The coating is a process in pharmaceutical industries in which a layer of polymer or film former is applied to the tablets. The coating is an old technique derived from the confectionery industries.
Purpose of coating
Tablets are coated for several reasons, some are as follows,
Environmental Protection
To protect the tablet from environmental factors like temperature, humidity, light, and oxidation, a layer of polymer is applied over the tablet.
Core tablets may take moisture from the environment or can be degraded by the heat if storage conditions are not appropriate so a layer of polymer is applied to protect the tablet from heat and also act as a moisture barrier.
To mask the taste
Some active pharmaceutical ingredients are very bitter and are difficult to swallow, because of bitterness so a layer of polymer is applied on the tablet which masks the taste by forming a layer around the tablet.
Product identification
Color coating is also used as an identification tool to differentiate different strengths of the same product. It helps in the identification of specific products and prevents the chances of a mix-up.
Dusting
Tablets are also coated to prevent dusting during blistering and to prevent cross-contamination. The coating also reduces friction during the blistering of tablets.
Ease in swallowing
The coating makes the product more palatable and easy to swallow.
Protection from Gastric acid
Some products are degraded by stomach acid or cause gastric irritation so they are covered by an acid-resistant polymer that protects the drug from acid and releases the drug in the small intestine, for example, enteric coating.
Drug Profile
some coating materials are used to change the release of drug profile e.g. sustained release.
Types of Coating
There are three types of tablet coating
- Film coating
- Enteric Coating
- Sugar Coating
Film Coating
In film coating, a thin layer of polymer is applied over the tablet and usually weight gain is 1-5% depending upon the requirements. The coating process of the film coating is a short time process.
Enteric Coating
Enteric coating polymer protects the drug from stomach acid and releases it in the small intestine. Weight gain of polymer is 10- 15% depending upon the formulation. The enteric coating process takes more time as compared to film coating.
Sugar Coating
It is a conventional type of coating and a layer of sugar is applied over tablets. In sugar coating weight gain is 50_80%. It is a multi-stage process and requires a lot of time which means that it is a lengthy process.
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Classification of coating on bases of Polymer.
The coating is divided into two types on the basis of the polymer used.
- Functional Coating
- Non-Functional Coating
Functional Coating
The functional coating is a type of coating in which the polymer alters the drug release profile.
for example
Enteric Coating
Modified Release coating
Non Functional Coating
It is the type of coating in which the polymer used does not alter the drug release profile. The polymer used only gives environmental protection or taste-masking properties.
Classification of coating on the basis of Solvent
On the bases of the solvent used in the coating process, the tablet coating is divided into three types
- Aqueous Coating
- Organic Solvent Coating
- Hydroalcoholic Solvent Coating
Aqueous Coating
In aqueous coating the solvent used is water. It is safe for both the operator and the environment. All the regulatory guidelines recommend, the use of water as a solvent in coating for environmental protection and operator safety.
Evaporation of aqueous solvents requires more heat. It is economical because water is used as a solvent.
Organic Solvent Coating
In organic solvent coating, organic solvents are used for example IPA, Rectified spirit, methylene chloride, etc. Evaporation of organic solvent requires less heat.
Organic solvents are costly so the cost of this process is increased.
Aqueous+organic Solvent Coating
In the Aqueous+organic coating, the solvent used is a mixture of water and organic solvent. Usually, 90% organic solvent and 10% water is used.
Principle of Coating
A solution or suspension of polymer or film former is sprayed over the moving bed of hot tablets by atomization, the solvent is evaporated by the heat leaving behind the thin layer of polymer or film former over the tablet.
This is a cyclic process and the cycle is repeated until the desired level of polymer film thickness over the tablet surface is achieved.
General Coating Process
The steps involved in the coating of tablets in the pharmaceutical industry are as follows,
- Line Clearance/Batch Identification
- Preparation of Coating Solution
- Tablets loading into a Coating pan
- Tablet Warming
- Application of Coating Solution
- Monitoring of Weight gain
- Tablet Drying
- Temperature Normalization/Cooling
- Unloading of Tablets
Line Clearance/Batch Identification
Line clearance is a process of assurance that there are no leftovers or traces of previous products in an area where the coating process is going to be performed.
In this step, the area is inspected by Production officer and IPQC officer and then the product or batch for coating is checked according to the batch manufacturing record and after assurance of all parameters, it is allowed to start the coating process.
Preparation of Coating Solution/Suspension
The coating solution is prepared according to the batch manufacturing record using a homogenizer or propeller mixer.
Tablets loading into a Coating pan
Tablets from containers are loaded into the coating pan and are de-dusted. For de-dusting pan is operated at low rpm for 5-10 minutes and the exhaust is fully opened.No heat is applied in this step.
Tablet Warming
After de-dusting of tablets, meaning removal of powder from the table, the tablets are warmed at a specific temperature depending on the nature of the product and solvent used.
IR Temperature gun is used to check the tablet bed temperature, some advanced coaters are equipped with product temperature sensors.
Application of Coating Solution
When tablets are warmed at the required temperature, the process of spray is started, the solution is delivered by a peristaltic pump and is atomized by air pressure through tiny holes of spray guns.
This atomized solution is applied over the moving bed of hot tablets. The solvent is evaporated and the solid content or polymer in the solution is begin to deposit on tablets.
Monitoring of Weight gain
During the process of spray the deposit weight gain on tablets is randomly observed and if any fluctuation is seen in weight gain the coating parameters are changed to ensure proper weight gain.
Tablet Drying
At the end of the spraying, tablets are dried for 5-10 minutes before removing from the coater.
Temperature Normalization/Cooling
After drying, cool the tablets for 10-15 minutes to normalize the temperature.
Unloading of Tablets
When the temperature is normalized, all the tablets are removed from the coater in containers by using an automatic discharging mechanism or by manual method.
The yield is calculated according to the batch manufacturing record and the batch is stored in the staging area.
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