Coating Parameters influence on tablets

Tablet Coating is a critical process and the success of the coating operation depends upon the operator’s skills to maintain the parameters of the tablet coating machine according to requirements as mentioned in the batch manufacturing record.

Manual coating pans require great care, the operator’s skills are more important for controlling coating operation in case of manual coating pan.

In automatic coating pans, the variation of results can easily be seen by the operator on the HMI of the tablet coater so can easily be adjusted by the operator.

For a smooth coating operation, the operator should be fully trained and aware of his role in controlling parameters of the coating machine as and when required according to variations.

Coating Machines

In pharmaceutical industries, many types of coating machines are used for coating tablets.

Conventional Coating Pans

The conventional type of tablet coaters are very simple and are operated manually. All coating parameters are controlled manually in this coater. Conventional coating pans are very simple and consist of a pan mounted on an axis. Heat is supplied by heating blowers and the air is exhausted by an exhaust duct mounted on the upper side of coating pans.

The coating solution is sprayed over the tablet bed by conventional spray guns fitted on a stand and are mounted over the coating pan. These type of coating pans produce a dusty environment in the coating area because it is not a close system. In conventional pans, heat is applied from the top head of the coater and exhaust air is also removed from the top head side of the coating pan.

Advance Coating Pans

Currently, Different types of tablet coaters are available with advanced heating and exhaust mechanisms and settings. Most commonly used coating pans are perforated coating pans mounted in a closed chamber. Perforated coating pan has many perforations which allow passage of hot air through the tablet bed and then the air is exhausted from the other side of the tablet coater.

In contrast to the manual or conventional coating pans where heat is supplied from the upper side, in perforated pans heat is applied from one side of the perforated pan and is removed from the other side, so the heating mechanism is more uniform. The coating solution is sprayed by advance type of spray guns which have a self-cleaning mechanism to prevent blockage. As it is a closed system so dust is not generated in the coating area.

Coating Parameters

There are different parameters which may affect the coating process. These parameters do not remain constant during all coating process and remain changing from time to time so great care is required to observe the coating parameters during all the time of the coating process.

If variation occurs in coating parameters they should be adjusted by the operator.

Following are critical coating parameters which should be observed during the coating operation,

Inlet air temperature
Outlet air temperature
Outlet butterfly valve /Under Pressure
Humidity Level
Tablet bed temperature
Pan Speed
Spray rate
Atomizing air pressure
Gun to gun distance
Tablet to gun distance

Inlet air temperature

Inlet air is supplied to the coating pan from outside. The environmental air is processed before supplying to the coating pan, so a processing unit is installed outside the coating area to process the air.

The processing unit consists of pre-filters and bag filters to filter the air. As the air comes in direct contact with product, HEPA filters are installed in the processing unit. The processed air is heated by a heating mechanism either by heating rods or by steam system.

Inlet air temperature plays an important role during coating operation because it evaporates the solvent and keeps tablets dry. Inlet air temperature is maintained in such a way that tablets should not become over wet or over dry. Too much heat evaporates the solvent before reaching the tablet surface and results in orange peel effect, logo filling, rough surface or reduction in weight gain of the tablet.

The low temperature will result in sticking, picking or twinning of tablets. Inlet air temperature should be observed critically during the coating operation.

Outlet Air Temperature

Outlet air temperature is also an important factor of the tablet coating operation. Outlet air temperature is less than 10-20 degree centigrade from inlet air temperature because of the heat exchange mechanism. Outside or return air should be filtered before it’s removal to the environment.

Outlet butterfly valve /Under Pressure

The return duct is attached with a valve called a butterfly valve. This valve controls the rate of air, discharged from the coating pan to the outside environment.

If valve opening is increased more air, flows outside and suction inside the coating pan is increased mean more negative pressure inside coating pan is produced which facilitate removal of dust and vapours from coater at a higher rate and if the opening of the valve is reduced, the air is removed at a reduced rate and suction inside the coating pan is also decreased mean inside become less negative and removal of dust and vapours become less.

Due to less removal of vapours from the coating pan, the humidity level inside the coating pan is increased. When the suction inside the coating pan is increased or more negative pressure is produced it pulls some coating solution to outside so it should be optimum.

Humidity Level

The humidity inside the coating chamber also affects the coating operation. When the humidity level inside the coater is increased tablet bed temperature is greatly affected due to gun to bed distance variations. When humidity level inside the coater is increased, tablet surface may become rough and friction between moving bed is increased so bed to gun distance is decreased, tablet bed temperature is also decreased and tablets become over wet so sticking picking and twining may occur due to increased humidity level inside the coating pan.

To remove humidity from the environmental air, a dehumidification system should be installed in an air processing unit. Inside coating pan humidity may increase when the exhaust outlet valve is not opened properly and tablet bed temperature is very high because it creates a humid environment inside the coater due to slow removal of vapours from the coating pan.

Tablet Bed Temperature

Tablet bed temperature is the product temperature means how hot the rotating tablets are. It is an important factor of coating operation because the tablet bed temperature indicates to the operators that how and when they have to increase the inlet air temperature.

Tablet bed temperature is measured by IR temperature gun or in advance coating pans a temperature sensor probe is installed inside the coating pan that gives product temperature on HMI. Tablet bed temperature depends on the type of solvent used. In the case of Aqueous coating, the tablet bed is kept high between 38-44 degree centigrade to evaporate the water. In case of Hydro-alcoholic Solvent Tablet bed temperature is kept 35-37 degree centigrade and in case of organic solvent, tablet bed temperature is kept 32-34 degree centigrade.

If the tablet bed temperature is dropped below the range, inlet air temperature is increased to bring tablet bed temperature in range and if tablet bed temp exceeds the limit then the inlet temperature is decreased to bring it back within range.

High tablet bed temperature results in rapid evaporation of solvent and loss of coating material which results in less weight gain of tablet and decreased coating efficiency.Low tablet bed temperature results in sticking, picking or twinning during the coating operation.

Spray Rate

Spray rate is the speed at which coating solution is applied to the moving tablet bed. It plays an important role in the coating operation. Spray rate is controlled by the peristaltic pump attached to the coating pan and the spray rate can be increased or decreased by changing the rpm of the peristaltic pump.

High spray rate results in sticking picking and twining and low spray rate results in a reduction in weight gain so spray rate and inlet temperature should be optimum. Spray rate, inlet air temperature and table bed temperature are interrelated with each other.

High spray rate results in reduction of tablet bed temperature so to increase the bed temperature inlet air temperature of the coater should be increased and if the spray rate is reduced, tablet bed temperature becomes high so to decrease the bed temperature inlet air temperature should be reduced.

Coating Suspension Viscosity

Viscosity Of coating solution or suspension also plays an important role in the coating operation. If the solid content in solution or suspension is high the viscosity of solution or suspension may become too high and it results in a viscous or thick solution.

Thick or viscous solution effects spray rate and atomization. The viscous solution may block the coating guns due to high solid content or proper flow is not achieved, so the solution is not showered properly over tablets.

The viscosity of coating solution or suspension should be such that it can pass through coating guns easily and smoothly without blocking spray guns.

Pan Speed

The speed at which tablets are rotated in a coating pan during coating operation also affects the coating process. High pan speed may cause attrition which breaks the tablet edges and tablet surface may become rough. Low pan speed may hinder proper tablet movement so pan speed should be optimum.

Pan speed is correlated with spray rate. If the spray rate is high, pan speed should also be high for proper rotation of tablets under spray guns. If the spray rate is high and pan speed is low, tablets may become over wet and result in sticking, picking and twining.

If the spray rate is low and pan speed is high, the spray is not properly sprinkled on each tablet and the proper coating material is not deposited on the tablet surface.

Atomizing air pressure

Atomizing air pressure determines the size of droplets of the coating solution. If atomizing air pressure is high fine droplets are produced which dry before reaching tablet surface and result in rough tablet surface or low weight gain of coating material over the tablet surface.

If atomizing air pressure is low the droplet size is large and it may result in over-wetting of tablets or sticking, picking and twining. So atomizing air pressure should remain optimum during the coating operation.

Gun to gun distance

Spray guns deliver the coating solution in the form of fine mist over the tablet surface.

Gun to gun distance is the distance between coating guns installed on a rod attached with a coating pan. Gun to gun distance should be such that it should not create any defect during the coating operation.

If gun to gun distance is less, the spray shower from one gun overlaps with the spray shower of another gun and the place where the overlapping occurs may become over wet. This over wetting results in sticking, picking or twining. If gun to gun distance is high, space or a gap is left between two showers and at that place spray is not showered properly over tablets so tablets at that area may become dry and result in a reduction in weight gain.

So gun to gun distance should be adjusted in such a way that the spray pattern should not overlap and no gap should be present between spray showers.

Spray Gun to Tablet bed distance

Spray gun to tablet bed distance is the distance between coating guns and moving bed of tablets. Spray gun to tablet bed distance is important for coating operation because as the distance increases less spray is applied over the tablet surface because of evaporation of the solvent and if the distance between spray guns and tablet bed becomes less, tablets become over wet resulting in sticking, picking or twining.

Summery

Tablet coating process is greatly affected by coating parameters and all the parameters are interrelated with each other. Coating defects like sticking, picking, rough surface, logo filling or twining may occur if any parameter during coating operation is ignored or not monitored properly.