Coating Tablet Defects: The Cause and The Remedies

Here is a list of common defects associated with coated tablets and some likely causes and the remedies.

Picking and sticking
This is when the coating removes a piece of the tablet from the core. Overwetting or examples or excessive film tackiness causes tablets to stick to each other or to the coating pan. On drying, at the point of contact, a piece of the film may remain adhered to the pan or to another tablet, giving a “picked” appearance to the tablet surface and resulting in a small exposed area of the core. It is caused by over-wetting the tablets, by under-drying, or by poor tablet quality.

REMEDY: A reduction in the liquid application rate or increase in the drying air temperature and air volume usually solves this problem. Excessive tackiness may be an indication of a poor formulation.


Twinning
This is the term for two tablets that stick together, and it’s a common problem with capsule shaped tablets.

REMEDY : Assuming you don’t wish to change the tablet shape, you can solve this problem by balancing the pan speed and spray rate. Try reducing the spray rate or increasing the pan speed. In some cases, it is necessary to modify the design of the tooling by very slightly changing the radius. The change is almost impossible to see, but it prevents the twinning problem.


Color Variation
This problem can be caused by processing conditions or the formulation. Improper mixing, uneven spray pattern and insufficient coating may result in color variation. The migration of soluble dyes, plasticizers and other additives during drying may give the coating a mottled or spotted appearance.

REMEDY:
1. The use of lake dyes eliminates dye migration.
2. A reformulation with different plasticizers and additives is the best way to solve film instabilities caused by the ingredients.


Orange Peel
This refers to a coating texture that resembles the surface of an orange. Inadequate spreading of the coating solution before drying causes a bumpy or “orange-peel” effect on the coating.
It is usually the result of high atomization pressure in combination with spray rates that are too high. This also indicates that spreading is impeded by too rapid drying or by high solution viscosity.

REMEDY: Thinning the solution with additional solvent may correct this problem.


Mottled color
This can happen when the coating solution is improperly prepared, the actual spray rate differs from the target rate, the tablet cores are cold, or the drying rate is out of specification.


Capping and Lamination
This is when the tablet separates in laminar fashion. Capping is partial or complete separation of top or bottom crowns of tablet main body. Lamination is separation of a tablet into two or more distinct layers. Friability test can be used to reveal these problems

The problem stems from improper tablet compression, but it may not reveal itself until you start coating. How you operate the coating system, however, can exacerbate the problem.

REMEDY : Be careful not to over-dry the tablets in the preheating stage. That can make the tablets brittle and promote capping.


Roughness
A rough or gritty surface is a defect often observed when coating is applied by a spray. Some of the droplets may dry too rapidly before reaching the tablet bed, resulting in the deposits on the tablet surface of “spray dried” particles instead of finely divided droplets of coating solution. Surface roughness also increases with pigment concentration and polymer concentration in the coating solution.

REMEDY: Moving the nozzle closer to the tablet bed and reducing the degree of atomization can decrease the roughness due to “spray drying”.


Hazing / Dull Film
This is sometimes called Bloom. It can occur when too high a processing temperature is used for a particular formulation. Dulling is particularly evident when cellulosic polymers are applied out of aqueous media at high processing temperatures. It can also occur if the coated tablets are exposed to high humidity conditions and partial salvation of film results.


Bridging
This occurs when the coating fills in the lettering or logo on the tablet and is typically caused by improper application of the solution, poor design of the tablet embossing, high coating viscosity, high percentage of solids in the solution, or improper atomization pressure. During drying, the film may shrink and pull away from the sharp corners of an intagliation or bisect, resulting in a “bridging” of the surface. This defect can be so severe that the monogram or bisect is completely obscured.

REMEDY: Increasing the plasticizer content or changing the plasticizer can decrease the incidence of bridging.


Filling
Filling is caused by applying too much solution, resulting in a thick film that fills and narrows the monogram or bisect. In addition, if the solution is applied too fast, Overwetting may cause the liquid to quickly fill and be retained in the monogram.

REMEDY: Judicious monitoring of the fluid application rate and thorough mixing of the tablets in the pan can prevent filling.


Erosion
This can be the result of soft or friable tablets (and the pan turning too fast), an over-wetted tablet surface, inadequate drying, or lack of tablet surface strength.


Peeling and frosting
This is a defect where the coating peels away from the tablet surface in a sheet. Peeling indicates that the coating solution did not lock into the tablet surface. This could be due to a defect in the coating solution, over-wetting, or high moisture content in the tablet core which prevented the coating to adhering.


Chipping
This is the result of high pan speed, a friable tablet core, or a coating solution that lacks a good plasticizer


Blistering
When coated tablets require further drying in ovens, too rapid evaporation of the solvent from the core and the effect of high temperature on the strength, elasticity and adhesion of the film may result in blistering.

REMEDY: Milder drying conditions are warranted in this case.


Cracking
It occurs if internal stresses in the film exceed the tensile strength of the film.

REMEDY: tensile strength of the film can be increased by Using higher molecular weight polymers or polymer blends.