coating process

The product coating uniformity in the Wurster column coating process is primarily determined by two factors — the coating-per-pass distribution and the circulation-time distribution. Experimental techniques were developed for a tablet-coating system to quantify these two factors. A magnetic-tracing technique was used to measure the circulation time, circulation-time distribution, and the number of passes made by the tablets during the coating process. The coating-per-pass and total-coating distributions were measured by a dye tracing technique. The circulation time decreased and the circulation-time distribution became narrower as the inlet air flow rate and the partition gap increased. A tail on the circulation-time distribution at low air flow rates and low partition gaps indicated the formation of dead/slow zones, which could result in non-uniform coating. Modifications to the distributor plate resulted in a narrow circulation-time distribution. The coating-per-pass distribution contributed significantly (>75%) to the total-coating uniformity. This was due to the broad coating-per-pass distribution, measured by coating the tablets for one pass only. The contribution due to the coating per pass increased with increase in the partition gap. The broad coating-per-pass distribution was due to differences in the proximity of the tablets to the spray zone, pulsing flow of the tablets, and tablet-to-tablet sheltering. High-speed video imaging in the spray zone confirmed the cause of the broad coating-per-pass distribution. Optimal total-coating uniformity requires process conditions that give narrow coating-per-pass and circulation-time distributions. Thus, the quantitative techniques developed serve as powerful tools to characterize and optimize the coating process.