Film coating process:
Film coating process plays a very important role in film coating and number of problems in tablet coating is related with process setup. There are number of variables which should be controlled in a process and we will understand how these variables are crucial for film coating process. We will also study various kinds of equipments, which are available for film coating.
Basic requirements of film coating:
· Removal of solvent
First requirement of film coating process is that coating suspension should be evenly distributed throughout the tablet bed. Spray guns are most commonly used for distribution of coating suspension. There are two different types of spray gun are used. Airless spray guns & Air atomized spray guns. One of the problems with airless spray guns are that spray rate cannot be controlled independent of air pressure. Thus in aqueous film coating air atomized spray guns are used.
There are three different types of guns, which are used such as 2 port guns, 3 port guns & 4 port guns. In 2 port guns, on port is for liquid and other port is for air pressure. In this gun actuation pressure to operate gun & atomization pressure is controlled by one pressure and thus both cannot be controlled independent of each other. In three port guns there are 2 ports for air pressure & 1 port for liquid. In this gun actuation pressure and atomization pressure can be controlled independently. In 3 port guns, spray width is controlled by a screw provided on gun which when opened allows air to flow and increase spray width. This can lead to variation in spray width from batch to batch. In 4 port guns there are 3 ports for air and 1 port for liquid. Thus actuation pressure, atomization pressure and spray width all three can be independently controlled by air pressure. This ensures uniformity of spray width from batch to batch.
Film coating process-spray rate
Spray rate monitoring is critical in aqueous film coating and if there is variation of spray rate in coating process, number of problems is experienced. Spray rate is primarily controlled through the use of peristaltic pumps. Peristaltic pumps are the simplest & easiest to clean. Spray rate can be controlled by the rpm of the pumps. Spray rate also depends upon tubing size used in pump thus depending on spray rate desired, appropriate tubing size should be selected.
Spray rate in a coating process can be easily monitored by keeping coating suspension on balance so that spray rate per minute can be calculated. It is very important to monitor at least first 3-5 batches and derive rpm of peristaltic pump, which gives required spray rate.
During the film coating, pan load should be optimum or maximum. If coating pan load is partial, lot of inlet air can go straight into exhaust duct and lot of energy is wasted. Due to partial load exhaust plenum is not fully covered. This reduces drying efficiency of coating pan. The process ting with partial load is more compared to full load. Mixing efficiency of coating pan also decreases with partial load and it requires more pan rpm than normally used. This is more critical in functional coatings such as enteric coating where edge wear or damage can result into failure of product in disintegration test.
In order to solve the problems associated with partial load, many machine manufacturers now use multipan approach. Thus with one coating pan different drum sizes are given to accommodate various batch sizes.
Spray gun position and setup
In coating pan where several guns are used it is very important to setup guns properly. Thus gun to gun distance and gun to bed distance is very important & need to be set properly to get required spray width. If guns are too closed kept and spray width is narrow, it leads to non-uniform coating. It requires more coating material to get uniform coating. If guns are too wide and spray widths are wide, they overlap each other. In this situation there is localized over wetting in the spray overlapping zone. Some spray also goes to pan wall, door & baffles. This also wastes coating material. Picking & sticking can take place. In an ideal setup spray pattern meets at the tablet surface and also covers almost entire tablet bed.
Gun validation is very important and should be done before starting spray. It is important to setup gun so that there is minimum variation between guns. In the beginning spray rate rather in terms of ml or weight for each gun should be recorded and guns should be adjusted if there is more than 10% variation.
During film coating process coating pan should be operated in negative pressure and positive process should be avoided. Each machine manufacturer gives coating pan having exhaust more than 1.5 times than inlet air blower.
There is adjustment for damper opening which allows us to set negative pressure. Negative pressure should keep between 10-25 mm during process.
If coating pan is operated under high negative pressure, there are chances of some spray directly going into exhaust. Due to this more coating material will be required to get uniformity. As we open door of coating pan several times in process, due to high negative process room air which is at 25°C goes inside coating pan & mixes with hot air (50-55°C) and thus decreases drying efficiency of coating pan. It is also very difficult to maintain bed temperature.
If coating pan is operated under positive pressure, drying does not take place and solvents are not effectively removed. When door of coating pan is opened several times, dust & hot air from coating pan contaminates room air, which is not allowed as per GMP. Due to these reasons, coating pan should never be operated in positive pressure.
Following are the list of process parameters that needs to be monitored throughout coating process.
· Inlet temperature
· Exhaust temperature
· Bed temperature
· Atomization air pressure
· Spray rate
· Gun to bed distance
· Gun to gun distance
· Negative pressure in pan
· RPM of coating pan
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