Download Tablet coating

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Tablet coating
Reasons
Many tablets today are coated after being pressed. Although sugar-coating was popular in
the past, the process has many drawbacks. Modern tablet coatings are polymer and
polysaccharide based, with plasticizers and pigments included. Tablet coatings must be
stable and strong enough to survive the handling of the tablet, must not make tablets stick
together during the coating process, and must follow the fine contours of embossed
characters or logos on tablets. Coatings can also facilitate printing on tablets, if required.
Coatings are necessary for tablets that have an unpleasant taste, and a smoother finish
makes large tablets easier to swallow. Tablet coatings are also useful to extend the shelflife of components that are sensitive to moisture or oxidation. Opaque materials like
titanium dioxide can protect light-sensitive actives from photo degradation. Cross
contamination is also reduced in the manufacturing plant as 'dusting' from tablets is
eliminated by coating. Colored coatings aid in the rapid identification of product by the
manufacturer, dispensing pharmacist and patient. Colored coatings mask any batchwise
differences in the appearance of raw materials (patient concern).
This is the last stage in tablet formulation and it is done to protect the tablet from
temperature and humidity constraints. It is also done to mask the taste, give it special
characteristics, distinction to the product, and prevent inadvertent contact with the drug
substance.
Functional film coatings are used to impart enteric or controlled release properties to the
coated tablet. If the active ingredient of a tablet is sensitive to acid, or is irritant to the
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stomach lining, an enteric coating can be used, which is resistant to stomach acid and
dissolves in the high pH of the intestines. Coatings are often chosen to control the rate of
dissolution of the drug in the gastro-intestinal tract. Some drugs will be absorbed better at
different points in the digestive system. If the highest percentage of absorption of a drug
takes place in the stomach, a coating that dissolves quickly and easily in acid will be
selected. If the rate of absorption is best in the large intestine or colon, then a coating that
is acid resistant and dissolves slowly would be used to ensure it reached that point before
dispersing. The area of the gastro-intestinal tract with the best absorption for any
particular drug is usually determined by clinical trials. Coating is also performed sustain
or control the drug release. The following are examples of modified release patterns
Enteric Release
Enteric coated dosage forms for delayed drug release profiles are a popular choice for the
delivery of drugs where either protection from the harsh, acidic, environment of the
stomach is required (proton pump inhibitors), or there is a need to reduce irritation of the
gastric lining of the stomach commonly associated with certain types of medications
(non-steroidal anti-inflammatories e.g. aspirin). In either case, enteric/delayed release
coatings are intended to prevent release of the drug until the dosage form has passed
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through the stomach, followed by rapid release of the drug in the proximal small
intestine. Such a release profile can be achieved through coating of multi-particulates or
tablets with pH-dependent polymeric coating systems.
Biphasic Release
In certain conditions (migraine and sleeping disorders), drug treatment may be
advantageous to be delivered in a bi-phasic manner rather than a single phase extended
release preparation. In the first phase of drug release, the immediate release dose fraction
(also called “loading-dose”) reaches a therapeutic drug level in the blood plasma quickly
after administration, while the second extended release phase (called the “maintenancedose”) provides the dose fraction, required to maintain an effective therapeutic level for a
prolonged period. Examples of such systems can be found as bilayer tablets or
combinations of immediate, and extended release multi-particulates.
First Order Release
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First order drug release is the dominant extended release profile found in the
pharmaceutical industry today (about 75% of all ER formulations). Several different
technologies can be utilized to achieve first order drug release profiles, such as;
hydrophilic or inert matrix systems and barrier membrane coated multi-particulate
systems.
Zero Order Release
The ability to deliver a drug at a rate which is independent of time and the concentration
of drug within a pharmaceutical dosage form is desirable. Zero order mechanism ensures
that a steady amount of drug is released over time, minimizing potential peak/trough
fluctuations and side effects, while maximizing the amount of time the drug
concentrations remain within the therapeutic window (efficacy). This is particularly
important for 24 hour delivery of drugs with a narrow therapeutic index. Osmotic tablet
formulations, coated tablet matrices, and the use of polymer combinations in hydrophilic
matrices can be utilized to provide zero order drug release profiles.
TYPES OF COATINGS
Sugar coating
Sugar coating is done by rolling the tablets in heavy syrup, in a similar process to candy
making. It is done to give tablets an attractive appearance and to make pill-taking less
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unpleasant. However, the process is tedious and time-consuming and it requires the
expertise of highly skill full technician. It also adds a substantial amount of weight to the
tablet which can create some problems in packaging and distribution.
Film coating
Film coating can be defined as a process in which thin (in the range of 20 - 200μm)
polymer – based coatings are applied on to appropriate drug-containing cores that can
be either tablets, beads, granules, capsules or drug powders and crystals. In
comparison to sugar coating, film coating is more durable, less bulky, and less time
consuming. But it creates more difficulty in hiding tablet appearance. Film coating is
based on using polymers (synthetic or polysaccharides) and nowadays is the most
widely
used to develop coated dosage forms. Can be done in volatile organic solvents or aqueous
medium. Water has the disadvantage of high latent heat of vaporization, while it is safer,
cheaper than organic solvents which can be even flammable thus presenting explosive
dangers. Accordingly, aqueous polymeric coatings are the most widely used nowadays.
But sometimes we may be forced to use organic solvent as in the case of a drug that is
rapidly degraded by water or highly moisture sensitive core, or when the core show
disintegration during coating as a result of water soluble ingredients. Even in such cases,
the use of organic solvent is routinely limited to a minimal amount by applying small
enough quantity for further possible aqueous coating, which is called as seal coat.
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Comparison between Film and Sugar Coating
FEATURES
FILM COATING
SUGAR COATING
Tablets
Appearance
Retain contour of original core. Usually
not as shiny as sugar coat type
Rounded with high degree of
Polish
Weight increase because
of coating material
2-3%
50-80%
Logo or ‘break lines’
Possible
Not possible
Operator training required
Process tends itself to automation and easy
training of operator
Considerable
Adaptability to GMP
High
Difficulty
Stages
Single
Multiple stages
Functional coating
Easily adaptable for controlled release
Not usually possible apart from enteric
coating
Process
3. Press coating:
Although less popular, it gained increased interest in the recent years for creating
modified released products. The process was developed originally to serve as
alternative for sugar and film coating in case of very water-sensitive drugs.
It can be also used for separating incompatible ingredients. The press coating also
offers potential for having a dual release pattern. The use of the process is limited by
the relative complexity of the mechanism used in the compression equipment.
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It involves the compaction of granular materials around a preformed tablet core using
specially designed tableting equipment. Compression coating is a dry process (no
solvents are used. This type of tablet (compression coated tablet) has two parts,
internal core and surrounding coat. The core is small porous soft tablet and prepared
in a small die at low compression force. For preparing final tablet, a bigger die cavity
is used in which first the coat material is filled to half and then core tablet is
mechanically transferred, again the remaining space is filled with coat material and
finally high compression force is applied.
Polymers used for film-coatings (film formers)
Film Formers (Non Functional Coating)
When polymers are applied to improve the product appearance, enhance stability,
improve handling properties or for taste masking. Such coatings are usually described
as non – functional film coatings because they do not interfere with the drug release
properties of the drug containing cores. Examples: Water soluble cellulose
derivatives. PVP, PEGs.
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Film Formers (Functional Coating)
Polymers that are used to modify the release of the active ingredient from the drug –
containing cores are described as functional coating polymers.
Modified release dosage forms are classified by the USP into two types:
1. Extended release: One that permits at least a twofold reduction in the
dosing frequency as compared to the situation in which the drug is
presented as a conventional dosage form.
2. Delayed release: One that releases the active ingredient at some time other
than promptly after administration (enteric coated products for example).
Examples: CAP, acrylate polymers, PVAP and Ethylcellulose.
 Film formers can be applied as aqueous solution (HPMC in water), aqueous
dispersion (Ethylcellulose in water) or organic solution using anhydrous solvent
(Ethylcellulose in isopropanol). This switch from the traditional organic solventbased systems occurred because of the many advantages associated with aqueous
dispersions over organic based polymeric solutions including:
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Low viscosity (even at a high solids content)
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Low tackiness
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The reduction of costs associated with explosion proof equipment and
solvent handling technology.
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1. Cellulose derivatives: Can be used for immediate release coatings (such as
HPMC), Sustained release coatings (such as Ethylcellulose) and enteric
coatings (Such as cellulose acetate phthalate). The properties of these
polymers are shown below
.
2. Poly methacrylates
A. Methacrylic acid and methacrylic ester copolymers
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Have free COOH groups and thus used for delayed release purposes by controlling
the ratio of acid to ester monomers and polymer mwt, the pH at which they dissolve
is controlled. Available under the trade name of Eudragit, such as Eudragit L 30 D.
B. Methacrylate ester copolymers
They have no free carboxylate and thus used mainly for sustained release purposes.
Eudragit NE30D is flexible and it can form at room temperature without any
plasticizer. The introduction of trimethylammonioethyl methacrylate chloride (TMCl)
modifies the permeability of methacrylic ester copolymers. Films of poly (EA-MMATAMCL) 1:2:0.2 (Eudragit RL) are more permeable and films of poly (EA-MMATAMCL) 1:2:0.1 (Eudragit RS) are less permeable than those of poly (EA-MMA) 2:1
(Eudragit NE30D). Accordingly, it is possible to modify the drug release rate by
selecting different permeable polymer.
3. Polyvinyl acetate: used for sustained release coatings
4. Polyvinyl acetate phthalate: used for enteric coating
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Equipment
1. Pan coaters:
Consists of circular pan that rotates with the tablet bed inside. Baffles attached
to the wall of the pan ensures good tablet mixing and uniform coating of the
tablets. Spraying gun with set of nozzles atomize the coating liquid as fine
droplets on the tablet bed to cover the whole width of the bed. Pan coaters
differ according to the drying system and modified to improve the drying rate
and consequently the coating efficiency. Three types are illustrated below and
discussed
A. Conventional standard coating pans:
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Consists of a circular metal pan with various shapes. Heat is introduced into the pan
and directed onto the tablet bed, and it is exhausted by ducts positioned through the
front of the pan. Most of the heat energy supplied is deflected off the surface.
B. Immersion sword pan
Was developed to improve the utilization of drying air. Drying air is introduced
through a perforated metal sword device that is immersed in the tablet bed. Since the
air intimately mixed with the wetted tablets, a more efficient drying environment is
provided.
C. Perforated pan systems were developed to further improve drying
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efficiency. Two commercial coaters are examples
-Accela coata:
Drying air is introduced through the perforation across from the tablet bed, which is
exhausted through perforations at the opposite side, and thus is passed through table
bed
-Driacoater:
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Introduces drying air through hollow perforated ribs located on the inside periphery
of the drum. As the coating pan rotates, the ribs dip into the tablet bed, and drying air
passes up through and fluidizes the tablet bed. Exhaust is from the back of the pan
Fluid bed drier:
Top spray Fluid bed
The spraying nozzle is located on top of the product chamber. The liquid is sprayed in
the opposite direction to material motion. If the spray formulation contains a volatile
solvent, evaporation of this solvent from the spray droplets occur, thereby increasing
the solid content and reducing the ability of the sprayed droplets to spread on the
particles being coated. It yields coated particles with significant void volume and
porosity. They are simple to operate and have higher production capacities than other
types of fluidized bed units
Although not applicable for tablets, small particles can be successfully coated in a top
spray granulator. The films formed in this process are not as uniform, but for the
release that are not dependent on membrane thickness or perfection (such as taste
masking); it is viable and simple approach. Thus it is NOT efficient of producing
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sustained release particles. The substrate is fluidized up to the Nozzle, which sprays
concurrently into the material. The high particle velocity and efficient heat transfer
allows for aqueous coating with little or no agglomeration.
Bottom spray
The spraying nozzle is located at the bottom of the product chamber. The liquid is
sprayed in the same direction as the material motion in the partition chamber. Such
layout allows more contact between the liquid and the solid.
Used successfully for tablet and pellets particularly for sustained release purposes.
The flow pattern is formed by a partition and an orifice plate, which controls the air
flow. The majority of air is diverted through the partition chamber, causing
fluidization and upward travel of the cores. As the particles or tablets exit the
partition chamber and enter the expansion zone, air velocity decreases and the cores
drop outside the partition chamber. The air in this down bed acts to cushion the
tablets as they travel downward to continue their cycling trough the coating zone. The
balance between the air inside and outside the partition and the gap between the
orifice plate and the partition chamber are critical for this cycling.
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The coating process
As shown in the following Figure, coating fluid (containing polymer, glidant,
colourant, plasticizer) is pumped by a peristaltic pump into spraying Nozzle that
spray the fluid as fine mist or droplets.
The film coating process requires a delicately balanced environment to form an
acceptable layer on the substrate. It consists of the following consecutive steps
1. Formation of appropriate size droplets (atomization)
2. Contact of these droplets with the substrates (adhesion)
3. Spreading and coalescence of the droplets
4. Evaporation of the solvent: drying must be rapid so that core penetration
of the solvent and dissolved coating material is minimized as what can
occur between aqueous polymeric liquid and core containing water soluble
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ingredients. Also the rate of evaporation must equal to the rate of spraying to
prevent over wetting and consequently preventing agglomeration of the cores.
Atomization:
The atomization process leads to the formation of small droplets. The size of these
droplets is critical for the coating process. In case of highly viscous solution such as
HPMC in water (gives high viscosity because it dissolves in water), higher atomizing air
pressure is required to obtain smaller droplets. Accordingly, too viscous polymeric
solutions are difficult to be sprayed into small droplets. Large droplet size of viscous
solutions leads to poor spreading on the tablet surface and hence rough surface. This
phenomena is called as ORANGE PEELING
the tablet surface will look like an orange peel with rough and low gloss (less shiny
appearance). A colored film with a smooth glossy surface will always appear darker and
more saturated in color then the same film with a rough, less glossy surface, thus
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roughness affects the visual perception of a colored tablet. Also the large droplets on
tablet surface are difficult to dry which may lead to localized over wetting and sticking.
On the other hand, polymeric dispersions (such as aqueous dispersion of ethyl cellulose
and Eudragit L 30 D aqueous dispersion) require lower air atomizing pressure to achieve
small droplet size as a result of low viscosity. However, the drying from these droplets is
faster than from polymeric solutions as shown in the following figure
particularly as the droplet size is smaller. Accordingly, high water evaporation from very
small droplet of polymeric dispersion may give rise to spray drying before adhesion to
tablet core. Spay drying also leads to poor spreading and roughness of the tablet surface
and consequently orange peeling. Also it reduces coating efficiency as a result of poor
adhesion and dust formation. ِThe orange peeling obtained with dispersion can be easily
solved by reducing the atomizing air pressure to achieve larger droplets and consequently
slower evaporation during spraying. Also reducing the distance between the nozzles and
the tablet bed will expose the droplets for less time of drying before deposition.
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According to the previous discussion, let us discuss factors that affect atomized
particle size and viscosity
1. Polymer solubility
Polymeric dispersions gives much lower viscosity than polymeric solutions at the
same concentration; the increase of viscosity with the increase in polymer
concentration is much more dramatic incase of polymer solutions than incase of
polymer dispersion, which is illustrated in the following Figure for ethyl cellulose
aqueous dispersion, organic ethyl cellulose solution and HPMC aqueous solution.
This implies that dispersions can be atomized into small droplets at higher
concentration than polymeric solution. For example Eudragit L 30 D and ethyl
cellulose dispersions can be atomized at concentrations up to 30%, while HPMC
aqueous solutions is typically sprayed at 6% which can be increased to 8-10%; higher
concentration may lead to orange peeling.
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2. Polymer molecular weight
One way to use higher concentration of water soluble polymer is decrease the
molecular weight of the polymer.
However, low molecular weigh give rise to low strength of the formed coat. Blend of
high molecular weigh and low molecular weight polymers is recommended.
3. Temperature
Polymer viscosity usually decreases with the increase in temperature, however,
gelation may happen at high temperature leading to sharp increase in viscosity.
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This is illustrated in the previous Figure, which show that the viscosity decrease until
almost 55 C, after which gelation happens. So you may think about heating the
solution to 50 C to reduce problematic viscosity, however, this way was proven to be
NOT successful because of cooling during pumping through the tubes and
particularly by the cooling expanding air during atomization.
4. Atomizing air pressure
Increasing the atomizing pressure leads to smaller droplet size particularly at low
viscosity.
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Spay variables
1. Rate of spraying
The rate of spraying depends on the mixing and drying efficiency of the system. The
proper rate at which the coating liquid should be applied depends on the mixing and
the drying efficiency of the system. Good mixing and efficient drying leads to higher
feed rate. Good mixing is achieved by the use of baffles. Tablet shape affects mixing;
Flat shapes leads to poor mixing, while as the shape is more spherical more optimum
mixing is achieved as with the biconcave tablets. Pan speed affects not only mixing
but also the velocity at which the tablets pass under the spray. Too Slow speed result
in localizes over wetting and sticking, while too high speed may not allow for enough
time for drying before the same tablet reintroduced to the spray. Pan speeds of 10-15
rpm are commonly used in the large pan coaters for nonaqeous coating, while slower
pan speeds (3-10 rpm) for aqueous coating (why). In any event, the rate of spraying
must equal the rate of drying to prevent over wetting or under wetting. Over wetting
leads to various problems in the coating process
A. Sticking of the tablets to each other, which is called twining, and to the
pan wall
B. Picking: when tablet sticking happens and falling apart happens, isolated
area of the film pull way from the tablet (picked by and left on the other
tablet) leading to a defect in the film.
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C. Cratering:
the coating liquid penetrates the surface of the tablet causing localized
disintegration of the core and disruption of coat, which appear such as
volcanic like craters. Generally it happens on the initial stages of the coating
process and becomes partially obscured as more film is deposited during
coating.
Continuous film formation
The film formation sequence differs according to the nature of the applied system
resulting in differences in the film structure and appearance, film permeability, the
need for post coating thermal treatment and the coat performance in modifying drug
release. The mechanism of film formation depends on type of the polymer liquid as
follows.
1. Polymer solution
A. Spraying of the coating solution onto a suitable substrate
B. The wetting and deformation of the sprayed droplets
C. The entanglement of the polymer chains
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D. As the solvent evaporates, the polymer gels
E. With further loss of solvent a continuous film is produced
2. Polymeric dispersion (pseudolatex system)
The coating liquid deposit on the coated cores as discrete spheres which should
coalesce into continuous film as the aqueous phase evaporates.
The formed film should have high elasticity and flexibility upon drying and should
not be hard and brittle. As the solvent evaporates the particle come close to each other.
Coalescence of the polymer particles depends on the intermolecular forces between
polymer chains.
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 Higher inter molecular forces results in poor fusion. Accordingly, plasticization is
usually required to reduce these intermolecular forces. Plasticizers are usually
high boiling organic solvents used to impart flexibility to otherwise hard and
brittle polymeric materials. Plasticizer generally cause a reduction in the cohesive
intermolecular forces along the polymer chains resulting in various changes in the
polymer properties, such as reduction in the glass transition or softening
temperature of the polymer. The plasticizer reduces the glass transition
temperature of the polymer; the glass transition temperature is the temperature
below which the polymer is characterized by ordered structure in which there is
minimal polymer chain movement. Above the glass transition temperature, the
polymer is in a rubbery state, which is usually characterized by amorphous
portions or regions with increased polymer chain movement and elasticity.
Accordingly, above the glass transition the polymer is rubbery (elastic and
flexible) and below the glass transition temperature, the polymer glassy (hard and
brittle). Plasticization can be achieved by the use of internal or external plactizinig
technique. Internal plasticizing pertains to the chemical modification of the basic
polymer that alters the physical properties of the polymer by controlling the
degree of substitution, the type of substitution and the chain length of the polymer.
An example is the highly flexible Eudragit NE 30 D, which does not require the
addition of external plasticizer. Most often the formulator uses external
nonvolatile plasticizer. In general, a product bed temperature that is 10 – 20 oC
above the Tg is usually recommended for optimal film formation.
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Spray Rate and Pressure:
In a study to examine the effect of the spray rate in a Wurster-insert fluidized bed
processor on the release of ibuprofen from ethylcellulose-coated tablets.
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Faster release rates from film-coated tablets were found to be associated
with higher spray rates.
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High spray rates can result in delayed coalescence of polymer spheres by
reducing bed temperatures resulting in poorly coalesced films.
–
The reduction in bed temperatures is due to the depletion of the applied
thermal energy by relatively large volumes of the aqueous vehicle for
evaporation.
 Optimum atomization pressure and droplet size are necessary to achieve a
uniform film coating.
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At high atomization pressures, very small droplets may dry before
contacting the substrate, a phenomenon termed spray drying, others may
contact the surface and dry before complete spreading of the polymer
spheres or coalescence occurs resulting in incomplete film formation
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At low atomization pressures, large droplets can overwhelm the
evaporative capacity of the system, causing over wetting. This may lead to
sticking of tablets in pans, or agglomeration and loss of fluidization in air
suspension coaters.
Coating Pan Speed:
Coating pan speed has been reported to have a major influence on the intra-tablet coating
uniformity in aqueous-based film coating. Spherical substrates, such as pellets, have the
best coating uniformity because there is no spatial orientation due to which all parts of
the surface are equally exposed to the spray coating over time. However, as the substrate
shape diverges from spherical to a flatter shape, as in tablets, it is more likely to have a
preferred spatial orientation. The flatter and broader the tablet, the more likely it is to
pass through the spray zone flat with the face exposed. The rotational exposure of all
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tablets required for uniform coating can be enhanced by increasing the motion of the
tablet bed by increasing the pan speed
Product Bed Temperature:
A low product bed temperature will result in a defective film and higher rates of drug
release because of drug migration into the film layer during the coating process and
incomplete film formation due to hardening of the uncoalesced polymer spheres. High
product bed temperatures results in:
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Faster evaporation of the dispersion medium of the coating droplets.
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Improper spreading of the polymer on the surface of the substrate
affecting the integrity of the film and its ability to control the drug release.
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Increased susceptibility to sticking problems
Formulation of polymeric coatings
1. Solvents: are used to dissolve or disperse the polymers and other additives and convey
them to substrate surface.
Ideal requirements
A. Should either dissolve/disperse polymer system
B. Should easily disperse other additives into solvent system
C. Small concentration of polymers (2-10%) should not in an extremely viscous
solution system creating processing problems
D. Should be colorless, tasteless, odorless, inexpensive, inert, nontoxic an
nonflammable
E. Rapid drying rate
F. No environmental pollution
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Mostly solvents are used either alone or in combination with water, ethanol, methanol,
isopropanol, chloroform, acetone, methylene chloride, etc. Water is more used because
no environmental and economic considerations. For drugs that readily hydrolyze in
presence of water, non aqueous solvents are used.
2. Film former
3. Plasticizers
Plasticizers are usually defined as high – boiling point organic solvents that are used to
impart flexibility to the otherwise hard or brittle polymeric materials. The addition of
plasticizers to the coating formulation will improve the integrity of the coat by reducing
the Tg of the coating polymer and enhancing the coalescence process. Plasticizers are
often low molecular weight compounds which need, to be effective, to distribute itself
between the polymers chains and interact with its functional groups thereby reducing the
interaction between the polymer chains and the cohesive intermolecular forces along
them thus softening the polymeric matrix.
They can be classified into
Water soluble
Glycols: glycerin, propylene glycol, low molecular weight PEG (PEG 200 and
400)
Triethyl citrate (Citroflex) and water soluble surfactants, such as Tweens
Water insoluble
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Organic acid esters: Acetyl triethyl citrate (ATEC) Acetyl tributyl citrate (ATBC)
Dibutyl sebacate (DBS) Diethyl phthalate (DEP)
Dibutyl citrate (DBC) Oils: Oleic acid, castor oil and coconut oil
Surfactants: Spans, Myvacet (acetylated mono glycerides)
Plasticizers do not necessarily need to be soluble in the solvent. If not soluble in water,
they can be emulsified using compatible emulsifier.
4. Surfactants (Tweens, Cetyl alcohol, SLS, Myvacet, Pluronic)
Surfactants can be “endogenous” to the polymeric aqueous dispersion since they are used
in the synthesis of some coating polymers (e.g. Nonoxynol 100 in Eudragit® NE 30D) by
emulsion polymerization. “Exogenous” surfactants may be added to the coating
formulation to facilitate the spreading of the coating droplets on the surface of the
substrate. Small amounts of non-ionic or anionic surfactants have been used to wet and
homogenize the coating mixtures and even act as plasticizers
Also used in case of aqueous dispersion to prevent sedimentation, and aggregation and
coalescence of the dispersed particles during shelf-life. They also reduce the contact
angle between powders for reconstitution and water, thus promote wetability and
dsipersability. In case of controlled release coatings water soluble surfactants were found
to increase the drug release rate of film coated granules by leaching into the release
media leaving a porous membrane.
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5. Antitack Agents (Glidants) Tack is the ability of a polymer to adhere to a substrate
with little contact pressure. During the application of the coating material in a coating
pan, pressure is induced by the movement of tablets.
Tumbling action of the coating pan:
-Enhances the film adhesion to the tablets surfaces but
-Causes some adherence and sticking between tablets and adherence and sticking to the
coating pan surface.
An unwanted and sometimes irreversible effect of tackiness is the agglomeration of
several coated units or, in the worst case, the whole batch which can occur at higher
product temperatures and higher plasticizer content.
In addition, the tackiness of polymeric films is important for subsequent curing step (post
coating thermal treatment) where the coated product is exposed to temperatures above the
coating polymer’s Tg.
Insoluble additives including talc, Mg stearate and colloidal silica have been used as antitack agents or anti-adherents to help reduce agglomeration or sticking of coated
substrates during and after coating. The effect of anti-tack agents on drug release is
basically a .stabilization effect by preventing damage to the film coats through sticking
during the coating curing steps.
6. Opaquent (Opicifiers): reduce the transparency of coat and to provide full coverage
the core colors. Most commonly used opicifier is titanium dioxide.
7. Colorants
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There are three groups of coloring substances generally used in colored tablet
coatings:
–
Water soluble dyes
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Water insoluble pigments
–
Water insoluble color lakes
A dye is a distinct chemical material, which exhibits coloring power when dissolved and
is water-soluble. Examples of dyes are Tertrazine, Sunset yellow, Erythrosine, and
Brilliant Blue. A pigment, such as iron oxides, generically is water insoluble material,
which colors by dispersion. The lakes are obtained by adsorbing dye onto insoluble
carrier such as aluminum to form insoluble form of the dye. Pigments and color lakes
are much more commonly used b0ecause of less migration during drying.