Tablet Coating

What is tablet coating?

Applying an outer layer of coating materials to the surface of dosage form (tablets, powders, granules, capsules) to achieve a specific purpose i.e. improve appearance, protect drug, improve patient acceptability or modify drug release

Coating can be applied to:

• Tablets

• Granules

• Capsules

• Powders

Why coat tablets?

• Protect drug from environmental degradation

• Mask unpleasant taste

• Prevent potential interaction between incompatible components

• Enable ease of handling

• Improve ease of swallowing

• Improve product appearance

• Improve product identification

• Increase product resistance to damage during handling

• Impart modified-release properties

Types of tablet coating

1. Sugar coating

2. Film coating

3. Compression coating

What is film coating?

Deposition of polymer-based liquid on the dosage form to form a thin film of coating

Can be used for tablets, capsules, powders and granules

Coating material comprises:

• Polymer

• Plasticizer

• Colourant

• Solvent

Advantages of film coating

• Can be used for other dosage forms apart from tablets

• Minimal weight increase due to coating materials, about 2-3%

• Single stage process

• Reduced processing time about 1.5 – 2 hours

• Formulation flexibility – can be easily adapted for modified or controlled release

• Coating allows branding and easy identification of product

• Can improve taste and odour masking

• Can improve appearance and swallowing

• Can protect tablets from moisture, light and oxygen

Disadvantages of film coating

• Retains contour of core dosage form

• Use of volatile organic solvents which are:

  • Flammable

  • Toxic

  • Environmental pollutants

• These have now been replaced with less volatile solvents and water

• Coating materials can be expensive

• Requires specialised coating equipment

• Coating defects can occur if process conditions aren’t controlled

• Water-based coatings may affect moisture-sensitive drugs

Film coating process

1. Spray coating liquid onto dosage form

2. Dry dosage form

3. Film coating adheres

• Wet atomised droplets of coating liquid sprayed onto dosage form

• Solvent evaporated and liquified droplets shrink on the surface of dosage form

• Droplets coalesce into a thin solid film

Film coating formation

For film coating to form:

• Coating formulation must progress from discrete particles to continuous film

• Importance of Glass transition temperature (Tg)

• Polymers and plasticizers have to undergo glass transition for film coating to form

  • High Tg means more rigidity and brittleness

  • Impact on physical state of API

  • Tg of polymer should be a lot lower than melting point of API and all excipients in tablet being coated

  • Low Tg allows film coating to be more readily formed

Types of film coating - immediate-release film coating

• Coating doesn’t affect drug release form final dosage form

• Coating is readily soluble in gastric fluid

• Immediate-release aesthetic film coating - improves product appearance

• Immediate-release barrier film coating:

  • Maintains product stability against environmental degradation

  • Provide taste masking, particularly important in paediatric dosage forms

Types of film coating - modified-release film coating

1. Gastro-resistant coating

• Coating is only soluble at pH above 5-6

• Prevents drug release in the stomach

2. Extended-release coating

• Coating are mostly insoluble in water

• Allows release of drug from dosage form to occur over a prolonged duration

Film coating formulation

Film-coating formulation contains:

• Polymer

• Plasticizer

• Colourants

These are the solid contents which make up to 5%-35%

• Solvent - main component, removed by drying

Process equipment:

• Conventional pan coating

• Contemporary side-vented pan coating

• Fluidised bed coating

What is the polymer?

Main solid content - 60%-70% of solid content

Polymer properties that affect coating process and film coating quality:

• Solubility

• Viscosity

• Permeability

• Mechanical properties

Polymer properties - solubility

• Determines solubility of film-coating formed

• Affects biopharmaceutics of dosage form

Polymer properties - viscosity

• Determines ease of film coating application

• Transfer and atomisation of coating liquid

• Quality of film coating formed

Polymer properties - permeability

• Determines resistance to external environment

• Other components in coating formulation

IR barrier film coatings:

• Taste masking

• Stability against moisture and other environmental factors

MR film coatings:

• Gastro-resistant formulations

Polymer properties - mechanical properties

• Determines quality of film coating formed

• Film strength - resistant to mechanical stress

• Film flexibility - resistance to cracking

• Film adhesion - ensures integrity of film coating

Table showing examples of IR film coating polymers and why they’re used (1)

Solubility in aqueous medium

Table showing examples of MR film coating polymers and why they’re used (2)

Limited or no aqueous solubility

What is a plasticizer? - requirements, mechanisms and examples

• 6 – 7% of solid content

• Impart flexibility on coating

• Reduces tendency for film coat to crack

• Improves film coat adhesion to substrate

Plasticizer requirements:

• Compatible with polymer

• Soluble in solvent system being used for polymer

• Retained in film after formation

Mechanism:

1. Plasticizers penetrate into the polymer mass

2. Separate polymer chains

3. Reduce polymer-polymer contact

4. Reduce rigidity

5. Increase ductility

Examples: glycerin, polyethylene glycols, propylene glycol, citrate esters – triethyl citrate, phthalate esters

What is a colourant?

• 15%-30% of solid content

• Improve product appearance

• Aid product identification

• Improve stability of coated products

• Pigments - water insoluble

They:

• Reduce permeability to moisture

• Chemically stable towards light

• Increase stability to moisture and light sensitive APIs

• Increase solid content but not viscosity – ease of application and drying

• Careful consideration in colour selection

• Patient acceptance

Examples: iron oxide, titanium dioxide, aluminium lakes

What is a solvent? - advantages, disadvantages and examples

• Dissolve or disperse coating materials

• Organic solvents

Advantage:

• Rapid drying times

Disadvantages:

• Operator safety

• Environmental hazard

• Solvent residues

• Cost

Aqueous coating formulations:

• Water

• Aqueous polymer dispersions - suspensions and lattices

When is use of solvent limited?

• Coating process will not accommodate the use of water e.g. due to poor drying

• Unacceptable adhesion with the use of water

• Product being coated will have stability issues with water introduced

• Polymer being used is water insoluble

Examples of organic solvents used:

• Methanol

• Dichloromethane

• Acetone

Ideal properties of film coated tablets

• Even coverage and colour of coating on surface of dosage form

• Void of defects

• Product appearance

• Product functionality or stability

• Compliant with compendial requirements of finished dosage form

What is sugar coating?

Applying multiple layers of sugar-based materials to the surface of a tablet

The coating is built up through several stages until the tablet has a smooth, often glossy finish

• Conventional pan coating technology

• Coating materials are readily soluble in water

• Rapidly disintegrate in water

• Immediate release of drug product

• Modified-release – if MR polymer applied to coated tablet during sealing

Advantages of sugar coating

• Improves product appearance

• Taste masking

• Ease of swallowing

• Can protect drug from environmental factors

Disadvantages of sugar coating

• Significant increase in tablet weight 50% -100%

• Multi-stage process

• Longer drying times

• Less applicable to functional coating

• Limited to the coating of tablets

• Branding

What is compression coating (press coating)?

Involves surrounding an already formed tablet core with a layer of dry powder, which is then compressed around the core to form an outer layer

• It does not require a liquid coating solution

• Process done on same tabletting machine

• Used to separate incompatible materials

• Inert layer introduced to maintain product stability

Advantages of compression coating

• Avoids exposure to water or organic solvents

• Suitable for moisture-sensitive or solvent-sensitive drugs

• Can separate incompatible ingredients

• Can be used to produce delayed-release formulations

Disadvantages of compression coating

• Requires specialised equipment

• More complex than conventional tablet compression

• Can increase tablet size

• Requires accurate positioning of the tablet core