In the part 3 of this module, we will learn the following on the tablets. In this module, we will study only important points for NIPER JEE. Kindly read the references given at the end, if you need more comprehensive information on any topic.
Equipments for tablet coating
1. Conventional coating pan
* Pellegrini pan system
* Immersion sword system
* Immersion tube system
2. Perforated pan system
* Accela-Cota system
* Hi-Coater system
* Glatt coater system
* Driacoated system
3. Fluidized bed coater
Important: Pan speeds of 10-15 RPM - non-aqueous film coating
Pan speeds of 3-10 RPM - aqueous film coating
Role of each step
Sealing/Water proofing: provides a moisture barrier and harden the tablet
Common materials used as sealants:
* Shellac (dissolution and disintegration time may increase over time due to polymerization - result in low bioavailability of tablets)
* Oleic acid
* Propylene glycol
* PEG 4000
* Cellulose acetate phthalate (CAP)
* Polyvinylacetate phthalate
Subcoating: round off the tablet edges and increase the tablet size.
This step involves the application of gum based solution followed by dusting with powder and then drying.
This step causes a 50-100% increase in the weight of tablets.
Solutions used for subcoating are gelatin, acacia, sugar, and corn syrup.
Subcoating/dusting powders are:
* Calcium carbonate
* Cane sugar
* Corn starch
* Calcium sulfate
Syrup coating (smoothing): smoothes out the subcoated surface. To give a tablet its colour.
Polishing: produces the characteristics gloss/luster.
* Carnauba wax
* White beewax
* Paraffin wax
* Naphtha as solvent for the wax
It is not used much now; have several disadvantages:
* The method relies totally on the skills of an operator
* It require other steps such as drying process to remove solvents
* Aqueous based coatings are not suitable for this method - overwetting of tablets
Are preferred method for coating due to increased efficiency and automation in the process.
Hydroxy Propyl Methyl Cellulose (HPMC)
It is a polymer of choice for air suspension and pan spray coating systems because of solubility characteristic in gastric fluid, organic and aqueous solvent system.
Disadvantage: when it is used alone, the polymer has tendency to bridge
or fill the debossed tablet surfaces. So mixture of HPMC and other polymers/ plasticizers is used.
Methyl Hydroxy Ethyl Cellulose (MHEC)
It is not frequently used as HPMC because soluble in fewer organic solvents.
Ethyl Cellulose (EC)
Insoluble in water and gastric fluids. Hence it is used in combination with water-soluble additives like HPMC and not alone.
Unplasticized ethyl cellulose films are brittle and require film modifiers to obtain an ideal film formulation.
Aquacoat is aqueous polymeric dispersion of ethyl cellulose manufactured by FMC Corporation, USA. It is used to achieve the modified release of the formulations.
Hydroxy Propyl Cellulose (HPC)
Soluble in water below 40 C (insoluble above 45 C), gastric fluid and many polar
* available in four viscosity grades i.e. K-15, K-30, K-60 and K-90.
* Average molecular weight of these grades is 10, 000, 40, 000, 160, 000 and
360, 000 respectively.
* K-30 is widely used as tablet binder and in tablet coating.
Polyethylene glycols (PEG)
Lower molecular weights PEG (200-600) are liquid at room temperature and are used as plasticizers. High molecular weights PEG (900-8000 series) are white, waxy solids at room temperature. Combination of PEG waxes with CAP gives films that are soluble
in gastric fluids.
* It is marketed under the name of Eudragit. Eudragit E is cationic co-polymer based on Poly(methyl)acrylates.
* Eudragit E is freely soluble and swellable in gastric fluid up to pH 5.
* Eudragit RL & RS are co-polymers with low content of quaternary ammonium groups.
* They produce films for delayed action (pH independent).
Ideal enteric polymer: should be soluble at PH 5 or above PH 5.
Cellulose acetate phthalate (CAP)
* Widely used
* Soluble only at PH above 6.
It is patented aqueous dispersion of CAP marketed by FMC Corporation, USA.
* Particle size of CAP in Aquateric
As organic solution (isopropanol), solid and aqueous dispersion
Available only as organic solution (isopropanol) and solid
Hydroxypropyl methylcellulose phthalate
Popular brand names: HP-50, HP-55, HP-55-S
It is soluble at lower PH (5 to 5.5) than other polymers.
To dissolve the polymers.
Ideal solvent is water.
Function: it change the physical properties of polymer.
* It changes the polymer-polymer interactions
* It optimizes the flexibility, tensile strength, and adhesion properties of the film.
Commonly used plasticizers:
* Castor oil
* Propylene Glycol (PG)
* Lower molecular weight (200-400 series) PEG
* Surfactants, etc.
For aqueous coating PEG and PG are more used while castor oil and spans used for organic-solvent based coating solution.
Concentration: (light shade)-0.01%- (dark shade )2%
Commonly used Colorants
* Lakes-most preferred
* Iron oxides
* Carminic acid
Other special coloring agents; which doesn't require milling equipments
Opalux: for sugar coating
Opaspray: for film coating
Opadry: film coating concentrate
They increase the film coverage of colors; thereby they decrease the amount of expensive colorants needed for the film coating.
Titanium dioxide (most commonly used)
Sticking and picking
It is defect where isolated areas of film are pulled away from the surface
when the tablet sticks together or to the coating pan (due to overwetting) resulting an exposed area of core of tablet.
* Increase the inlet air temperature
* Decrease the rates of application of coating solution
Definition: roughness of film coating. The polymer is dried before reaching the base of coating pan.
Reducing the degree of atomization
Moving the atomizer deep into the coating pan
Orange peel effect
Definition: It is surface defect resulting in the film being rough and nonglossy.
Appearance is similar to that of an orange.
* Use mild drying conditions
* Use additional solvents to decrease viscosity of solution.
Bridging and filing
Definition: the film shrink and pull away from the bisects of tablet
Increasing the concentration of plasticizer or change the plasticizer.
Definition: too rapid drying of solvents in polymer
* Use mild drying conditions
Definition: coating becomes dull immediately or after prolonged storage at high temperatures
Also called blooming.
Decrease plasticizer concentration and increase molecular weight of plasticizer
Definition: A defect which involves variation in colour of the film
Proper mixing, reformulation with different plasticizers and additives or use
mild drying conditions
Definition: It is defect in which the film either cracks across the crown of the tablet
(cracking) or splits around the edges of the tablet (Splitting)
Adjusting the plasticizer type and concentration
References and further reading
Banker, G. S., & Anderson, N. R. (1991). The theory and practice of industrial pharmacy; Lachman, L. 3rd edition.
Remington, J. P. (2006). Remington: The science and practice of pharmacy (Vol. 1). D. B. Troy, & P. Beringer (Eds.). Lippincott Williams & Wilkins.