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COATING TECHNOLOGY AND ITS RECENT ADVANCES – A REVIEW


Tablet coating defect: (1-30)

Sr. No.

Tablet Defect

Definition

Reason

Remedies

1

Blistering

Local detachment of film from the substrate forming blister.

Entrapment of gases in or underneath the film due to overheating either during spraying or at the end of the coating run

Use mild drying condition

2

Chipping

The film becomes chipped and dented, usually at the edges of the tablet

Decrease in fluidizing air or speed of rotation of the drum in pan coating

Not to over dry the tablet in preheating stage.

3

Cratering

Volcanic-like craters appears exposing the tablet surface

The coating solution penetrates the surface of the tablet, often at the crown where the surface is more porous, causing localized disintegration of the core and disruption of the coating.

Use efficient and optimum drying conditions.

Increase viscosity of coating solution to decrease spray application rate

4

Picking

Isolated areas of film are pulled away from the surface when the tablet sticks together and then part

Over wet tablet bed where adjacent tablets can stick together and then break apart.

Use optimum and efficient drying conditions or increase the inlet air temperature

5

Pitting

Pits occur in the surface of a tablet core without any visible disruption of the film coating

Temperature of the tablet core is greater than the melting point of the materials used in the tablet formulation.

Modifying the drying (inlet air) temperature such that the temperature of the tablet core is not greater than the melting point of the batch of additives used.


6

Blooming

coating becomes dull immediately or after prolonged storage at high temperatures

It is due to collection on the surface of low molecular weight ingredients included in the coating formulation

Decrease plasticizer concentration and increase molecular weight of plasticizer.

7

Blushing

whitish specks or haziness in the film

It is thought to be due to precipitated polymer exacerbated by the use of high coating temperature at or above the thermal gelation temperature of the polymers.

Decrease the drying air temperature

Avoid use of sorbitol with Hydroxy Propyl Cellulose, Hydroxy Propyl Methyl Cellulose, Methyl Cellulose and Cellulose ethers

8

Twinning

Two tablets stick together

Common problem with capsule shaped tablets

By balancing the pan speed and spray rate.

9

Colour variation

Variation in the colour of the film

Alteration of the frequency and duration of appearance of tablets in the spray zone or the size/shape of the spray zone

Geometric mixing, reformulation with different plasticizers and additives or used mild drying conditions

10

Orange peel effect

Film becomes rough and non glossy. Appearance is similar to that of orange

Inadequate spreading of the coating solution before drying

Use mild drying conditions.

Use additional solvents to decrease viscosity of solution

11

Mottling

Uneven distribution of colour on the surface of tablet with dark and light patches.

Due to different colouration of the excipient or the degradation product of the tablet is coloured.

Coating solution is prepare properly in sufficient quantity.

12

Cracking or Splitting

Film either cracks across the crown of the tablet(cracking), or splits around the edges of the tablet (splitting).

Internal stress in the film exceeds tensile strength of the film.

Use of high molecular weight polymer increase the tensile strength of the film.

EVALUATION PARAMETER FOR COATED TABLETS: (1-4)
1. Water vapour permeability:
If the coating is used as seal coat or to provide some physical protection for a tablet containing a water unstable drug, then the water vapour permeability test should be carried out.

2. Film tensile strength: This test is carried out when the effect of varying concentration of plasticizers or additives is being evaluated. Tensile strength testing is one of the better ways to optimize the level of additives in the formulation. Strips of the film are tested on a tensile strength tester by applying a known force at a constant rate. The elasticity and the tensile strength/ breaking stress of the films are evaluated.

3. Coated tablet evaluations:
* Adhesion test with tensile strength tester
* Diametric crushing strength of coated tablet:
i. Temperature and humidity may cause film defects
ii. Quantification of film surface roughness, hardness & colour uniformity.
iii. Mass variance of the coated tablets and variance of the tablet API content.

4. Stability studies must be conducted on the coated tablets to determine if temperature and humidity changes will cause film defects. Exposure of coated tablet to elevated humidity and measurement of tablet weight gain provide relative information on the protection provided by the film.

5. In process quality control:
a) Weight of tablet
b) Crushing strength
c) Tablet thickness
d) Disintegration time

References:
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