Tablet coating, Types of coating, Coating instruments, advantages disadvantages, coating problems & remedies.

1. TABLET COATING
Poonam Nikam
Assistant Professor
Pharmaceutics Departmet

2. Tablets Coating
Reasons Behind Coating of Tablets:
To mask the taste, odour or colour of the drug. Improving the
product appearance, particularly where there are visible
differences in tablet core ingredients from batch to batch.
Provide physical protection, facilitates handling, particularly in
high speed packaging / filling lines.
To provide chemical protection from its surrounding
environment (particularly air, moisture and light).
To control the release of drug from the tablet e.g. sustained
release tablets, repeat action tablets.
To protect the drug from the gastric environment of the
stomach with an acid resistant enteric coating.

3. Components Considered in
Tablet Coating
Tablet Properties: - Shape, Tolerance, Surface area.
 Tablet to be coated must possess the proper physical
characteristics like spherical shape and uniform surface.
 To tolerate attrition of tablets during coating process they must
be resistant to abrasion and chipping.
 As the tablet surfaces that are brittle and soften in presence of
heat or effected by coating composition and tend to become
rough in the early stages of coating process are unacceptable for
film coating.
Coating process: -
A. Coating equipment
B. Coating parameters.
C. Facility & ancillary equipment.
D. Automation of coating process.
Coating composition: - which involves polymers, color, plasticizer,
solvent.

4. Types of Coating-
Coating
Film coating
Conventional Film
coating
Functional Film
coating
Delayed release
Film coating
Extended release
Film coating
Sugar coating

5. Coating formation
1. Coating solutions:
Coating solutions contain the coating material (polymers or sugar), the
coating solvent and other excipients that are required to improve the
performance of the tablet coating,
e.g. colourants/opacifiers, plasticisers (to render the film flexible).
The choice of the solvent/solvent blend is according to the
physicochemical properties of the coating material (i.e. the
compatibility of the material with the solvent); however, other
considerations include the volatility and the flammability of the
solvent.
The concentration of the coating material within the solution is also a
consideration. Increasing the concentration of coating material within
the solvent will reduce the processing time; however, by increasing the
concentration of material, the viscosity of the solution may be
unacceptably high to achieve the correct spray properties during
coating.

6. 2. Coating emulsions:
• More recently emulsions have been developed as tablet coating
systems. In these the polymer is dissolved in a volatile organic phase
(with plasticizer and colourants/opacifiers, as required) and this is
emulsified within an external aqueous phase.
• The initial stage in the coating process involves the deposition and
subsequent spreading of the atomized coating solution/emulsion on
the surface of the tablet (or granule).
• To achieve a uniform surface distribution of the coating
solution/emulsion on the tablet, consideration of the wetting
properties of the solution/emulsion on the surface of the tablet is
required. Following spreading, evaporation of the solvent initially
enables coalescence of the organic droplets, and hence initial film
formation on the surface of the tablet.
• As drying continues, the saturation solubility of the coating material
in the solvent is exceeded and the solid coating is formed on the
surface of the tablet. It should be noted that contact, spreading,
droplet coalescence and solvent evaporation occur almost
instantaneously.

7. (A) Sugar Coating
1) Sealing-
Objectives- (i) To prevent moisture penetration into the tablet
core, a seal coat is applied and (ii) To strengthen the tablet core
without a seal coat, the over wetted tablets would absorb
excess moisture, leading to tablet softening, and may affect the
physical and chemical stability.
Ingredients
• Alcoholic solutions of Shellac (10 – 30% solid) or alcoholic
solution of zein,
• Alcoholic solution of cellulose acetate phthalate (CAP) or
alcoholic solution of polyvinyl acetate phthalate.

8. (A) Sugar Coating
2) Sub-coating-
Objectives-To round the edges and build up the tablet size. Sugar
coating can increase the tablet weight by 50 to 100% at this step.
Method:- The sub-coating step consists of alternately applying a
sticky binder solution to the tablets followed by a dusting of sub-
coating powders and then drying. Subsequent coatings are applied
in the same manner until the tablet edges have been covered and
the desired thickness is achieved.
3) Smoothing (Syruping)-
Objectives-To cover and fill in the imperfections in the tablet
surface caused by the sub- coating step.
Ingredients-Simple syrup solution (approximately 60–70%(w/w)).
Often the smoothing syrups contain a low percentage of titanium
dioxide (1–5%) as an opacifier. This gives a very bright and
reflective background for the subsequent coloring step.

9. (A) Sugar Coating
4) Colour coating-
Objective-To impart an elegant and uniform colour.
Ingredient-Syrup (60 – 70% sucrose) containing the desired color.
Method-Syrup solutions containing the dyes are coated upto 60
individual applications until the desired color is achieved. After each
application of color, the coatings are dried. In the finishing step a few
clear coats of syrup may be applied.
5) Polishing-
Objective-To produce the desired luster on the surface of the tablet.
Ingredients-Mixtures of waxes (like beeswax, carnauba wax, candella
wax or hard paraffin).
Method-Either this mixture of waxes is applied as powder or as
dispersions in various organic solvents in a polishing pan (canvas line
pan).
6) Printing-In order to identify sugar-coated tablets often it is necessary
to print them, using pharmaceutical grade ink, by means of a process of
offset rotogravure.

10. (B) Film Coating
 Film coating adds 2 to 5% to the tablet weight.
 Film coating is a complex process that involves the
application of thin (in the range of 20-200 μm)
polymer-based coatings to an appropriate substrate
(tablets, pellets, granules, capsules, powders, and
crystals) under conditions that permit:
1. Balance between (and control of) the coating liquid,
addition rate and drying process.
2. Uniformity of distribution of the coating liquid
across the surface of product being coated.
3. Optimization of the quality (both visual and
functional) of the final coated product.

11. Advantage-
 Substantial reduction in quantity of coating applied (2-4% for
film coating, compared with 50-100% for sugar coating).
 Faster processing times and Improvement in process efficiency
and output.
 Greater flexibility in optimizing formulations as a result of the
availability of a wide range of coating materials and systems.
 Ability to be applied a wide range of pharmaceutical products.

12. Tablet Coating in Practice:
1)Pan-pour method-
Viscous coating materials are directly added from some
container into the rotating pan moving with the tablet bed.
Tablets are subjected to alternate solution application,
mixing and then drying.
Disadvantages:
• The method is relatively slow and it relies heavily on the
skill of the operator.
• Tablets always require additional drying to remove the
latent solvent.
• Aqueous film coating is not suitable for this method
because localized over wetting will produce
physicochemical instability.

13. 2) Pan-spray method-
• The pan coating system is generically composed of a
metal pan (drum) into which the tablets are placed and
that may be rotated at a range of speeds.
• The coating solution is sprayed on to the surface of the
tablets within the pan whilst the drum is rotated.
• Simultaneously warm air is passed over the surface of
the tablets to facilitate the evaporation of the solvent in
which the coating material has been dissolved.
Control of the coating process is obtained by modifying
the following parameters:
• Rotation rate of the drum/pan
• Airflow rate
• Temperature of the air
• Concentration of sugar/polymer within the coating
solution/emulsion.

14. • More recently, pan coaters have been developed in
which the pan is perforated
(e.g. the Accela-Cota and Hi-Coater systems).
• In these systems the warmed air is passed into the drum
and through the tablet bed before being exhausted (with
the solvent from the coating solution) via the perforated
drum.
• In the Driacoater system, the drum is composed of a
series of perforated fins (typically 8 per drum) from
which the warmed air is provided. As the drum rotates,
the tablets in the tablet bed are mixed by and collected
on the fins before being suspended in the warmed air.
• The tablets are then dropped into the tablet bed and the
process is repeated.
• The warmed air is then exited from the rear of the pan.

15. (a) Pan variables:
Uniform mixing is essential to deposit the same quantity of film on
each tablet.
1. Pan design or baffling: Some tablet shapes mixes freely while
other shapes may require a specific baffling arrangement to ensure
adequate mixing.
Disadvantages: Baffles may produce chipping and breakage if not
selected properly.
(b) Pan speed:
• Pan speed affects mixing and the velocity at which the tablet pass
under the spray.
• Too slow speed cause localized over-wetting resulting in tablets
sticking to each other or to the pan.
• Too high speeds may not allow enough time for drying before the
same tablets are reintroduced to the spray. This results in a rough
coating appearance on the tablets.
Optimum pan speed: 10 – 15 rpm for non-aqueous film coating.
3 – 10 rpm for aqueous film coating.

16. 3) Fluidized bed process (air suspension coating)
This process have been successfully used for rapid coating of
tablets, granules and capsules.
Process variables are as follows:
(a) Chamber design and air flow rate controls the fluidization
pattern, (b) Tablet shape, size and density, (c) Volume and rate of
air flow either too high rate produce attrition and breakage of
tablets or too low rate mass does not move fast enough through
the spray region over-wetting occurs and (d) Inlet and exhaust
air temperature.
Examples-
Non-enteric materials: e.g. Hydroxypropyl methylcellulose
(HPMC), Methyl hydroxy ethyl cellulose (MHEC), Ethyl cellulose
(EC), Polyvinyl pyrrolidone (PVP), Sodium carboxymethyl cellulose
(Sod. CMC), Polyethylene glycols (PEG), Acrylate polymers e.g.
Eudragit E
Enteric materials: e.g. Cellulose acetate phthalate (CAP), Acrylate
polymers (Eudragit L, S), Hydroxypropyl methylcellulose phthalate
(HPMCP), Polyvinyl acetate phthalate (PVAP).

17. (c) Spray variables
1) Rate of liquid application.
2) Spray pattern.
3) Degree of atomization
These three spray variables are interdependent. For spraying two types of
systems are there:
(a)High-pressure, airless system and (b) low-pressure, air atomization system.
(d) Process air variables
(temperature, volume, rate) are required for optimum drying of the coating by
evaporation of the solvent. The balance between the supply and exhaust air
flow should be such that all the dust and solvent are confined within the
coating system
(C) Enteric Coating
1) Pan-pour method.
2) Pan-spray method.
3) Fluidized bed process (air suspension coating)

18. Coating Pans
Conventional coating Pans:
• Pelligrini pans
• Immersion sword type pan
• Immersion tube type pan
Perforated coating Pans:
• Accela-cota
• Hi-coater
• Dria coater
• Glatt Pan Coating
• Huttlin Butterfly pan
• Dumolin Ida.x coating pan

27. Driacoater

31. Problems Associated with
Tablet Coatings:
There are several problems associated with tablet coatings,
including:
1. Poor adhesion of the coating to the tablet;
2. Tablet abrasion;
3. Filling tablet markings;
4. Rough surface;
5. Formation of cracks in the coating; and
6. Variations in the colour of the coating.

32. 1. Poor Adhesion of the
Coating to the Tablet:
This phenomenon may be due to:
• High relative humidity within the coating chamber when
coating tablets using an organic solvent system.
• High coating spray rate.
• Concentration of polymer in the coating solution/emulsion is
too low.
• Temperature of air is too low, resulting in a slow rate of solvent
evaporation (particularly valid for coating systems that employ
solvents of low vapour pressure, e.g. water).
• Air fluidization rate or pan rotation rate is too slow.
• The tablet substrate has minimal curvature. Typically curved
surfaces are easier to coat than flat surfaces.

33. 2. Tablet Abrasion:
The coating process involves exposing the tablets to shearing
stresses that are generated as a result of collisions with other
tablets and also with the walls of the coating chamber. This may
result in damage to the tablet surface. This problem may occur
due to:
• Inappropriate tablet hardness.
• Irregular tablet shape.
• Tablet bed is too heavy during coating.
• The speed of rotation of the pan or the air fluidization rate is
excessive.

34. 3. Filling Tablet Markings:
Manufacturers may wish to identify their product with a
particular mark/name (performed by using a tablet punch that
has been embossed with the specified mark).
If the coating conditions are unsuitable, the coating will
excessively deposit within the mark/name and, in so doing, the
marking will be partially obscured.
This may occur due to:
• The use of deep markings.
• Use of an excessive volume of coating solution.
• Air temperature is too low.
• Pan rotation speed/fluidization flow rate is too low.

35. 4. Rough Surface:
• One of the major problems of tablet coating is the production of tablets that
exhibit a rough surface.
• This phenomenon is often associated with drying of the coating droplets prior to
reaching the surface of the tablet. To correct this problem the spray rate may be
increased and the inlet air temperature decreased.
5. Formation of Cracks in the Coating:
• The formation of cracks in tablet coatings is principally due to the use of an
inappropriate coating formulation.
• Plasticizers are employed to lower the glass transition temperature of polymer
coatings. This in turn renders the film more flexible and less brittle.
• Therefore cracking in polymer coatings may indicate that either the plasticizer
concentration should be increased or, alternatively, a different plasticizer that is
more compatible with the polymer chosen for the coating should be considered.
• In certain situations cracking of polymer coats may occur due to the use of a
polymer that has a low stress resistance and is therefore prone to stress failure.
To rectify this situation either the molecular weight of the polymer should be
increased or, alternatively, a different polymer should be used that has a greater
resistance to the applied stress (i.e. an increased ultimate tensile strength).

36. 6. Variations in the Colour of the
Coating:
Tablets that have been coated with a polymer containing a
colourant should show uniform colour. Variations in the colour
of a tablet coating may be due to:
• Improper mixing of the colour within the coating formulation.
• Uneven coating process, resulting in regional differences in the
thickness of the applied coating.
• Migration of coloured components within the tablet core into
the coating. This may be resolved by the use of a coloured
coating that will mask the effects of the migration or by the use
of a coating in which the components within the table core are
insoluble.

37. Sr.
No.
Reason Cause & Remedies
1 Blistering:
It is local detachment of film from the
substrate forming blister.
Reason: Entrapment of gases in or
underneath the film due to
overheating either during spraying or
at the end of the coating run.
Cause: Effect of temperature on the
strength, elasticity and adhesion of the
film.
Remedy: Use mild drying condition.
2 Cratering:
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.
Causes:
• Inefficient drying.
• Higher rate of application of coating
solution.
Remedies:
• Use efficient and optimum drying
conditions.
• Increase viscosity of coating solution
to decrease spray application rate.
ProblemsAndRemediesForTabletCoating

38. Sr.
No.
Reason Remedies
3 Picking
It is defect where isolated areas of film
are pulled away from the surface when
the tablet sticks together and then part.
Reason: Conditions similar to cratering
that produces an overly wet tablet bed
where adjacent tablets can stick together
and then break apart.
Causes:
• Inefficient drying.
• Higher rate of application of coating
solution.
Remedies:
• Use optimum and efficient drying
conditions or increase the inlet air
temperature.
• Decrease the rater of application of
coating solution by increasing viscosity of
coating solution.
4 Pitting
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.
Cause: Inappropriate drying (inlet air)
temperature.
Remedy: Dispensing with preheating
procedures at the initiation of coating and
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.

39. Sr.
No.
Reason Remedies
5 Blooming
It is defect where coating becomes
dull immediately or after prolonged
storage at high temperatures.
Reason: It is due to collection on the
surface of low molecular weight
ingredients included in the coating
formulation. In most circumstances
the ingredient will be plasticizer.
Cause: High concentration and low molecular
weight of plasticizer.
Remedy: Decrease plasticizer concentration and
increase molecular weight of plasticizer.
6 Blushing
It is defect best described as whitish
specks or haziness in the film.
Reason: 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.
Causes:
• High coating temperature.
• Use of sorbitol in formulation which causes
largest fall in the thermal gelation
temperature of the Hydroxy Propyl Cellulose,
Hydroxy Propyl Methyl Cellulose,
Methyl Cellulose and Cellulose ethers.
Remedies:
• Decrease the drying air temperature.
• Avoid use of sorbitol with Hydroxy Propyl
Cellulose, Hydroxy Propyl Methyl
Cellulose, Methyl Cellulose and Cellulose ethers.

40. Sr.
No.
Reason Remedies
7 Colour Variation
It is a defect which involves variation in colour
of the film.
Reason: Alteration of the frequency and
duration of appearance of tablets in the spray
zone or the size/shape of the spray zone.
Cause: Improper mixing, uneven
spray pattern, insufficient coating,
migration of soluble dyes-plasticizers
and other additives during drying.
Remedy: Go for geometric mixing,
reformulation with different
plasticizers and additives or use mild
drying conditions.
8 Infilling
It is defect that renders the intagliations
indistinctness.
Reason: Inability of foam, formed by air
spraying of a polymer solution, to break. The
foam droplets on the surface of the tablet
breakdown readily due to attrition but the
intagliations form a protected area allowing the
foam to accumulate and set. Once the foam
has accumulated to a level approaching the
outer contour of the tablet surface, normal
attrition can occur allowing the structure to be
covered with a continuous film.
Cause: Bubble or foam formation
because of air spraying of a polymer
solution.
Remedy: Add alcohol or use spray
nozzle capable of finer atomization.

41. Sr.
No.
Reason Remedies
9 Orange Peel/Roughness
It is surface defect resulting in the film being
rough and non-glossy. Appearance is
similar to that of an orange.
Reason: Inadequate spreading of the coating
solution before drying.
Causes:
• Rapid Drying.
• High solution viscosity
Remedies:
• Use mild drying conditions.
• Use additional solvents to decrease
viscosity of solution.
10 Cracking/Splitting
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.
Cause of Cracking/Splitting:
• Use of higher molecular weight
polymers or polymeric blends.
• Use lower molecular weight
polymers or polymeric blends. Also
adjust plasticizer type and
concentration.
11 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

42. Reference
1. Lieberman HA, Rieger MM, Banker GS. “Pharmaceutical Dosage
Forms: Disperse System”, vol.3; Second Edition,473-511
2. The theory and practice of industrial pharmacy by Leon Lachman,
Herbert A. Liberman, Joseph L. Kanig; Third edition
3. Aulton, ME. “Pharmaceutics, The Science of Dosage Form Design”,
2nd edition, Churchill Livingstone, London, 2002, pp 309-322.
4. Lachman L, Lieberman HA, Kanig JL. “The Theory and Practice of
Industrial Pharmacy”, 3rd edition, Varghese Publishing House, Bombay,
1991, pp 457-477.
5. Niazi SK. “Handbook of Pharmaceutical Manufacturing Formulation:
Liquid products”, CRC Press LLC, 2004.
6. Remington-The science and practice of pharmacy 21st edition pg
323,740-744.
7. Lachman L, Lieberman HA, Kanig JL. ‘Theory and Practice of Industrial
pharmacy’- Varghese publishing house, third edition-pg no-511.
8. Edward J, Pittsburgh B, Pennsylvania,’ Pharmaceutical Packaging
Handbook’ USA Informa health care.