Film Coating Process of Tablet in Pharmaceutical Manufacturing

Learn about film coating process, difference between Aqueous and Solvent (non-aqueous) film coating and parameters.

Film coating Involves spraying a solution of polymer + pigments + plasticizers on to a rotated, mixed tablet bed forms a thin, uniform film on tablet surface.

Tablet film coating is performed two types, one is aqueous film coating (generally water is used as a solvent) and non-aqueous film coating (generally organic solvents are used.) Some problems are associated with the non-aqueous film coating like safety of employees (as most of the solvents dangerous smells, and they are not good to breathe) atmosphere pollution etc. But key problem is with the approval of the regulatory authority. High quality aqueous film coating must be smooth, uniform and adhere satisfactorily to the tablet surface and ensure chemical stability of a drug.
Film coating

Minimal weight gain (2-3% as opposed to 60-80% for sugar coating)

Single stage process an so generally quicker

Coat is less likely affect disintegration

Expensive equipment & plant requires large space

Easy to automate (less reliance on skilled operator / easier to meet GMP requirements of SOPs, validation etc.)

High installation & energy costs.

Aqueous vs Solvent Film Coating

Aqueous Film Coating

Aqueous based process are now the most common

-No safety issues (e.g. can be allow mobile vessels)

-can release to atmosphere

Take longer and may lead to mechanical damages as tablets are tumbled for longer

Requires more efficient drying air plant

Advance in drying efficiency has allowed aqueous processes to be developed

Even for moisture sensitive products

Solvent Film Coating

-Solvent based coating solution / suspension (e.g. alcohols, methylene chloride)

—Rapid drying time due to inherent volatility

—Can be applied to moisture sensitive products

Operator safety issues

Requires modification to equipment and facility (flame-proofing / intrinsically safe)

Environmentally responsible disposal is expensive (solvent recovery)

Can impart taste / smell to the product

Common component of film coating 

Solvents

1. Water

2. Solvents – Alcohols (e.g. ethanol, isopropanol)

Esters- (e.g. ethyl acetate, ethyl lactate)

Chlorinated hydrocarbons (e.g. methylene chloride, 1:1:! trichloroethane)

Polymers / Film Formers

Form a clear, non-tacky, mechanically strong film

Must be soluble in the chosen solvent and also allow release of drug in the body

Must be compatible with the core contents most common example is Hydroxy propyl methyl cellulose (HPMC)

Plasticisers

Modify the properties of the polymer to assist the coating process (e.g. by reducing brittleness of the coat)

Need to be miscible with the polymer e.g. propylene glycol, polyethylene glycol (PEG)

Colourants

Water insoluble pigments or water soluble dyes e.g. yellow or red iron oxide, aluminium lake dyes

Opacifiers

Provide a “hiding” function preventing the coat from being see through

Used to mask core colour and / or provide light protection e.g. titanium dioxide

Film coating Process

The coater drum rotates and the baffles in the drum mix the tablets

Warm air is drawn through the bed as it enters (inlet air) and exit (outlet air) the drum through perforations

The distance from the spray guns to the tablet bed is measured and the spray adjusted to achieve and even (non overlapping) coverage across 75% of the bed

The coating suspension is pumped through the spray guns with compressed (atomising) air which forms a spray of fine droplets

As suspension droplets hit the tablet surface they should spread into a film before the solvent is rapidly removed by the throughput air

The aim is gradually buildup the coat as the tablets pass beneath the spray guns.

Parameters in Film Coating Tablet Process

Coating at any scale involves the understanding of the interactions between the product being coated, the film coating formulation, the equipment installation and the processing parameters. When a film coating process is transferred and scaled from the laboratory to production equipment many factors must be well thought-out.

The variable inputs to a film coating process derived from differences in equipment installations include but are not limited to:

1. air volume
2. inlet air temperature
3. humidity control
4. nozzle orifice size
5. numbers of spray guns
6. the distance from spray gun to spray gun
7. the distance of the spray guns to the tablets
8. Spray rate
9. Atomizing air pressure
10. the coating pan dimensions
11. Rotating speed of pan