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TABLET COATING

Anju K Johnmpharm

DEFINITION

Tablet coating is the application of a coating material to the exterior of a tablet with the intention of conferring benefits and properties to the dosage form over the uncoated variety.

Also applicable to modified release dosage forms &also to hard-shell and soft elastic capsules (less extent)

REASONS FOR COATING TABLETS• protection particularly from

light and moisture.• Mechanical strength,reduse

cross condamination, dusting prevented, mask tastes,easier to swallow

• Coloured coating rapid identification, patient compliance, in marketing brand identification

• Functional film coatings used to impart enteric or controlled-release properties

Types of tablet coatingThree main types are in use:• Film coating• Sugar coating• Press coating

Sugar coating • multistage process• Increase bulk,mask taste,odour

1. Sealing of the tablet cores2. Subcoating3. Smoothing4. Colouring5. Polishing6. Printing.

Multistage process 1. Sealing tablet core- application of a water

impermeable polymer such as Shellac, cellulose acetate phthalate and polyvinyl acetate phthalate, which protects the core from moisture, increasing its shelf life.

2. Sub coating -by adding bulking agents such as calcium carbonate or talc in combination with sucrose solution.

3. Smoothing process -remove rough layers formed in step 2 with the application of sucrose syrup.

4. Colouring - for aesthetic purposes often titanium based pigments are included.

5. Polishing - effectively polished to give characteristic shine, commonly using beeswax, carnauba wax.

6. Printing -indelible ink for characterisation.

Example of sugar coated tablets Brufen® POM• Available in 200mg and

400mg strength

Premarin® POM• Conjugated oestrogens

625mcg (maroon) and 1.25mcg (yellow)

Colofac ® P• Mebeverine hydrochloride

100mg Round, white, sugar coated

Kalms ® GSL• 45mg Hops powder,90mg

Gentian powdered extract, and 135mg Valerian powdered extract

Film coating• Modern approach to coating tablets,

capsules, or pellets by surrounding them with a thin layer of polymeric material. 

• Process: Single stage process, which involves spraying a coating solution containing the following;

1.Polymer 2.Solvent 3.Plasticizer4.Colourant

Film coating

Advantages single step process in relatively short period of time. Process enables functional coatings to be incorporated into the dosage form.

Disadvantages There are environmental and safety implications of using organic solvents as well as their financial expense.

FILM COATING POLYMERSIdeal characteristics of a film coating

polymerSolubilityFor conventional film coating the polymer should have good solubility in aqueous fluids to facilitate the dissolution of the active ingredient from the finished dosage form. However, where a modified-release action is required then a polymer system of low water solubility or permeability will be chosen.

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Viscosity

polymers should have a low viscosity for a given concentration. This will permit the easy, trouble-free spraying of their solutions in industrial film coating equipment.

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Permeability

Film coating can be used to optimize the shelf-life of a tablet preparation, as some polymers are efficient barriers against the permeability of water vapour or other atmospheric gases. These properties vary widely between the individual polymers.

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Mechanical propertiespolymer must be • one with adequate strength to

withstand the impact and abrasion encountered in normal handling. (development of cracks )

• comply with the relevant regulatory and pharmacopoeial requirements

Shellac • Material of natural origin- purified resinous

secretion of the insect Laccifer lacca.

• Oldest known material used for enteric coatings.

• Suited for drug targeting in the distal small intestine as soluble at pH 7.0

• Its use is now less popular in commercial pharmaceutical applications for enteric coatings. Due to poor batch to batch reproducibility, which is a crucial requirement.

Shellac

Cellulose acetate phthalate (CAP)

Chemical name: Cellulose acetate phthalate Trade name: CAP, Aquateric Application form: organic or aqueous

dispersion Functional groups: acetyl, phthalyl Soluble above pH: 6Additional remarks: sensitive to hydrolysis,

5-30% plasticizer required.

Polyvinyl acetate phthalate (PVAP)

• Chemical name: polyvinyl acetate phthalate

• Trade name: Opadry enteric (aqueous), Coloron

• Application form: organic solution, aqueous dispersion.

• Functional groups: acetyl, phthalate, vinylacetat :crotonic acid ratio 90:10.

• Soluble above pH: 5• Additional remarks: Plasticizer is

required.

Acrylic polymers • Chemical name: Methacrylic • Trade name: Eudragit® • Application form: organic solution

or aqueous dispersion.• Functional groups: methyacrylic

acid • Soluble above pH: 5 * depends on

co- polymers used.

Polymer dissolution Factors affecting the release of a drug from a polymer:

• Thickness of the coating material• pH• Other excipients • Ionic state

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Plasticizers to modify the physical properties of

the polymer to decrease film brittleness. Examples of plasticizers are:polyols, such as polyethylene glycol

400organic esters, such as diethyl

phthalateoils/glycerides, such as fractionated

coconut oil. only water-miscible plasticizers can

be used for aqueous-based spray systems.

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Colourantswater-insoluble colours (pigments). Pigments have certain advantages

over water-soluble colours: they tend to be more chemically stable towards light, provide better opacity and covering power, and optimize the impermeability of a given film to water vapour.

Examples of colourants are:iron oxide pigments titanium dioxide aluminium Lakes.

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Solvents

• water is used as polymer solvent • The disadvantages of organic solvents

for the process:organic solvent vapor into the

atmosphere is ecologically unacceptable, and efficient solvent vapor removal from gaseous effluent is expensive.

Explosive ,so safety prblem

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Basic process requirements for film coating

1. adequate means of atomizing the spray liquid for application to the tablet cores.

2. adequate mixing and agitation of the tablet bed.

3. sufficient heat input in the form of drying air to Provide the latent heat of evaporation of the solvent.

4-good exhaust facilities to remove dust- and solvent-laden air.

Functional coatings

Functional coatings are coatings, which perform a pharmaceutical function. These include;

Enteric coating

Controlled release coating

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Enteric coatingThis technique is used to protect

the tablet core from disintegration in the acid environment of the stomach for one or more of the following reasons:

1. Prevention of acid attack on active constituents unstable at low pH

2. To protect the stomach from the irritant effect of certain drugs

3. To facilitate absorption of a drug that is preferentially absorbed distal to the stomach

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Enteric film coatingThe enteric polymers (CAP,PVAP,

suitable acrylic derivative ) are capable of forming a direct film in a film-coating process. Sufficient weight of enteric polymer must be used to ensure an efficient enteric effect. This is normally two or three times that required for a simple film coating.

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Enteric sugar coating

The sealing coat is modified to comprise one of the enteric polymers in sufficient quantity to pass the enteric test for disintegration. The subcoating and subsequent coating steps are then as for conventional sugar coating.

Examples of enteric coated OTC products

• Enteric coated aspirin E.g. Micropirin® 75mg EC tablets

• Enteric coated peppermint oil E.g. Colpermin®

The ideal properties of enteric coated material

Permeable to intestinal fluidCompatibility with coating solution

and drug Formation of continuous filmNontoxicCheap and ease of applicationAbility to be readily printedResistance to gastric fluids

Controlled-release coatings After film coating these particles

are filled into hard gelatin shells, or occasionally compressed directly into tablets by a process which permits minimal rupture of the applied film.

The coatings involved use polymers with restricted water solubility or permeability, and include ethylcellulose and modified acrylate derivatives.

Polymers used in pharmaceutical formulations as coating materials.

Polymer Trade name Application Shellac EmCoat 120 N

Marcoat 125 Enteric Coatings Taste/Odor Masking

Cellulose acetate Aquacoat CPD®  Sepifilm™ LP Klucel® Aquacoat® ECD Metolose®

Enteric Coatings Taste masking Sustained release coating Sub coat moisture and barrier sealant pellet coating

Polyvinylacetate phthalate Sureteric® Enteric Coatings

Methacrylate Eudragit®  Enteric Coatings Sustained Release Taste Masking Moisture protection Rapidly disintegrating Films

Press coating process involves compaction of coating material around a preformed core. The technique differs from sugar and film coating process. Advantages:This process enables incompatible materials to be formulated together, such that one chemical or more is placed in the core and the other (s) in the coating material. Disadvantages :Formulation and processing of the coating layer requires some care and relative complexities of the mechanism used in the compressing equipment.

Coating equipments• coating pans

• fluid beds

Compression Coating Machines

Accela-Cota: It is a prototype of perforated cylindrical drum providing high drying air capacity. Therefore it is preferred for film coating.

Hi-coater system: The drying air is directed into the drum is passed through the tablet bed, and is exhausted through the perforations in the drum.

Tablet coating problems• Picking and sticking•Bridging•Capping•Erosion•Peeling and frosting•Mottled color•Chipping•Orange peel•Twinning

Picking and sticking: This is when the coating removes a piece of the tablet from the core.

• Caused by over-wetting the tablets, by under-drying, or by poor tablet quality.

Bridging: This occurs when the coating fills in the lettering or logo on the tablet.– improper application of the

solution, –poor design of the tablet

embossing,

Capping: This is when the tablet separates in laminar fashion.

• The problem stems from – improper tablet compression–over-dry the tablets in the

preheating stage. That can make the tablets brittle and promote capping.

Erosion: This can be the result of soft tablets, an over-wetted tablet surface, Inadequate drying or lack of tablet surface strength.

Peeling and frosting: This is a defect where the coating peels away from the tablet surface in a sheet.

• This could be due to a defect in the ,–coating solution,–over-wetting or –high moisture content in the tablet core

Chipping: This is the result of high pan speed, a friable tablet core, or a coating solution that lacks a good plasticizer.

 

Mottled color: This can happen when the ,–coating solution is improperly

prepared– the actual spray rate differs from the

target rateOrange peel: This refers to a coating

texture that resembles the surface of an orange.

• It is usually the result of high atomization pressure in combination with spray rates that are too high.

• By thinning the solution prevented

Twinning: This is the term for two tablets that stick together, and it’s a common problem with capsule shaped tablets.

–We can solve this problem by balancing the pan speed and spray rate.

–Try reducing the spray rate or increasing the pan speed.

REFERENCE• Pharmaceutical dosage forms:

Tablets volume III,edited by A.Lieberman,Leon Lachman,and Joseph B.Schwartz.

• Dispensing for pharmaceutical students by Cooper and Gunn’s.