Industrial pharmacy lecture notes on tablet coating process

1. 4. Tablet Coating
Berhanemeskel W.G, Asst. Prof.
Department of Pharmaceutics
School of Pharmacy
College of Medicine and Health Sciences
University of Gondar
May 2009
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2. Presentation outline
• Historic perspective
• Objectives
• Process and equipment
• Types of tablet coating
• Evaluation
• Coating optimization
• Specialized coating
• Future trends
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3. Historical perspective
• The earliest references to coated solid dosage forms
appears in early Islamic drug literature 850 A.D, coated
pills (Rhazes)
• In the 16th century in the French publication coating as
means of masking taste of medicine
• In mid 1800- sugar coating pills in French publication,
as a means of masking
• Around 1814- patent was issued in the USA
• In 1950’s- film coated tablets were marketed by Abbott
Laboratory
• In same year (1950) Dr Dale Wurster, a professor at the
University of Wisconsin, Patented an air suspension
coater (fluidized bed) 3

4. Objectives of tablet coating
 To mask the taste, or color of the drug
 To provide physical and chemical protection for the
drug
 To control the release of the drug from the tablet
 To protect the drug from the gastric environment of
stomach
 To avoid chemical incompatibilities or to provide
sequential drug release
 To improve the pharmaceutical elegance by use of
special colors and contrasting printing
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5. Process and Equipment
 The coating process can best be described by initially
discussing the key factors that it comprises and then
showing their complex interactions
 There are three primary components involved in
tablet coating:
1. Tablet properties
2. Coating process
• Coating equipment
• Parameters of the coating process
• Facility and ancillary equipment
• Automation in coating processes
3. Coating compositions
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6. Tablet properties
 The tablets must be resistant to abrasion and
chipping
 Smooth surface
 Ideal physical shape i.e. spherical
 Chemical nature of drug and other ingredients
(stability)
 Degree of wetability
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7. Coating process
 Tablet coating is the application of a coating
composition to a moving bed of tablets with
the concurrent use of heated air to facilitate
evaporation of the solvent
 The distribution of the coating is
accomplished by the movement of the tablets
either of the two mentioned below to the
application of the coating composition
1. Perpendicular (coating pan) or
2. Vertical (air suspension coater)
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8. Equipments
• Most coating processes use one of three
general types of equipment:
– The standard coating pan
• Pouring method
• Spray method
– The perforated coating pan
– Fluidized bed (air suspension) coat
• The general trend has been toward
– Energy efficient
– Automated systems
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9. Conventional pan system
 Consists of a circular metal pan rotated on its horizontal axis
by a motor
 Heated air is supplied into the pan and onto the tablet bed
surfaces and is exhausted by means of ducts positioned
through the front of the pan
 Coating solutions are applied to the tablets by ladling or
spraying the material onto the rotating tablet bed
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10. Spraying of Coating Solution 10

11. Perforated pan systems
 Consists of a perforated or partially perforated drum
that is rotated on its horizontal axis in an enclosed
housing
 Drying air is directed into the drum, is passed through
the tablet bed, and its exhausted through perforations
in the drum
 Coating solutions sprayed through spraying nozzles
that are positioned inside the drum
 Are efficient drying systems with high coating
capacity and can be completely automated
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12. 12

13. Fluidized Bed (Air suspension)
 The basic principle of fluidized - bed coating is to
suspend tablets in a moving hot - air stream in the
upward direction during the coating process.
 The coating material is sprayed through a nozzle from
the top, the side, or the bottom into the fluidized bed.
 The solvent in the solution is removed from the
coating by the hot - air stream, which also carries the
coated tablets/particles.
 Fluidized - bed coating provides better coating
uniformity due to good solid – fluid mixing and
minimizes formation of agglomerates.
 Friable tablet cores difficult to coat (attrition)
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14. Spray application systems
 The two basic types of systems used to apply a finely divided
(atomized) spray of coating solutions or suspensions onto
tablets are:
1. High pressure (airless)
2. Low pressure, air atomized
Airless spray system
 Liquid is pumped at high pressure 250 to 3000 pounds per
square inch gauge (psig) through a small orifice (0.009 –
0.20in id.) in the fluid nozzle.
 Degree of atomization and the spray rate are controlled by
– The fluid pressure
– Orifice size and
– Viscosity of the liquid
Liquid feed 14

15. Low pressure air atomized system
 Liquid is pumped through somewhat larger orifice
(0.020 inch to 0.060inch id) at relatively low pressures
(5 to 50 psig).
 Low pressure (10 to 100 psig) air contacts the liquid
stream at the tip of the atomizer and a finally divided
spray is produce
 The degree of atomization is controlled by
– The fluid pressure
– Air pressure
– Air cap design
– Fluid cap orifice
– Viscosity of the liquid
Air stream
Liquid feed
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16. Parameters of the coating process
 Air capacity
 Quantity of air flowing
 Temperature of air
 Quantity of water in the inlet air
 Tablet surface area
 Coating composition
 Equipment efficiency
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17. Facility and ancillary equipment
• It should meet the requirements of cGMP
– Adequate space
– Specialized ventilation
– Colloid mill or ball mill
– Jacketed tanks
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18. Tablet coating types
 The coating process is the last critical step in
the tablet production cycle.
 The type of process chosen depends on
– The type of coating that is applied
– Durability (toughness) of the tablet core and
– The economics of the process
• Types
– Sugar coating
– Film coating
• Non-enteric coating
• Enteric coating 18

19. Sugar coating
 It involves several steps ranges from a few
hours to a few days
 The sugar coating process involves building up
layers of coating material on the tablet cores
as they are tumbled in a revolving pan by
repetitively applying a coating solution or
suspension and drying off the solvent.
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20. Steps of Sugar Coating
A. Water proofing and sealing coats
 For tablets containing components that may absorb moisture or
be adversely affected on contact with moisture
 Sealing materials: shellac, CAP, Zein
B. Sub-coating
 provide a good bridge between the main coating and the sealed
core as well as rounding off any sharp corners. This is normally a
two—step procedure:
 The 1st step involves the application of a warm subcoat syrup
(containing acacia and/or gelatin) that rapidly distributes
uniformly over the tablets and eventually becomes partially dry
and tacky.
 2nd step a sub-coat powder (containing material such as calcium
carbonate, talc, kaolin, starch and acacia) is dusted evenly over
the tablets after which the pan is allowed to rotate until the coat
is hard and dry.
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21.  5 to 10 additional coatings of a very thick syrup
are applied to the rolling tablets for the
purpose of
 completing the rounding of the tablets
 smoothing the coating and
 Imparting the desired color of the tablet
 The syrup may be colored, a suitable dye may
be added to the syrup during this step of the
coating process as well as during the next step.
 It requires the most skill in pouring syrup
C. Syrup/Smoothing or “grossing”
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22.  A polishing pan is used which is usually of cylindrical
design with canvas side walls.
 Polishing materials used are
 Beeswax
 Carnauba or
 Warm solutions of these waxes in naphthalene or other
suitable volatile oils.
 Tablets are left to roll until a high luster is produced
 They are normally “racked” to allow any traces of
solvent to evaporate before being sent to the inspection
and packing operations.
D. Finishing and Polishing
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23. Advantages of sugar coating
 It utilizes inexpensive and readily available raw materials.
 Constituent raw materials are widely accepted—no regulatory
problems.
 No complex equipment or services are required.
 Results are aesthetically pleasing and have wide consumer
acceptability.
 Simplicity of equipment and ready availability of raw materials
make sugar coating an ideal coating method for developing
countries.
Disadvantages of Sugar Coating
 The process is tedious and time consuming .
 Requiring the expertise of a highly skilled technician .
 Increase size and weight of the original tablets .
 Not convenient to swallow as are small tablets .
 The sugar coating may vary slightly from batch to batch within the batch.
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24. Film coating has the following advantages:
 The film process is simpler
 It is easier to automate
 It is more rapid than sugar coating.
 Weight gain of only 2-6% are involved as
opposed to more than 50% with sugar
coating.
 Moisture involvement can be avoided, if
necessary through the use of nonaqueous
solvents.
 Distinctive identification tablet markings are
not obscured by film coats.
2. Film Coating
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25. Methods
A. Pan pour method
 Requires additional drying steps to remove
latent solvents
 Replaced by newer coating techniques
 Not suitable for aqueous based coating
B. Pan-spray methods
 Next evolution in improving the efficiency of
film coating process
 Use of appropriate nozzles-so that entire
width of the tablet bed can be covered
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26. Process variables
 The variables to be controlled in pan-
spray film coating processes are:
1. Pan variables (pan design/baffling, speed,
pan load)
2. Process air (air quality, temperature, air
flow rate/volume/balance)
3. Spray variables (spray rate, degree of
atomization, spray pattern, nozzle to bed
distance)
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27. Materials used in film coating
• The coating materials may be
– A physical deposition of the material on the tablet
substrate e.g. sugar, shellac and wax coating
– A continuous film
• Examples of coating materials
– Synthetic polymers
– Solvents
– Plasticizers
– Colorants
– Opaquant-extenders etc
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28. An ideal coating material should have the following attributes:
• Good solubility in solvent of choice for coating preparation
• Solubility required for the intended use e.g.
– Free water solubility
– Slow water solubility or
– pH dependent solubility (enteric coated)
• Capacity to produce an elegant looking product
• Stability in the presence of heat, light, moisture, air and substrate being
used
• Essentially no color, taste or odor
• Compatibility with common coating solution additives.
• Non-toxicity with no pharmacologic activity and ease of application to the
particles or tablets.
• Resistance to cracking and provision of adequate moisture, light, odor, or
drug sublimation barrier when desired
• No bridging or filling of the debossed tablet surfaces by the film former
• Ease of printing procedure on high speed equipment
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29. Classification of film formers
• The available film formers can be classified
into
– Non-enteric and
– Enteric
• Non-enteric materials:
– E.g. HPMC, MHEC, EC, HPC, povidone, Na HMC,
PEG, acrylate polymers
• Enteric materials
– E.g. CAP, Acrylate polymers, HPMC Phthalate,
Polyvinyl acetate Phthalate
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30. Solvents for making film coating
• Water, Ethanol, Isopropanol, Acetone,
Methylene chloride, CCl4
• Ideal solvent characteristics includes
– Should either dissolve or disperse the polymer
system or other like plasticizers
– It should result optimum viscosity around 300cps
for 2 to 10% polymers
– Should be colorless, tasteless, odorless, non toxic,
inert non inflammable, in expensive
– Should have rapid drying rate
– No environment impact
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31. Plasticizers
 Used to modify the film to have flexible and
tensile strength
Common plasticizers
1. Water soluble: e.g. PEG (200-400 low mwt),
Glycerin, propylene glycol, polysorbates
(tweens)
2. Organic solvents: e.g. caster oil, organic acid
esters, sorbitan esters (spans)
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32. Quality control
 After coating, the tablets should be inspected
and tested for appearance and performance
1. Appearance
2. Performance (bioavailability/rate of
absorption)
3. Mechanical strength
4. Stability (ambient and stress conditions)
5. Film defects.
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33. Examples of film defects
1. Sticking and picking (in pan coating)
• Causes
– over wetting,
– tackiness of coating materials
• Solutions
– Decrease rate of application of coating solution
– Increase drying air temperature
– Increase volume of air that circulate to dry
– Change the composition of coating
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34. 2. Roughness or grittiness (in case of spraying)
Causes
 Droplets (small droplets)
 Drying too rapidly before reaching the tablet bed
 Increased pigment or polymer composition
Solution
 Decrease nozzle to bed distance
 Decrease the degree of atomization
 Reduce the composition of pigment or polymer
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35. 3. Peeling (orange peel effect)
Causes
– Inadequate spreading of the coating solution
before drying or
– too rapid drying or high viscosity
Solution
– Adequate spreading of the coating solution
– Lower drying rate
– Thin the solution
– Increase adhesion with the use of plasticizer
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36. 4. Bridging and filling: in case of embossed or engraved tablets
Causes for Bridging
• Shrinkage of film intanglation during drying or
• Formulation problem
Solution
– Change plasticizer
– Increase plasticizer concentration
Cause for filling
– Application of too much solution
– Fast application of solution (over wetting fill the monogram)
Solution
– Optimize application rate and mixing of tablets
5. Blistering (burning or swelling)
Cause: high temp and rapid evaporation
Solution: reduce temp.
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37. 6. Hazing (dull, bloom or cloudiness)
Causes
– too high processing temp.
– Too high humidity conditions
Solution: optimize temp. and humidity
7. Color variation of the film (mottling)
Causes- processing condition or formulation
– Improper mixing
– Uneven spray pattern
– Insufficient coating
– Migrating soluble dyes
– Type of plasticizers and other additives
Solution: optimize processing and formulation and additives, dyes
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38. 8. Cracking
Cause
– stress (internal) > tensile strength of the film
Solution
– Use high mwt polymer or combination polymer to
increase tensile strength of the film
– Adjust plasticizer : polymer ration
– Change the plasticizer

39. Specialized coating techniques
1. Press coating (compression coating): not used
commonly now a days
2. Electrostatic coating (deposition)
3. Dip coating
4. Vacuum coating
• The system is coated under reduced pressure
(vacuum) important in heat sensitive or oxidizable
e.g. nitrogen
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