Film coating is a critical layer on tablets. Most tablet manufacturers are aware of the tremendous responsibility of the coating solution for tablets. However, the scientific complexity behind them is often overlooked.
If you are a tablet manufacturer, to make better product decisions, it is essential to understand the science behind the pharmaceutical tablet coating.
The science behind film coating on tablets
Film coating works on a simple scientific principle: applying a thin and even layer of polymer-based liquid onto the surface of a tablet. But within this simple-looking science lies a series of well-controlled and well-thought-out physical and chemical processes.
Polymer science:
Polymer forms the base of the coating material of tablets. The choice of your polymer depends on the desired function of the coating, for example:
- For immediate release – hydrophilic polymers
- For sustained release – hydrophobic polymers
- For enteric release – pH-dependent polymers
Besides the type of polymer, even the properties of the coating material of the tablet, such as molecular weight, affect the film strength, permeability, and dissolution.
Components of tablet coating material ( additives):
Besides polymers, other components of tablet coating material affect the appearance, function, and performance of the coating. These include
- Plasticizers
- Colorants/Pigments
- Solvents
- Opacifiers
- Anti-tacking agents
Read More: Why Excipients Matter in Pharmaceuticals.
Tablet film coating is also heavily influenced by the mechanism behind the coating. Let us understand that in detail.
Film coating mechanism
When we spray a coating solution onto a tablet, a film is formed on the tablet’s surface. But it doesn’t appear instantly. The steps involved in the tablet film coating mechanism are:
- Spraying
- Compaction and deformation
- Coalescence
- Film Formation
Let us understand this in detail.
Spray atomization:
The coating solution is sprayed onto the tablets that are being rotated simultaneously in a typical coating setup, such as a pan fluid bed, using a nozzle system. As the coating spread, the polymer molecules start sticking to each other because of cohesive forces. This ensures an even distribution of coating across the tablets.
Compaction and deformation:
The water or solvent from the coating evaporates, and the viscosity of the solution increases. Polymer chains move closer, increasing the adhesion. This is the compaction and deformation stage, ie, the stage where particles rearrange and pack tightly together.
Coalescence
As spraying continues, new droplets keep landing on top of older layers. The polymer chains with good cohesive properties fuse (coalescence). This leads to the formation of a continuous, uniform coating solution for tablets.
Film formation:
Once most of the water is evaporated, the polymer chains lock together, forming a strong and continuous film coating on tablets.
Read More: Everything You Need to Know About Thin Film Coating Materials
Key principles behind the tablet coating mechanism:
- Cohesion: Polymer molecules must stick well to each other.
- Diffusion: They need to move slightly to merge into a smooth layer.
- Coalescence: Layers must fuse to form one continuous film.
- Drying & evaporation kinetics: Evaporation must be fast enough to form the film but slow enough to allow polymer coalescence. Hence, controlled heat and airflow are used during the process.
- Heat: Heat plays an essential role in drying, improving the mobility of the polymer chains, and boosting the coalescence process.
- Plasticizer content: The amount of plasticizer affects flexibility and coalescence.
Even minor flaws in any of these key principles can lead to coating failures. Every step in the tablet coating systems, from formulation to spray technique, affects the performance of tablets.
Read: Common Challenges in Tablet Coating and How to Overcome Them
Conclusion:
Pharmaceutical tablet coating is a functional science. It is a balance of material science, thermodynamics, surface chemistry, and process control.
At Novo Excipients, we are committed to delivering advanced excipient and tablet coating solutions that meet the evolving needs of the global pharma industry.
For more information on our film coating solutions, please contact our team of experts.