An Overview on Tablet Coating

Tablet coating:
Coating is a process by which an essentially dry, outer layer of coating material is applied to the surface of a dosage form in order to confer specific benefits that broadly ranges from facilitating product identification to modifying drug release from the dosage form. Coating may be applied to multiple range of oral solid dosage form, including tablets, capsules, multi-particulates and drug crystals.

 RATIONALE FOR COATING A SOLID DOSAGE FORM :

Coating of a solid dosage form is designed to perform a specific function. For example; protection against moisture, taste masking pH, time controlled.

Tablets can be easily coated and a variety of products are available in the market. Generally the coating process gives the following advantages:

1. Increased bioavailability
2. Improved patient acceptance
3. Formulation stability

The rationale for coating s solid dosage form such as tablet can be categorized into three main headings:
1. Therapy
2.Technology
3. Marketing

Therapy:
1. To minimize irritation of the esophagus and stomach.
2. Minimize inactivation of active ingredient in the stomach due to acidic environment.
3. Control the duration and site of drug release.
4. Improve patient compliance e.g. easier to swallow, masks unpleasant taste.

Technology:
1. Separate the reactive component in the tablet.
2. Minimize dust formation and contamination with respect to tablets.
3. Easily identify a branded product.
4. Facilitates their handling on packaging equipment and on high speed automated filling.
5. Improve drug stability e.g. protect the drug from oxygen, moisture and light, the three key causes of drug degradation.

Marketing:
1. Mask the unattractive and mottled tablet core.
2. Mask unpleasant taste.
3. Improve product stability.

Primary components involved in tablet coating:

·         Tablet properties

·         Coating process

·         Coating equipment's

·         Parameters of the coating process

·         Facility and ancillary equipment's

·         Automation in coating processes

Advantages of tablet coating:

• Tablet coatings must not make tablets stick together during the coating process must follow the fine contours of embossed characters or logos on tablets and must be stable and strong enough to survive the handling of the tablet.
• Printing on tablets can also be done by coatings, if required. Coatings are necessary for tablets giving a smoother finish, makes large tablets easier to swallow and also to mask the unpleasant taste.

 Disadvantages of tablet coating

• Limitations of sugar coating such as relatively high cost, long coating time and high bulk have led to the use of other coating materials.
• However the process of coating is tedious and time-consuming and it requires the expertise of highly skilled technician.

Materials used in film coating

Sr.no	Material	Type	Example
1	Film formers	Enteric, Non enteric	HPMC, Ethyl cellulose, Sod. CMC, Polyethylene Glycol, Cellulose acetate phthalate, HPMCP
2	Solvent	-------	IPA and Methylene chloride.
3	Plasticizers	Internal plasticizing External plasticizing	Castor oil, Propylene glycol, Glycerin, surfectants, polysorbates.
4	Colorants	Inorganic colorants Natural colorants.	FD and C colors, D and C colors, iron oxides, Carmel, turmeric.
5	Opacifiers	----------	Titanium dioxide, talc, aluminum silicate.

 METHOD OF TABLET COATING:
Techniques used in tablet coating: Generally three method are used for tablet coating
1. Sugar coating
2. Film coating
3. Enteric coating

1. Sugar coating: Traditionally sugar coating form the bulk of the coated tablet, and it is a multistage process involves six separate operations:

• Seal tablet core: Application of water impermeable polymer such as Shellac, Cellulose acetate phthalate and Polyvinyl acetate phthalate, which protect the core tablet from moisture, harden the tablet surface and increased its shelf life.

• Sub coating: It causes a rapid build up to round off the tablet edges by adding the building agents such as Calcium carbonate or talc in combination with sucrose solution.

• Smoothing: Smoothes out the sub-coated surface with the application of sucrose syrup and increases the tablet size to the predetermine size.
• Coloring: Gives the tablet its color and finished size, often titanium based pigments are used.

• Polishing: Effectively polished to give characteristic shine, commonly used beeswax, carnauba wax.

• Printing: Indelible link for characterization.

2.  Film Coating: It is a modern approach to coating tablets, capsules or pellets by surrounding them with a thin layer of polymeric material. It a single stage process, which involves spraying a coating solution containing the following:

• Polymer
• Plasticizer
• Colorant
• Solvent

The solution is sprayed onto a rotating tablet bed followed by drying which facilitates the removal of solvent leaving behind the deposition of thin film of coating materials around each tablet.

* Polymer used in film coating:
1. Enteric polymers: Cellulose acetate phthalate, Polyvinyl acetate phthalate, Shellac, Polymethacrylates.

2. Non-enteric polymers: Hypromellose, Hydroxyethyl cellulose, Carboxymethyl cellulose, Polyethylene glycol.

* Plasticizer used in film coating:
1. Polyols type: Glycerol, Propylene glycol, Polyethylene glycol

2. Organic esters: Phthalate and citrate esters

3. Oil/Glycerides: Castor oil, Fractionated oil.

* Colorant used in film coating: 1. Organic dyes and their lakes: Sunset yellow, Tartazine, Erythrosine

2. Inorganic colors: Iron oxide yellow, Titanium oxide

3. Natural Colors: Riboflavin, Carmine

* Solvents used in film coating:
1. Organic solvents: Methylene chloride, Isopropyl alcohol

2. Inorganic solvents: Water

Ideal Properties of Film Coating Material:

• Solubility in solvent of choice for coating preparation
• Solubility requirement for the intended use e.g. free water solubility, slow water solubility or pH- dependent solubility.
• Capacity to produce an elegant looking product.
• High stability against heat, light, moisture, air and the substrate being coated.
• No inherent color, taste or odor.
• High compatibility with other coating solution additives.
• Non-toxic with no pharmacological activity.
• High resistance to cracking.
• Film former should not give bridging or filling of the debossed tablet.
• Compatible to printing procedure. 

Merits of film coating over sugar coating:

Film Coating	Sugar Coating
Tablet appearance:
· Retain shape of original core	· Rounded with high degree or polish
· Small weight increase of 2-3% due to coating material	· Larger weight increase 30-50% due to coating material
· Logo or break lines possible	· Logo or break lines are possible
Process:
· Can be automated e.g. Accela cota	· Difficult to automated e.g.Taditional coating pan
· Easy training operation	· Cosiderable training operation required
· Single stage process	· Multistage process
· Easily adaptable for controlled release allows for functional coating.	· Not able to be used for controlled release apart from enteric coating.

Tablet coating defect:

S. No.	Tablet Defect	Definition	Reason	Remedies
1	Blistering	Local detachment of film from the substrate forming blister.	Entrapment of gases in or underneath the film due to overheating either during spraying or at the end of the coating run	Use mild drying condition
2	Chipping	The film becomes chipped and dented, usually at the edges of the tablet	Decrease in fluidizing air or speed of rotation of the drum in pan coating	Not to over dry the tablet in preheating stage.
3	Cratering	Volcanic-like craters appears exposing the tablet surface	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.	Use efficient and optimum drying conditions. Increase viscosity of coating solution to decrease spray application rate
4	Picking	Isolated areas of film are pulled away from the surface when the tablet sticks together and then part	Over wet tablet bed where adjacent tablets can stick together and then break apart.	Use optimum and efficient drying conditions or increase the inlet air temperature
5	Pitting	Pits occur in the surface of a tablet core without any visible disruption of the film coating	Temperature of the tablet core is greater than the melting point of the materials used in the tablet formulation.	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.
6	Blooming	coating becomes dull immediately or after prolonged storage at high temperatures	It is due to collection on the surface of low molecular weight ingredients included in the coating formulation	Decrease plasticizer concentration and increase molecular weight of plasticizer.
7	Blushing	whitish specks or haziness in the film	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.	Decrease the drying air temperature Avoid use of sorbitol with Hydroxy Propyl Cellulose, Hydroxy Propyl Methyl Cellulose, Methyl Cellulose and Cellulose ethers
8	Twinning	Two tablets stick together	Common problem with capsule shaped tablets	By balancing the pan speed and spray rate.
9	Colour variation	Variation in the colour of the film	Alteration of the frequency and duration of appearance of tablets in the spray zone or the size/shape of the spray zone	Geometric mixing, reformulation with different plasticizers and additives or used mild drying conditions
10	Orange peel effect	Film becomes rough and non glossy. Appearance is similar to that of orange	Inadequate spreading of the coating solution before drying	Use mild drying conditions. Use additional solvents to decrease viscosity of solution
11	Mottling	Uneven distribution of colour on the surface of tablet with dark and light patches.	Due to different colouration of the excipient or the degradation product of the tablet is coloured.	Coating solution is prepare properly in sufficient quantity.
12	Cracking or Splitting	Film either cracks across the crown of the tablet(cracking), or splits around the edges of the tablet (splitting).	Internal stress in the film exceeds tensile strength of the film.	Use of high molecular weight polymer increase the tensile strength of the film.