The coating process is important step in the manufacture of modern pharmaceutical tablets. The coating itself can serve several functions – it can strengthen the tablet, control the release of active ingredients, improve its taste, provide colour, make it easier to package and handle, and protect it from moisture. Sugar coating was one of the earliest methods developed to coat tablets and is still practised for some products, especially confectionery. Other methods include fluidized bed coating, dry coating, but the most widely used method in the pharmaceutical and nutraceutical industry today is that of film coating. This article describes the essential process steps and equipment required for film coating tablets.
Basic Film Coating Equipment
At the heart of a modern film coating facility is the coating pan or drum and a spraying system. Also, required is an air handling unit and dust collector, and the associated controls. Depending on the application, a humidifier or dehumidifier may also be necessary. The modern coating pan is usually a perforated rotating drum that is housed within a cabinet. The cabinet provides the means of controlling the environment within the drum i.e., temperature and pressure, air flow rate and rotational speed of the drum.
The film coating process begins with the loading of a batch of tablets into the coating drum. These are heated in a stream of flowing air and time allowed for dust to settle and a steady temperature of the outlet air to be reached (typically 40 to 46°C). This usually takes about 15 minutes. Spray guns within the drum are then activated to create a fine mist of coating solution that dries as soon as it reaches the surface of the tablets. As the water or solvent evaporates from the mist, it leaves the solids behind to form a thin film. The secret of film coating is very similar to spray-painting – it is best to spray lightly and evenly so that successive thin coatings are allowed to build up to create a robust dense layer on the surface of the tablet. The time taken to achieve this can range from a few minutes to several hours and is governed by the particular characteristics of the coating material, as well as the process parameters such as drum speed, temperature, pressure and spray rate. Above all these parameters must be carefully controlled for the whole length of the coating process. Disruption of spray rate, for example, will often produce defective tablets.
Spraying and distribution
The first requirement during the coating process is that the coating suspension must be distributed evenly on the tablet bed. To do this spraying guns are used. Most often these are atomizers that use compressed air to atomize the coating solution or suspension. There are three common atomizer gun types – 2-port, 3-port and 4-port. The simple 2-port gun has one port for the liquid coating solution and the other is for the atomizing air. The only control available for a 2-port gun is the pressure of air on the one port. With a 3-port system, the atomization and spray activation pressures can be controlled independently which allows the spray width to be controlled. With the 4-port gun, three ports are for air and one for liquid, and these allow for the independent control of activation pressure, atomization pressure and spray width.
Spray Rate – Film Coating Process
Monitoring the spray delivery rate, is crucial especially in aqueous film coating as variations in the spray rate during the coating process can lead to several problems, such as picking and sticking, orange peeling and frosting.and erosion, all of which are caused by over-wetting of the tablets. The spray rate or supply rate of coating solution is usually controlled using peristaltic pumps. This type of pump allows for accurate control of rate which depends only on the tubing size used in the pump and the speed of rotation of the pump. Peristaltic pumps are very easy to clean and tubes can be changed easily, to minimize cross contamination between different coating materials.
The spray rate can be monitored by checking the usage of coating solution (e.g., following the weight change of a reservoir with time). By checking the first 3 to 5 batches in this way, the rotation speed of the peristaltic pump can be verified.
During the coating process, the pan must be fully or optimally loaded to prevent air from going straight to the bed to the exhaust duct, and thereby waste of energy and reducing drying efficiency. Mixing efficiency of the coating pan also decreases with partially loaded pans, requiring an increased rotation speed of the coating drum. This is important especially for functional such as enteric coatings where increased rotation can cause edge wear on the tablets leading to failure during disintegration tests.
To avoid issues caused by non optimal loading of the coating pan, manufacturers often use a multi-pan approach where different drum sizes are used to accommodate different batch sizes.
Spray Gun – Position and Setup
Several spray guns may be located in the coating drum and each one of them must be set up properly. To get the required spray width, it is important to optimize the gun-to-gun distance and the gun-to-bed distance. If the guns are too close, the spray width would be narrow resulting to non-uniform coating. Conversely, if the guns and the spray widths are too wide, there would be overlap of spray area – some spray may hit non targeted areas such as the door or drum wall. This results in wastage of the coating material and may lead to other problems such as picking and sticking. Ideally, the spray pattern must meet the tablet surface and cover almost the entire tablet bed.
Before starting the spraying process, it is important to set up the guns correctly and carry out validation of the sprays to make sure that there is a minimum variation between the guns. At the start of operations, the spray rate should be checked for each gun and recorded. The guns should be adjusted if there is a variation of more than 10% in spray rate between each of the guns.
During the coating process, the pressure in the drum should always be less than the inlet air pressure. However, high negative pressures must be avoided to ensure that some spray is not lost via the air exhaust port. It is important therefore to keep a check on the pressure within the drum during a coating operation. At the same time, a positive pressure in the drum must also be avoided as this may cause contamination of the environment when the drum door is opened due to the release of dust particles and hot air, both of which are prohibited by GMP.
Process parameters to observe during when setting up coating machines and during the coating process:
· Atomization air pressure
· Batch size
· Bed temperature
· Drum loading—when tablets are first loaded, check broken, capped or chipped tablets. Do defects occur when the drum is first loaded or when rotated?
· Exhaust temperature
· Gun to bed distance
· Gun to gun distance
· Gun nozzles – must be kept clean and free of product build-up.
· Inlet temperature
· Negative pressure in pan
· RPM of coating pan
· Solution preparation
· Spray gun calibration
· Spray rate
· Tablet quality