Coating Process Optimization


In modern pharmaceutical plants many tablets require coating before they are pressed. These tablet coatings are usually polymer and polysaccharide in nature and need to be strong and stable enough to survive handling. The surface and finish of the tablets need to be smooth in order to allow them to be swallowed easily. The coating also helps mask any unpleasant tastes or odours associated with the active ingredients. Colourings are added to improve their appearance, and the coating allows an identifying mark of the manufacturer to be stamped into the tablet which is a common requirement of regulatory authorities.

Coatings are also important in other industries such as confectionery where they are made up of sugar or vegetable oil and are used to seal off the softer cores of the candies.

Tablet coating usually takes place within a perforated rotating drum. There are angled baffles placed in the drum that, along with the air flow inside, help mix the tablet bed. This allows the tablets to be lifted and turned from the peripheries to the centres of the drum to ensure that all surfaces of the tablets are evenly and uniformly coated with the sprayed coating or deposited coating. The atmosphere within this rotating drum needs to be closely controlled for temperature and dew point or relative humidity to ensure that the coating is uniform and smooth in appearance.

Once the coating is applied the drying of the coating takes place by drawing heated air through the tablet bed from a fan. This air flow also requires temperature, humidity and volume control to ensure controlled drying and extraction. Monitoring the dew point of both the inflow and outflow of this air gives an indication of the progress of the drying process. This allows the drying time to be kept to a minimum, reducing overall costs of the drying process.

Knowledge of the water activity of pharmaceutical solids (proteins, drugs, and excipients) is essential to obtain a solid dosage form with optimal chemical, physical, microbial, and shelf-life properties. Water activity (aw) influences the chemical stability, microbial stability, flow properties, compaction, hardness, and dissolution rate of dosage forms of pharmaceuticals, proteins, biopharmaceuticals, nutraceuticals, and phytochemicals.