Coating Tablet Defects: The Cause and the Remedies
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
Sticking and twining
ًWhen tablets that stick together (twining) or to the pan wall. Twining is a common problem
with capsule shaped tablets. Usually caused by over wetting
REMEDY: Assuming you do not 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.
Picking
Definitioin: Overwetting 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.
Cratering
Definition:
It is defect of film coating whereby volcanic-like craters appears exposing the
tablet surface.
Reason:
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. Usually happens for hydrophilic cores with aqueous coating.
Remedy: Avoid over wetting or seal coat the hydrophilic core
Pitting
Definition:
It is defect whereby pits occur in the surface of a tablet core without any
visible disruption of the film coating.
Reason:
Temperature of the tablet core is greater than the melting point of the
materials used in the tablet formulation.
Remedy: avoid low melting ingredient or use drying temperature lower than melting point
of the low melting ingredient.
Cracking/Splitting
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)
Reason: Internal stress in the film exceeds tensile strength of the film.
Remedy: Increase the tensile strength by replacing low molecular weight polymer with high molecular weight polymer or use a blend of high and molecular weight
polymer. Internal stresses in the film can be minimized by adjusting the plasticizer
type and concentration, and the pigment type and concentration.
Hazing / Dull Film
Definition: 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.
Color Variation
Definition: 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 or pigments 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/Roughness
Definition:
It is surface defect resulting in the film being rough and nonglossy. Appearance is similar to
that of an orange.
The Causes And Remedies Of Orange Peel/Roughness
Sr. No. CAUSES
REMEDIES
1.
Rapid Drying
Use mild drying conditions
2.
High solution viscosity
Use additional solvents to
decrease viscosity of solution.
Bridging and Filling
Bridging caused by inappropriate formulation or inappropriate logodesign with less
adhesion between coating suspension and tablet surface. During drying, the film may
shrink and pull away from the sharp comers of a bisect, resulting in a "bridging" of the
surface depression. This defect can be so severe that the monogram or bisect is
completely obscured. This mainly represents a problem the formulation. Increasing the
plasticizer content or changing the plasticizer can decrease the incidence of bridging.
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.
Judicious monitoring of the fluid application rate and thorough mixing of the tablets in
the pan can prevent filling.