UNDERSTANDING COATING:
Coating can be defined as a physical deposition of a layer on Tablet surface for the purpose of masking the Taste, odour, appearance, physical / chemical protection or to control the drug release.
Coating process is the application of coating composition to a moving bed of Tablets in a coating pan by concurrent use of heated air to facilitate evaporation of the solvent.
NEED OF COATING:
- To mask the Taste, Odor, or color of the drug
- To provide physical and chemical protection for the drug
- To control the release of the drug from the Tablets
- To protect the drug from the gastric environment of the stomach with an acid-resistant enteric coating
- To incorporate another drug or formula adjuvant to avoid chemical incompatibilities or to provide sequential drug release.
- To improve the pharmaceutical elegance by use of special colors and contrasting printing.
COATING COMPOSITIONS:
- Chemical nature and different characteristic of Film former.
- Viscosity may affect the spreading of coating liquid
- Percentage Solid content generally affects the Tablet surface and coating efficiency.
- Coating liquid attributes included physical appearance, viscosity, specific gravity, microbial count, pH (for pH-dependent coating liquids), and active drug stability (if in coating).
- Coating solution hold time study-Changes that might occur included color change, evaporation of volatile solvents or microbial growth.
TYPES OF TABLET COATING:
- Sugar Coating
- Enteric Coating
- Film Coating
- Modified Release Coating
SUGAR COATING:
Sugar Coating involves applying several layers of sugar on the surface of the Tablets and pills which in turn provides better protection and Taste for these Tables and pills.
The introduction of the process air evaporates the fluid and dries the sugar coating. When the sugar-coating solution is added continuously, the particles remain in the process until the required coating thickness is achieved.
Sugar coating process is a multistage stage process involving 5 separate operations. These are
- Seal Coating
- Sub Coating
- Smoothing
- Colour Coating
- Polishing
Seal Coating: Seal coating is to offer the initial protection to the cores and to prevent the migration of ingredients from core to coating.
Sub Coating: Sub coating provides the means for rounding off the Tablet edges and building up the core weight.
Smoothing: Smoothing is usually achieved by the application of plan 70% w/w syrup.
Colour Coating: Use of appropriate colorants which are dissolved in the coating syrup, allows achieving the desired color.
Polishing: Desired luster is obtained at the final stage of sugar coating. Polishing is the application of powdered wax.
ENTERIC COATING:
- Enteric coating is an outer layer applied to the Tablet
- The layer enables the Tablet or pill to be dissolved in the intestine rather than the stomach
- Enteric coating is specifically designed to delay the release of the API till the right time and the right place
- Protection from local irritation in stomach.
Raw Materials Used in the Enteric Coating:
The ideal properties of enteric coated material are
- Permeable to intestinal fluid
- Compatibility with coating solution and drug
- Formation of continuous film
- Non-toxic
- Cheap and ease of application
- Ability to be readily printed
- Resistance to gastric fluids
Enteric polymers are designed to resist the acidic nature of the stomach contents yet dissolve readily in the duodenum.
- Cellulose acetate phthalate
- Polyvinyl acetate phthalate
- Shellac
- Methacrylic acid copolymer
- Eudragit RS and RL
FILM COATING:
Film coating is nothing but the application of a film coating composition to a moving bed of Tablets with the concurrent use of heated air to facilitate evaporation of the solvent.
The Process involves spraying a coating solution containing the following:
- Polymer
- Solvent
- Plasticizer
- Colorant
The solution is sprayed onto a rotating Tablet bed followed by drying, which facilitates the removal of the solvent leaving behind the deposition of thin film of coating materials around each tablet.
Film coating is the most commonly used process for Tablet coating.
Film Coating is the preferred technique for tablet coating because of advantages like
- Retains shape of original core
- Logo or ‘break lines’ will not be distorted during coating
- Simple & automated process
- Easily adaptable for controlled release allows for functional coatings
Essential Tablet Requirements for Coating:
- Tablets must be resistant to abrasion and chipping (hardness and friability should be good)
- Surface defects should not be there as coating is done on all exposed surface
- The physical shape of the tablet is important (to avoid twinning formation)
MODIFIED RELEASE COATING:
Modified release preparations have been designed in such a way that the rate or place at which the active pharmaceutical ingredients are released has been modified.
Coating can be applied directly to the Tablet or indirectly by coating small pellets before compression.
Layering of active drug on to neutral core (bead) followed by one or more rate controlling functional members, allows controlled/modified release.
COATING MATERIALS:
- Polymers
- Solvents
- Plasticizers
- Opaquant extenders
- Colorants
Ideal characteristics of coating materials:
- Solubility in the coating solution
- Capacity to produce elegant looking product
- Stability in presence of heat, moisture and air
- Essentially no odour or taste
- Compatibility with common coating solution additives
- Nontoxic and ease of application
- Resistance to cracking and should act as barrier
Polymers:
Polymer is the basic material used for formation of the Tablet coat
Polymers can be:
- Film Coating – such as Hydroxypropyl methylcellulose, Methyl Hydroxyethyl Cellulose, Ethyl cellulose, Hydroxypropylcellulose, Povidone
- Enteric – such as Cellulose Acetate Phthalate, Acrylate Polymers, Hydroxypropyl Methylcellulose Phthalate, Polyvinyl Acetate Phthalate.
- Enteric are non-aqueous
Solvents:
Primary function is to dissolve or disperse coating materials.
Important considerations of the Ideal solvent system are:
- Should dissolve or disperse the polymer system.
- Should easily disperse other coating solution components.
- Should have no color/taste/ odor,
- should be nontoxic, inert & non-flammable
- Should rapidly dry
- Should be environment friendly
- Examples: water, ethanol, methanol, isopropanol, methylene di-chloride, Acetone
Plasticizers:
Plasticizer provides elasticity to the polymer and helps to spread the polymer uniformly on the tablet surface
- They help to provide flexibility, tensile strength or adhesion properties of the resulting film
Plasticizers can be water soluble or insoluble
Water soluble
- Polyethylene glycol
- Propylene glycol
Water insoluble
- Tributyl citrate
- Acetylated monoglyceride
- Dibutyl sebacate
Opaquant Extenders:
Opaquant provide the opaqueness to the Tablet.
These are fine inorganic powders added with the colorants to provide
- Opaqueness to the Tablet surface
- Uniform film coverage
Examples of Opaquant extenders:
- Silicates (talc, aluminum silicate)
- Carbonates (magnesium carbonate)
- Sulfates (calcium sulfate)
- Oxides (Mg oxides)
Colorants:
Colorants are used to provide distinctive color and elegance
They may be soluble in the solvent system or suspended as insoluble in solvents.
To achieve proper distribution of suspended colorants in the coating solutions requires the use of fine-powdered colorants (< 10 microns)
Two types of colorants are used:
- Synthetic dyes
- Lakes of dyes