A Comprehensive Review on Pharmaceutical Film Coating: Past, Present, and Future

Figures & data

Figure 1 Different FC techniques and processes (A) Conventional FC pan, (B) Fluid bed FC, (C) Phases of FC, (D) Phases of microencapsulation.

Table 1 Some of the Most Used Components in Functional and Non-Functional FC

Function	Material Name
Functional Film-Forming Polymer	Cellulose Acetate Phthalate
	Hydroxy Propyl Methyl Cellulose Phthalate
	Cellulose Acetate Trimellate
	Ethyl Cellulose
	Methacrylic Acid Copolymer
	Shellac
Non-Functional Film Forming Polymer	Hydroxy Propyl Methyl Cellulose
	Hydroxy Propyl Cellulose
	Polyvinyl Pyrrolidone
	Polyvinyl Alcohol
	High Molecular Weight Polyethylene Glycol
Solvent or Vehicle	Water, Ethanol, Methylene Chloride
Plasticizers	Propylene Glycol, Polyethylene Glycols, Diethyl Phthalate, Fractionated Coconut Oil, Castor Oil
Colourants	Water-soluble Dyes (FD&C Yellow 5) Water-insoluble (FD & C Yellow 5 Lake) Inorganic Pigments (Iron Oxide Titanium Dioxide) Natural Colourants (Beta Carotene)

Table 2 Summary of the Various Coating Defects, Their Causes, and Their Remedies

Defect	Definition	Cause	Remedy
Blistering	It occurs when film elasticity or adhesive properties are compromised and the film detaches from the tablet core.	High temperatures during the drying or at the end of the coating process. **	Use moderate temperature during drying stages.
Chipping and Edge Erosion	The film becomes dented and chipped especially on the edges of the tablet	The sharp edges on tablets. *** High core friability. *** A decrease in the rotation speed of the coating pan during the coating process**. Spray rate too low. ** A poor polymer or coating formulation (ie incorrect amount of plasticizer). * Suspension solids concentration too low. *	Change the tablet shape/design. Investigate core improvement. Fix the rotation speed of the coating pan. Increase the spray rate. Adjust the proportion of plasticizer in the coating solution or select a polymer with a higher molecular weight. Increase suspension solids concentration (if possible).
Cratering	It is a volcanic crater that appears on the tablet which in turn results in the exposure of the tablet’s surface.	Insufficient drying time to seal the film. ** A high volume of the coating solution is applied. **	Optimize the efficiency and time of the drying step.
Picking and sticking	It may occur when part of the film sticks to the coating pan or each other’s causing detachment of coat pieces from the core.	Spray rate too high and a drying air volume too low. ** Poor film adhesion. * Poor adhesion to the core. ***	Increase the airflow rate and temperature. Also, decrease the rate of the spraying coating liquid. Select a high adhesion film coating (ie low solids film coating formulation). Use high adhesion core excipients.
Pitting	It is a distortion of the core of the tablet without any visible sign of cracking of the film coat.	It may occur if the tablet core becomes hotter than the melting point of the materials used in its formulation. **,***	Optimize the temperature of the inlet air such that the temperature of the tablet core does not exceed the melting point of the and use additive or excipients with higher melting points.
Blooming	It is the dulling of the tablet colour after a long period of storage at a high temperature	It occurs due to the use of too much plasticizer or the use of a low molecular weight plasticizer. *	Decrease the amount and increase the molecular weight of the plasticizer in the coating formulation.
Blushing	It is the haziness or the appearance of white specks in the film.	Precipitation of the particles of the polymer in the film, due to excessive heat during coating. ** Gelation of the polymer caused by certain combinations with other excipients. *	Decrease the drying temperature. Do not use sorbitol with cellulosic polymers such as hydroxypropyl cellulose, hydroxy methylcellulose, methylcellulose, and cellulose ethers.
Mottling or Tablet to Tablet Colour Variation	Variation in the colour of tablets within a batch.	Poor tablet shape. *** Suspension solids concentration is too high. ** To poor mixing, uneven spray patterns of the machinery. *	Optimise the shape of cores. Decrease the concentration of solids in the suspension. Aim for even geometric mixing.
Orange Peel Roughness	The tablet has a rough surface, rather than a glossy texture which looks like an “Orange Peel”.	It may occur due to a core tablet erosion. *** It may also occur due to a too high spray pressure combined with a fast spray rate, which results in an uneven coating of the tablet.** Poor film coating formulation, (eg low film mechanical strength, poor adhesion to core, poorly plasticized. *	Improve tablet core: reduce hygroscopic ingredients, reduce friability, choose better core shape. Optimize the spraying pressure and rate. Select an optimized film coating.
Cracking (Splitting)	It is the splitting of the film coat especially on the crown area or splits around the edges.	The core and coating have different thermal expansion properties. *** Lack of relaxation time between tablet compression and coating. *** Poorly plasticized film coating. ** Expansion of core due to heating. *	Reduce mineral type fillers in the core formulation. Extend time between tableting and coating. Optimize the amount of plasticizer. Avoid high tablet bed temperatures Use lower molecular weight polymers or polymeric blends. Also, adjust the plasticiser type and concentration.
Twinning		Inappropriate tablet shape (flat surfaces). *** Pan speed too low. ** Atomization air pressure too low. ** Spray rate too high Spray guns too close to the bed. ** Insufficient drying. **	Choose a tablet design that eliminates flat surfaces. Increase pan speed. Increase atomization air pressure. Decrease spray rate Increase gun-to-bed distance. Increase drying (through higher inlet temperature or higher inlet airflow rate).
Logo Bridging	It is the disappearance of the tablet logo or bridging of monograms present on the surface of the tablet core.	Low adhesion core ingredients. *** Poor logo design. *** Spray rate too high. ** Product temperature too low. ** Atomization air pressure too low. ** Poorly plasticized film coating a low adhesion film coating. *	Use high adhesion core materials. Use logo design optimized for film coating. Decrease the spray rate. Increase product temperature. Increase atomization air pressure. Optimize adhesiveness and plasticization of the film coating.

Notes: * coating formulation faults, ** process and equipment faults, *** Tablet core faults.

Figure 2 Continuous FC process with PAT.