Tablet coating is a very critical process performed in pharmaceutical industries & it is the process where a thin coat of polymer and other excipients is applied over the tablets.
To apply the coat over the tablet surface we prepare the suspension of polymer and other required excipients in a suitable solvent either water or organic solvent.
To maintain the product quality and to ensure good coat over tablet there must be a critical control of different parameters & heating or temperature control is one of the most important tablet coating parameters.
As we are familiar with, the basic principle of tablet coating is to evaporate the solvent, for evaporation heat or temperature is the most important parameter.
Imagine a situation of tablet coating without heat meaning we simply spray the coating suspension over the tablet but do not apply heat or temperature so what will happen?
It is obvious that without heat the solvent will not be evaporated and it will result in sticking, picking and ultimately the dissolution of tablets in the coating pan and our batch will be destroyed.
In this article, we will discuss the following
- How Heat Is Applied?
- Heating Methods?
- Role Of Temperature For Tablet Coating
- Practical Case Study
How Heat Is Applied?
To apply heat over the tablets, the tablet coating pan is designed in such a way that it contains perforations and we supply hot air from outside to inside the tablet coating pan.This hot air results in evaporation of solvent and solid contents are deposited over the tablet surface.
Heating Methods
There are two methods which can be utilized to heat the air and one is direct heating method and other is the indirect heating method.
Direct Heating Method
In a direct heating method the electric heaters are installed in the air handling unit of the tablet coating pan.The air after filtration passes through the electric heaters and becomes hot.The temperature of heaters is controlled from the HMI of the tablet coating pan.
Indirect Heating Method
When we talk about indirect heating methods then it means that instead of electric heaters we have heating coils installed in the air handling unit.These coils are heated with the steam and when air passes through these coils it becomes hot.
Role Of Temperature For Tablet Coating
As we discussed previously without heat the evaporation of solvent will not be possible so now we will learn how temperature plays its role during the process of tablet coating and it must be controlled & monitored to ensure good product quality without any coating defects.
We categorise the temperature during tablet coating into three main types,
- Inlet Temperature
- Outlet Temperature
- Bed Temperature
Inlet temperature means the temperature of air which is coming inside the tablet coating pan from outside and outlet temperature means the temperature of air which leaves the tablet coater after evaporation.The bed temperature is the temperature of tablet surface.
Temperature Relation
Now have a look what is relation between inlet air temperature, outlet air temperature and tablet bed temperature. The key controlling parameter is the inlet air temperature and the other two are directly related to the inlet air temperature means if the inlet air temperature is decreased there will be a drop in outlet air temperature and tablet bed temperature as well. So the controlling and monitoring of inlet air temperature is very critical as if there is an issue with inlet air temperature various tablet coating defects will be observed.
Practical Example
Now we are familiar that for tablet coating process the heating is very necessary to evaporate to the liquid content of the coating suspension.For heating, the atmospheric air is processed and is heated by direct or in direct method as discussed above.The hot air is supplied to the tablet coating pan where it not only evaporate the liquid content but also keep the surface of tablets hot during tablet coating process.
In simple words when the surface of a tablet receives a coating solution it is immediately dried to keep the tablet intact.
The amount of required heat varies depending on the type of solvent used in the tablet coating suspension.For aqueous solution we need more heat and more organic solvent we need less heat.
Suppose we are dealing with a film coating where our suspension contains an aqueous solution e.g water.
Imagine to evaporate water and to keep our tablet dry we maintain about 38°C to 44 °C degrees of tablet bed temperature.
Suppose we set the temperature of inlet air at 70°C and when this hot air comes in the coating pan it keeps dying our tablets and maintains its bed temperature at 40°C.
We are familiar that evaporation results in a cooling effect so when this hot air leaves the tablet coating pan its temperature is decreased due to evaporation & heat exchange mechanism & it is about 15 to 20°C less than the temperature of incoming air.
If we maintain all above-mentioned parameters our process will be very smooth but imagine what will happen if we reduce the temperature of inlet air from 70 °C to 50 °C?
Due to decreased incoming air temperature the temperature of tablets surface will also decrease and it will be indicated by reduced outlet air temperature as well.
Now what will happen to product quality?
The first defect which will appear due to less heating temperature will be the sticking and afterwards picking will be observed.The ultimate reason of these defects is that due to less heat the tablets are not dried properly after getting spray coat of coating suspension and starts to stick with each other or with coating pan.
Now the solution is to increase the temperature of incoming air but what will happen if we increase the temperature more than recommended value.
Case Study 2
Suppose we increase the temperature of incoming air from 70°C to 90°C.
More heat will be supplied inside the tablet coating pan which will raise the tablet bed temperature as well.
What will happen due to excessive heat?
The tablet bed temperature will be very high suppose it reaches to 50°C and drying of tablet and evaporation of solvent will be rapid so solvent will be evaporated before reaching to tablets surface & it will result in loss of solid contents instead of depositing over the tablet surface.
Due to loss of solid contents,the required film will not be formed over tablet surface & If the tablet film will not be formed over the tablet surface then a roughness of the tablet surface will be observed due to rubbing with the tablet coating pan.
The desired weight gain will not be achieved and colour variation will be observed in the case of colour tablets.
Conclusion
The strict supervision and critical control of heating parameters is mandatory during the process of tablet coating because overheating and under heating both will result in tablet coating defects and product quality will also be affected.
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