The film coating process involves the deposition of a thin layer of film-forming polymer on a product substrate. The process can be carried out using conventional baking equipment, but today more sophisticated equipment is used to achieve efficient drying, a high degree of automation, and a coating time.
The coating formulation contains a polymer solubilized in a suitable solvent along with other additives such as plasticizers and pigments. This solution is sprayed onto a bed of rotating or fluidized tablets. The drying conditions cause the removal of the solvent, leaving a thin deposit of coating material around each core of the tablet.
There are a number of variables that need to be controlled in a process and we will understand how these variables are crucial to the film coating process. We will also study various types of equipment, which are available for film coating.
Basic requirements of film coating.
- Distribution
- Mixing
- Drying
- Removal of solvent
Film Coating Process
Now we discuss step by step Film Coating Process in Pharmaceuticals and how to perform these steps.
The first requirement of the film coating process is that the coating suspension is evenly distributed throughout the tablet bed. Spray guns are most often used for coating slurry distribution. Two different types of spray guns are used. Airless spray guns and air atomized spray guns. One of the problems with airless spray guns is that the spray speed cannot be controlled independently of the air pressure. Therefore, air-atomized spray guns are used in aqueous film coating.
Types of Guns:
- 2 Port Gun
- 3 Ports Guns
- 4 Ports Guns
There are three different types of guns, which are used, 2-port guns, 3-port guns, and 4-port guns. On 2-port guns, the port is for liquid and the other port is for air pressure. In this gun, the actuation pressure for operating the gun and the atomizing pressure are controlled by one pressure, and therefore both cannot be controlled independently of each other. On three-port guns, there are 2 ports for air pressure and 1 port for liquid. In this gun, actuation pressure and atomization pressure can be controlled independently. On 3-port guns, the spray width is controlled by a screw provided on the gun which, when opened, allows air to flow through and increases the spray width. This can cause variations in spray width from batch to batch. On 4-port guns, there are 3 ports for air and 1 port for liquid. Therefore, the actuation pressure, atomization pressure, and spray width can be independently controlled by air pressure. This ensures uniformity of spray width from batch to batch.
What is Film Coating Process Spray Rate?
Controlling the spray rate is critical in aqueous film coating and if there is a variation in spray rate in the coating process, several problems are experienced. The spray rate is controlled primarily through the use of peristaltic pumps. Peristaltic pumps are the simplest and easiest to clean. The spray rate can be controlled by the rpm of the pumps. The spray rate also depends on the size of the tubing used in the pump, so depending on the desired spray rate, the appropriate tubing size should be selected.
The spray rate in a coating process can be easily controlled by keeping the coating suspension in equilibrium in order to calculate the spray rate per minute. It is very important to monitor at least the first 3-5 batches and derive the rpm from a peristaltic pump, which provides the required spray rate.
During film coating, the tray load should be optimal or maximum. If the load on the coating pan is partial, a large amount of intake air can go directly to the exhaust duct and a lot of energy is wasted. Due to the partial load, the exhaust plenum is not completely covered. This reduces the drying efficiency of the coating pan. The partial load process is more compared to full load. The mixing efficiency of the coating pan also decreases at part load and requires more pan rpm than is normally used. This is most critical in functional coatings such as enteric coating where edge wear or damage can result in product failure in the disintegration test.
To solve the problems associated with partial loading, many machine builders now use the multi-plate approach. Therefore, with a coating tray, different drum sizes are provided to accommodate various batch sizes.
Spray Gun Position and Configuration
In a coating pan where multiple guns are used, it is very important to set the guns correctly. Therefore, the distance between the gun and the gun and the distance between the gun and the bed is very important and must be set correctly to obtain the required spray width. If the guns are kept too close and the spray width is narrow uneven coating results. Requires more coating material to obtain a uniform coating. If the guns are too wide and the spray widths are wide, they overlap each other. In this situation, there is localized over-wetting in the spray overlap zone. Some spray also goes to the tray wall, door, and baffles. This also wastes coating material. They can be chopped and glued. In an ideal configuration, the spray pattern is on the surface of the tablet and also covers almost the entire bed of the tablet.
Gun validation is very important and should be done before you start spraying. It is important to configure a weapon so that there is minimal variation between weapons. At first the spray rate should be recorded rather than in terms of ml or weight for each gun and guns should be adjusted if there is more than 10% variation.
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During the film coating process, the container should be operated under negative pressure and the positive process should be avoided. Every machine builder provides a coating pan that has an exhaust more than 1.5 times that of the inlet air blower.
There is an adjustment for the gate opening that allows us to establish a negative pressure. The negative pressure should be kept between 10-25mm during the process.
If the coating pan is operated under high negative pressure, there is the possibility that some spray will enter the exhaust directly. Because of this, more coating material will be required to achieve uniformity. As we open the coating tray door several times in the process, due to the high negative, the air from the process room which is 25 ° C enters the coating tray and mixes with hot air (50- 55 ° C) and therefore reducing drying. coating pan efficiency. It is also very difficult to maintain the temperature of the bed.
If the coating pan is operated at positive pressure, drying is not performed and solvents are not effectively removed. When the coating tray door is opened multiple times, dust and hot air from the coating tray contaminates the air in the room, which is not allowed according to GMP. For these reasons, the coating pan should never be operated under positive pressure.
Process parameters
Below is the list of process parameters that should be monitored throughout the coating process.
- Inlet temperature
- Exhaust temperature
- Bed temperature
- Atomization air pressure
- Spray rate
- Gun to bed distance
- Gun to gun distance
- Negative pressure in the pan
- RPM of coating pan
Further Information
Ideal characteristics of film-coated tablets
- The film-coated tablets should have a uniform coverage with a uniform color coverage on the surface of each dosage unit within a batch and from batch to batch.
Film-coated tablets must be free from defects that affect the functionality and stability of the finished product. - Film-coated tablets must meet the specifications of the finished product and any relevant compendium requirements.
Types of film coatings:
Film coatings can be classified in various ways, but it is common practice to do so in terms of the drug delivery system. Therefore, film coatings can be classified as immediate release film coatings or modified release film coatings.
1-Immediate-release film coatings
Immediate release film coatings also known as non-functional or conventional film coatings are typically reserved for situations where it is necessary to improve product appearance, ease of ingestion, product stability, and taste masking. The coatings do not have a measurable effect on the biopharmaceutical properties of the coated dosage forms.
2- Modified-release film coatings
Modified release (or functional) film coatings are used when it is necessary to modify the release characteristics of the drug. Modified release film coatings can be further classified as delayed-release coatings (eg, enteric / gastro-resistant coating) or extended-release coatings.
