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| Introduction of Tablet Film Coating |
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The modern coating technique has been developed over the years from the use of sugar to film coating which provides a pleasant taste and attractive colourful appearance to tablets, which were unpleasant to swallow due to their bitterness.
To answer the question “Why are tablets coated?”
A number of reasons can be suggested; some of them are as follows:
The core contains a substance (API), which imparts a bitter taste in the mouth or has an unpleasant odour.
- The core contains a substance (API), which is unstable in the presence of light and subject to atmospheric oxidation,
i.e. a coating is needed to improve the stability.
- The coated tablet is packed on a high-speed packaging unit. The coating reduces friction and increases the production rate.
- To modify the drug release profile e.g. enteric coating, delayed release coating etc.
- To separate incompatible substances by using the coat to contain one of them or to coat a pallet, which was previously
compressed in to a core?
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| The Advantages of Film Coating over Traditional Sugar Coating Process |
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Film coating has several advantages over the old process of Sugar coating. Some of them are as follows :
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Short process Film Coating is a much shorter process in comparison with Sugar coating. Normally, 70-80% reduction in time
can be achieved.
In film coating, the weight gain over the tablets is around 2-3% and reaches only up to 7-8% for delayed release systems
whereas in Sugar Coating a minimum weight gain of 40-50% is a normal phenomenon.
No logo bridging: The film thickness is very thin and hence tablet logos / break lines are defined very clearly without filling.
Auto process: Sugar Coating is a highly skilled operation however film coating can easily be performed with proper training.
Single-stage Operation: Sugar Coating involves multistage operation extending over 24-32 hours while film coating is a single
step process normally completed in 3-4 hours.
Protection to API: Film coating protects the API present in the tablets from moisture & light and enhances its stability.
Ease of packing: Being non-bulky Film coated tablets it can be packed in Blister, Alu-Alu, PVC-PVDC and Paper Poly packing
material. |
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| Film Coating Materials and Their Properties |
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- A film coating is a thin polymer based coat applied to a solid dosage form such as a tablet, granule or pellets. The thickness of
such coating is approximately around 50-100 µm. Under close examination, the film structure can be seen relatively non
homogeneous and quite distinct in appearance, for example, from a film resulting from casting a polymer solution on a flat
surface. This non-homogeneous character results from the deliberate addition of insoluble ingredients such as pigments and
by virtue of the fact that the film itself is built up in an intermittent fashion during the coating process.
- Film coating formulation usually contains the following components:
1. Plasticizer-Makes the film flexible.
2. Anti-adhesives - Makes coating powder free flowing
3. Colourants - Food permitted for aesthetic look to the tablets
4. Solvents - Aqueous or organic to be use for coating process
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- The vast majority of the polymers used in film coating are either cellulose derivatives, such as the cellulose ethers; or acrylic
polymers and copolymers.
- Polymers can further be classified under two main categories:
1. Polymers for non-functional Film Coating.
2. Polymers for Enteric Film Coating.
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| Cellulose ethers |
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| Hydroxypropyl Methyl Cellulose (HPMC) |
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| This polymer provides the basis of coating with the cellulose ethers and its usage date back to the early days of the film coating. It is soluble in both aqueous media and the organic solvent systems normally used for film coating. HPMC provides aqueous soluble films, which can be colored by the use of pigments or used in the absence of pigments to form clear films. The polymer affords relatively ease processing due to its non-tacky nature. A typical low-viscosity polymer can be sprayed from an aqueous solution containing around 10-15%w/w polymer solids. |
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| This polymer is used rarely in film coating because of the lack of commercial availability of low viscosity material. |
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| Hydroxypropyl cellulose (HPC) |
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| This has the property of being soluble in both aqueous and alcoholic medium. Its films tend to be rather tacky, which possess restraints on rapid coating. HPC films also suffer from being weak. This polymer is often used in combination with other polymer to provide additional adhesion to the substrate. |
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| Polyvinyl Alcohol is used as film forming agent. It is soluble only in aqueous medium used for film coating. PVA provides aqueous soluble films, which can be colored by the use of pigments or used in the absence of pigments to form clear films. |
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| Enteric polymers are designed to resist the acidic nature of the stomach contents, yet dissolve readily in the intestine.
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| Cellulose Acetate Phthalate (CAP) |
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| This is the oldest and most widely used synthetic enteric coating polymer patented.
CAP is a white free flowing powder usually with a slightly odour of Acetic Acid. CAP is in insoluble in water and alcohols. |
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| Hydroxypropyl Methyl Cellulose Phthalate (HPMCP) |
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HPMCP is another polymer for enteric coating and more stable than CAP. It is more flexible polymer and compatible with
many plasticizers like Diethyl Phthalate, Acetylated Monoglyceride or Triacetin, offering formulator a wide choice.
HPMCP is a white powder or granular material, insoluble in water but soluble in aqueous alkalis and common non-aqueous
film coating solvent system. |
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| Methacrylic Acid Copolymers |
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| This polymer is available in various grades and forms of powder or liquid. This polymer can bind large quantities of pigments. |
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Plasticizers are simply relatively low molecular weight materials, which have the capacity to alter the physical properties of a
polymer. Generally the effect is to make the polymeric film softer and more pliable.
It is generally considered that the mechanism of action for a Plasticizer is for the plasticizer molecules to interpose
themselves between the individual polymer strands thus breaking down to large extent polymer-polymer interactions. This
action is facilitated as the polymer-plasticizer interaction which is considered to be stronger than the polymer-polymer
interaction.
The action of plasticizer is to lower the glass transition temperature.
For any given polymer, a plasticizer needs to be carefully chosen based on physico-chemical properties of the polymer,
chemical incompatibilities of plasticizer with drug and polymer and solvent system chosen. |
Classification of Plasticizer
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| Polyols
Propylene glycol
Glycerol /glycerine Polyethylene glycols |
| Citratre Esters (Triethyl Citrate, Acetyl Triethyl)
Triacetine
Phthalate Esters (Diethyl Phthalate, Dibutyl Phthalate) Dibutyl Sebacate |
| Castor oil
Acetylated Monoglyceride Fractionated Coconut Oil |
Colourants / Opacifiers :
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These groups of materials are commonly used as ingredients in film-coating formulae. They obviously contribute to
the
aesthetic appeal of the product.
Colourants also aid in the identification of individual products by patients, particularly those who are taking multiple
medications.
They reinforce brand imaging by a manufacturer and thereby decrease the risk of imitation.
Colourants for film-coated tablets have greater or lesser extent opacifying properties, which are useful when it is desired to
optimize the ability of the coating to protect the active ingredient against the action of light.
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Classification Organic dyes and their lakes
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| Sunset yellow |
Quinoline yellow |
| Tartrazine |
Brilliant blue |
| Indigo carmine |
Ponceau 4R |
| Carmosine |
Erythrosine |
Lakes are water-insoluble complexes of dyes with hydrated Alumina.
Formulator should take the fact in consideration while developing new formulation that some of the dyes and its lakes show high sensitivity towards light like Erythrosine, Indigo Carmine, Brilliant Blue etc. The selection of any of colourants in the formulation should be based on regulatory acceptance.
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| 1) Titanium dioxide, Calcium carbonate |
| 2) Iron oxide yellow, red and black |
| 3) They are most stable towards light compare to dyes. |
| 4) Another advantage is their wide regulatory acceptance. |
| 5) One drawback against above advantages is that the limited range of colours is achieved. |
6) MICA BASED PEARLSCEENT PIGMENTS (CANDURIN LUSTER): Chemically it is Potassium Aluminium Silicate which is inert &
non-toxic imparts the pearlescent/metallic effect to tablets and enhances its aesthetic look. |
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The major additives being used are :
> Magnesium Stearate
> Silicon Dioxide
> Talcum
Anti-tacking activity:
Tackiness is a concept that is widely used to describe the forces involved in the separation of two parallel surfaces
by a thin film of liquid. Such consideration is important during the coating process. An excess tackiness can cause
troublesome adhesion of tablets to each other or to the coating vessels.
Increasing the pigment concentration and decreasing the pigment particle size, the effect of tack is being reduced at high
polymer concentration. But at low polymer concentration only talc is effective in reducing tack. |
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These materials perform a necessary function in conveying the coating materials to the surface of the tablet or particle. The
major classes of solvents capable of being used are :
1. Water
2. Acohols
3. Ketones
4. Esters
5. Chlorinated Hydrocarbons
Solvents has to interact well with the chosen polymer. This is needed as high polymer solvent interaction permits
Film-properties such as adhesion and mechanical strength to be optimized. Solvents should not have volatility
problem. |
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