DEFINITIONTablet 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,odour1. Sealing of the tablet cores2. Subcoating 1. Seal tablet core3. Smoothing 2. Sub coating 3. Smoothing 4. Colouring4. Colouring 5. Polishing 6. Printing5. Polishing6. Printing.
Multistage process1. 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 tabletsBrufen® POM• Available in 200mg and 400mg strengthPremarin® POM• Conjugated oestrogens 625mcg (maroon) and 1.25mcg (yellow)Colofac ® P• Mebeverine hydrochloride 100mg Round, white, sugar coatedKalms ® 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.Polymer2.Solvent3.Plasticizer
Film coatingAdvantagessingle step process in relatively short period oftime. Process enables functional coatings to beincorporated into the dosage form.DisadvantagesThere are environmental and safetyimplications of using organic solvents as wellas their financial expense.
FILM COATING POLYMERSIdeal characteristics of a film coating polymer Solubility For 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.
Viscositypolymers 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.
PermeabilityFilm 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.
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.
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
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.
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.
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
Basic process requirements for film coating1. 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 coatingControlled release coating
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 pH2. To protect the stomach from the irritant effect of certain drugs3. To facilitate absorption of a drug that is preferentially absorbed distal to the stomach
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.
Enteric sugar coatingThe 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 materialPermeable to intestinal fluidCompatibility with coating solution and drugFormation 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 pharmaceuticalformulations as coating materials. Polymer Trade name ApplicationShellac EmCoat 120 N Enteric Coatings Marcoat 125 Taste/Odor MaskingCellulose acetate Aquacoat CPD® Enteric Coatings Sepifilm™ LP Taste masking Klucel® Sustained release coating Aquacoat® ECD Sub coat moisture and barrier Metolose® sealant pellet coatingPolyvinylacetate phthalate Sureteric® Enteric CoatingsMethacrylate Eudragit® Enteric Coatings Sustained Release Taste Masking Moisture protection Rapidly disintegrating Films
Press coatingprocess involves compaction of coating materialaround a preformed core. The technique differsfrom sugar and film coating process.Advantages:This process enables incompatiblematerials to be formulated together, such thatone chemical or more is placed in the core andthe other (s) in the coating material.Disadvantages :Formulation and processing ofthe coating layer requires some care and relativecomplexities of the mechanism used in thecompressing equipment.
Accela-Cota: It is a prototype of perforatedcylindrical drum providing high drying aircapacity. Therefore it is preferred for filmcoating.
Hi-coater system: The drying air is directedinto the drum is passed through the tabletbed, and is exhausted through theperforations 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 coreChipping: 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.