Tablet coating in pharmaceutical industry

Tablet coating in pharmaceutical industry is the final line of process in manufacturing tablet form of oral dosage. It gives the tablet its final finished properties necessary for its therapeutic, physical and marketing requirements. After coating, the tablets are packaged with the packaging material for distribution to their target markets

What is tablet coating in pharmaceutical industry?

Tablet coating in pharmaceutical industry  is the coating of tablets with the required liquid material, in a specialized equipment called Tablet coating equipment. It sprays the liquid on a bed of tablets in a rotating pan. As the liquid is sprayed, it resides on the tablet’s surface. Even distribution of coating material is ensured through the continuous movement of tablets with the effect of a rotating pan. It shuffles the entire batch of tablet upside and down during every rotation and ensures every tablet gets the sprayed liquid from every dimension.

The liquid to be coated is sprayed in an atomized form, creating uniformly sized drops in an organized pattern. The atomization properties of liquid play a critical role in the success of the coating process,  physical appearance, and product properties. It can damage the entire batch of tablets if atomization fails to produce a satisfactory spray pattern.

What are the operating parameters of tablet coating in pharmaceutical industry?

The equipment’s operating properties directly impact the tablet’s final properties. It is necessary to select optimized parameters for every product to achieve the desired properties.

Some parameters that directly impact the product include the following

Inlet Air Temperature

Inlet air temperature is the temperature of air that passes through the tablet bed from the inlet of coating equipment. Commonly, the temperature of inlet air is at a higher value, used to dry the tablets, when applied with the spraying liquid.

Determination of optimum inlet air temperature is necessary for an effective drying process and the final product. Otherwise, it will result in over-wetting or over-drying the tablet. As a result, tablet coating equipment will not produce tablets of the required quality, physical attributes, and therapeutic effect.

Pan Speed

Pan speed is the speed at which the pan of the coating equipment rotates. Measured in revolution per minute or hour, pan speed indicates the number of revolutions a coating pan completes during the tablet coating process.

Too little pan speed could result in over-spraying of the tablets, causing the tablets to stick with each other. It is also possible that tablets are not coated sufficiently.

If the pan speed is too high, it will move the tablets at high speed, causing them to jump and possibly come out of the pan. It will also result in abrasion, causing scratches on the tablets and damaging their physical appearance.

Commonly, the pan speed is directly proportional to the spray rate – the higher the spray rate, the higher the pan speed, and vice versa.

Spray Rate

Spray rate is the amount of liquid sprayed per unit time on a tablet bed through a spray gun, and it is the major factor in determining the coating process for a particular product.

The spray rate must be designed adequately, as lower spray results in insufficient coating and decreases weight. Higher spray rates results in tablet sticking with each other.

The spray rate also affects the bed temperature inside the coating pan, due to the liquid’s natural negative impact on high temperatures.

If the bed temperature is not compensated for the spray rate, the coating process will not be effective, leaving behind tablets having insufficient coating characteristics. Sot the bed temperature must be adjusted for every increase or decrease in the spray rate – a higher spray rate decreases the bed temperature and increases the bed temperature when the spray rate decreases.

Atomizing Air Pressure

Atomizing air is supplied to the spray gun that combines with the liquid to form spray drops. Increasing the atomization pressure results in fine drops while decreasing the atomization pressure increases the drop size.

If atomization pressure is too high, it will result in very fine particles that completely or partially evaporate due to inlet temperature. As a result, tablets will not get adequate liquid, and their surface will become rough and have low tablet weight.

Similarly, if the atomization pressure is too low, it will result in bulky drops from the spray gun, over-wetting the tablets, and tablets will stick to each other.

Atomization pressure is indicated in bar or psi.

Important components of equipment for tablet coating in pharmaceutical industry

Equipment for tablet coating in pharmaceutical industry consists of many sub-components designed to perform specific functions. These parts are critical and essential for an effective coating process.

Let’s discuss some important components and functions they perform

Peristaltic Pump

The peristaltic Pump is a positive displacement pump that transfers the liquid by compressing and decompressing it in compressible tubing. The Pump consists of shoes or a roller that compresses the compressible tubing, which in turn exerts pressure on the liquid inside the compressible tubing, causing the liquid to move forward.

The liquid is sucked in through the compressible tubing and under the effect of the Pump’s roller or shoes, liquid moves from one end to another end.

The main advantage of this Pump is that it provides a high degree of contamination control, as the product does not come in contact with any parts of the Pump only come in contact with the tube.

Additionally, the peristaltic Pump does not have mechanical parts such as valves, pistons or gears found in other types of Pumps. This further increases the contamination control and also makes it maintenance-free. A peristaltic pump transfers liquid ( to be sprayed) from a vessel or container to the spray gun. Through a peristaltic pump, we can increase or decrease the liquid pressure that affects the drop formation of the spray stream.

The peristaltic Pump can be controlled through the main controller, which gives the signal to the Pump for controlling the liquid flow, such as flow rate and start/stop the Pump. It can also be controlled through a local control panel on the Pump.

Spray gun

The spray gun is a mechanical assembly with a hole in the middle. It is used to spray liquid on tablets bed with various spray patterns. The spray pattern depends on atomization air pressure, spray rate and inlet temperature.

A peristaltic pump is used to supply liquid to the spray gun.

Coating Pan

A coating pan is a round metallic structure containing tablets to be coated. They are perforated at one side to allow air passage through the pan and tablets. The pan rotates in a fixed revolution,

moving the tablet inside. The rotation of the pan enables the tablets to be coated from every dimension, and air through the tablet bed helps in the timely drying of the sprayed liquid.

The inlet air, which is at a high temperature, passes through the pan, from the perforate section, and exits into the exhaust of the tablet coating pan.

The coating pan is fitted with mechanical vanes called baffles. It facilitates the coating process by ensuring the smooth tumbling of tablets without breakage or damage. It also facilitates smooth interaction between coating liquid and tablets.

Air inlet section

• The air inlet section is used to supply air to the coating pan. It consists of
•  Supply fan to supply air to the coating pan
• Filter to remove contaminants to make it clean the air, as this air will come in direct contact with the tablet

• Heating mechanism to increase the air temperature

The air inlet section is connected to the coating pan with duct sizes of suitable sizes.

Exhaust air section

 An exhaust air section is used to suck the air from the tablet coating bed. It creates a negative pressure inside the pan and ensures that the coating liquid is effectively pointed towards the tablet bed.

The exhaust air section is connected to the coating pan through ducts of suitable sizes. It consists of an exhaust fan and filter to collect any chemicals or traces of the product and prevent them from entering the environment.

Sampling Port 

The sampling port in tablet coating in pharmaceutical industry is a specialized opening from the coating pan that facilitates the operator in taking out small quantities of tablets for testing purposes. it helps the operator to collect tablets without stopping the coating machine.

Working principle of equipment for tablet coating in pharmaceutical industry

Let’s discuss how the coating is performed in a coating pan

• The coating pan rotates, and the tablets inside also move and tumble. Inlet air passes through the tablet bed and exits through the perforated section.
• A peristaltic pump is turned ON, which starts the liquid supply to the spray gun, and atomizing air is supplied to the spray gun.
• As the pan rotates and liquid is sprayed on hot inlet air facilitates quick evaporation of the solvent. This air exhausts from through the exhaust air system
• The operator continuously monitors the tablet quality collected through the sampling port.
• When the spraying liquid becomes empty, and the tablet has the adequate product specification, the coating pan is stopped, and tablets are taken out from the pan