What will you gain?1.9.1Introduction1.9.2Aspects of tablet coating1.9.3Basic principle of tablet coating1.9.4Type of tablet coating process1.9.4.1 Sugar coating1.9.4.2 Film coating1.9.4.3 Enteric coating1.9.4.3.1 Enteric sugar coating1.9.4.3.2 Enteric film coating1.9.4.3.3Controlled release coating1.9.4.4 Specialized coating1.9.4.4.1 Compressed coating1.9.4.4.2 Electrostatic coating1.9.4.4.3 Dip coating1.9.4.4.4 Vacuum film coating1.9.5Equipments1.9.6Processing parameters

1.9.1 Introduction (1-3,5)Coatedtablets are defined as tablets covered with one or more layers of mixture ofvarious substances such as natural or synthetic resins ,gums ,inactive andinsoluble filler, sugar, plasticizer, polyhydric alcohol ,waxes,authorized colouring materialand some times flavoring material .Coatingmay also contain active ingredient. Substances used for coating are usuallyapplied as solution or suspension under conditions where vehicle evaporates.1.9.2 Aspects oftablet coatingI. Therapyi) Avoid irritation of oesophagus and stomach ii) Avoid bad taste iii) Avoid inactivation of drug in the stomach iv) Improve drug effectivenessv) Prolong dosing intervalvi) Improve dosing intervalvii) Improve patient complianceII. Technology i) Reduce influenceof moisture ii) Avoid dust formation iii) Reduce influence of atmosphere iv) Improve drug stability v) Prolong shelve life III. Marketing i) Avoid bad taste ii) Improve product identity iii) Improve appearance and acceptability 1.9.3 Basic principle of tabletcoating The principle of tablet coating is relativelysimple. Tablet coating is theapplication of coating composition to moving bed of tablets with concurrent useof heated air to facilitate evaporation of solvent. Basicprinciples involvei) Insulation whichinfluences the release pattern as little as possible and does not markedlychange the appearance.ii) Modified release with specific requirement andrelease mechanism adapted to body function in the digestive tractiii) Colour coating which provides insulation or iscombined with modified release coating.1.9.4 Type of tablet coating process1.9.4.1 Sugar coating (1,3,5)Compressedtablets may be coated with coloured or uncoloured sugar layer. The coating iswater soluble and quickly dissolves after swallowing. The sugarcoat protectsthe enclosed drug from the environment and provides a barrier to objectionabletaste or order. The sugar coat also enhances the appearance of the compressedtablet and permit imprinting manufacturings information. Sugar coatingprovides a combination of insulation, taste masking, smoothing the tablet core,colouring and modified release. The disadvantages of sugar coating are the timeand expertise required in the coating process and thus increases size, weightand shipping costs.Sugar coating process involves five separateoperations:I. Sealing/Water proofing: provides a moisture barrier and harden the tablet surface.II. Subcoating:causes a rapid buildup to round off the tablet edges.III. Grossing/Smoothing:smoothes out the subcoated surface and increases the tablet size topredetermine dimension.IV. Colouring: givesthe tablet its colour and finished size.V. Polishing:produces the characteristics gloss.I. Sealing/WaterproofingPriorto applying any sugar/water syrup, the tablet cores must be sealed, thoroughlydried and free of all residual solvents. The seal coat provides a moisture barrier and hardnessthe surface of the tablet in order to minimize attritional effects. Coretablets having very rapid disintegration rates conceivably could start thedisintegration process during the initial phase of sugar coating. The sealantsare generally water-insoluble polymers/film formers applied from an organicsolvent solution. The quantities of material applied as a sealing coat willdepend primarily on the tablet porosity, since highly porous tablets will tendto soak up the first application of solution, thus preventing it from spreadinguniformly across the surface of every tablet in the batch. Hence, one or morefurther application of resin solution may be required to ensure that the tabletcores are sealed effectively. Common materials used as a sealant include Shellac, Zine,Cellulose acetate phthalate (CAP), Polyvinylacetate phthalate,Hyroxylpropylcellulose, Hyroxypropylmethylcellulose etc.II. SubcoatingSubcoating is the actual start of the sugar coatingprocess and provides the rapid buildup necessary to round up the tablet edge.It also acts as the foundation for the smoothing and colour coats.Generally two methods are used for subcoating:i) The applicationof gum based solution followed by dusting with powder and then drying. Thisroutine is repeated until the desired shape is achieved.ii) The application of a suspension of dry powder ingum/sucrose solution followed by drying.Thus subcoating is a sandwich of alternate layer of gumand powder. It is necessary to remove the bulk o the water after eachapplication of coating syrup.Table.23. Typical Binder Solution Formulation For Subcoating(1)%W/W%W/W

Gelatin63.3

Gum acacia (powdered)88.7

Sucrose (powdered)4555.3

Distilled waterto100to100

Table.24. Typical Dusting Powder Formulation For Subcoating(1)%W/W%W/W

Calciumcarbonate 40.0-

Titaniumdioxide5.01.0

Talc,asbestos free25.061.0

Sucrose(powdered )28.038.6

Gumacacia (powdered)2.0-

Table.25. Typical Suspension Subcoating Formulation(1)%W/W

Sucrose40.0

Calciumcarbonate20.0

Talc,asbestos free12.0

Gumacacia(powdered)2.0

Titaniumdioxide1.0

Distilledwater25.0

III. Grossing/ smoothingThe grossing/smoothing process is specifically for smoothing and filing theirregularity on the surface generated during subcoating. It also increases thetablet size to a predetermined dimension.If the subcoating is rough with highamount of irregularities then the use of grossing syrup containing suspendedsolids will provide more rapid buildup and better filling qualities. Smoothingusually can be accomplished by the application of a simple syrup solution(approximately 60-70 % sugar solid). This syrup generally contains pigments,starch, gelatin, acacia or opacifier if required.Small quantities of coloursuspension can be applied to impart a tint of the desired colour when there areirregularities in coating.IV.Colour coatingThis stage is often critical in the successfulcompletion of a sugar coating process and involves the multiple application ofsyrup solution (60-70 % sugar solid) containing the requisite colouring matter.Mainly soluble dyes were used in the sugar coating to achieve the desiredcolour, since the soluble dye will migrate to the surface during drying. Butnow a days the insoluble certified lakes have virtually replaced the solubledyes in pharmaceutical tablet coating. The most efficient process for colourcoating involves the use of a predispersed opacified lake suspension. V. PolishingSugar-coated tablets needs to be polished to achievea final elegance. Polishing is achieved by applying the mixture of waxes likebeeswax, carnubawax, candelila wax or hard paraffin wax to tablets in polishingpan.1.9.4.2 Film CoatingFilm coating is more favored over sugar coating.Table.26. Comparison Between Film Coating And Sugar Coating(1)FEATURESFILM COATINGSUGAR COATING

Tablet:AppearanceWeightincrease because of coating materialLogoor break linesRetain contour of original core.Usually not as shiny as sugar coat type2-3%PossibleRounded with high degree of polish30-50%Not possible

ProcessOperatortraining requiredAdaptabilityto GMPProcessstagesFunctionalcoatingsProcess tends itself to automation and easy training of operatorHighUsually single stageEasily adaptable for controlled releaseConsiderableDifficulty may ariseMultistage processNot usually possible apart from enteric coating

Process description (1)Film coating is deposition of a thin film of polymer surrounding thetablet core. Conventional pan equipmentsmay be used but now a days more sophisticated equipments are employed to havea high degree of automation and coating time.The polymer is solubilized into solvent.Other additives like plasticizers and pigments are added. Resulting solution is sprayed onto a rotatedtablet bed. The drying conditions causeremoval of the solvent, giving thin deposition of coating material around eachtablet core. Process details (1)Usually spray process is employed in preparation of filmcoated tablets. Accela cota is theprototype of perforated cylindrical drum providing high drying aircapacity. Fluidized bed equipment hasmade considerable impact where tablets are moving in a stream of air passingthrough the perforated bottom of a cylindrical column. With a smaller cylindrical insert, the streamof cores is rising in the center of the device together with a spray mistapplied in the middle of the bottom. Forfluidized bed coating, very hard tablets (hardness > 20 N) have to be used.Basic process requirements for film coating (2)The fundamental requirements are independent of theactual type of equipments being used and include adequate means of atomizingthe spray liquid for application to the tablet core, adequate mixing andagitation of tablet bed, sufficient heat input in the form of drying air toprovide the latent heat of evaporation of the solvent. This is particularlyimportant with aqueous-based spraying and good exhaust facilities to removedust and solvent laden air.Development of film coating formulations (1)If the following questions are answered concomitantlythen one can go for film coating:i) Is it necessaryto mask objectionable taste, colour and odor?ii) Is it necessaryto control drug release?iii) What tablets size, shape, or colour constrains mustbe placed on the developmental work?Colour, shape and size of final coated tablet areimportant for marketing and these properties have a significant influence onthe marketing strategies. An experiencedformulator usually takes the pragmatic approach and develops a coatingformulations modification of one that has performed well in the past. Spraying or casting films can preliminarilyscreen film formulations. Cast films cabis prepared by spreading the coating composition on teflon, glass or aluminumfoil surface using a spreading bar to get a uniform film thickness. Sprayed films can be obtained by mounting aplastic-coated surface in a spray hood or coating pan. Coating formula optimization (1)Basic formula is obtained from past experience or from various sourcesin the literature. Modifications arerequired to improve adhesion of the coating to the core, to decrease bridgingof installations, to increase coating hardness, etc. Usually concentration of colorant andopaquant are fixed to get predetermined shade.Common modification is to alter polymer-to-plasticizer ratio or additionof different plasticizer/ polymer.Experimentation of this type can be best achieved by fractionalMaterials used in film coating (1,13)I. Film formers, which may be enteric or nonentericII. SolventsIII. PlasticizersIV. Colourants V. Opaquant-ExtendersVI. Miscellaneous coating solution componentsI. Filmformers (1)Ideal requirements of film coating materials aresummarized below:i) Solubility insolvent of choice for coating preparationii) Solubility requirement for the intended use e.g. freewater-solubility, slow water-solubility or pH -dependent solubilityiii) Capacity to produce an elegant looking productiv) High stability against heat, light, moisture, air andthe substrate being coatedv) No inherent colour, taste or odorvi) High compatibility with other coating solutionadditivesvii) Nontoxic with no pharmacological activityviii) High resistanceto crackingix) Film former should not give bridging or filling ofthe debossed tabletx) Compatible to printing procedureCommonly used film formersare as followi. HydroxyPropyl Methyl Cellulose (HPMC)Itis available in different viscosity grades. It is a polymer of choice for airsuspension and pan spray coating systems because of solubility characteristicin gastric fluid, organic and aqueous solvent system. Advantagesinclude: it does not affect tablet disintegration and drug availability, it ischeap, flexible, highly resistant to heat, light and moisture, it has no tasteand odor, colour and other additives can be easily incorporated.Disadvantageincludes: when it is used alone, thepolymer has tendency to bridge or fill the debossed tablet surfaces. So mixture of HPMC and other polymers/plasticizers is used.ii. MethylHydroxy Ethyl Cellulose (MHEC)It is available in wide variety of viscosity grades. It is not frequently used as HPMC becausesoluble in fewer organic solvents.iii. EthylCellulose (EC)Depending on the degree of ethoxy substitution, differentviscosity grades are available. It is completely insoluble in water and gastricfluids. Hence it is used in combinationwith water-soluble additives like HPMC and not alone. Unplasticized ethyl cellulose films arebrittle and require film modifiers to obtain an acceptable filmformulation. Aqua coat is aqueouspolymeric dispersion utilizing ethyl cellulose.These pseudolatex systems contain high solids, low viscositycompositions that have coating properties quite different from regular ethylcellulose solution.iv. HydroxyPropyl Cellulose (HPC)It is soluble in water below 40oc (insolubleabove 45 oC), gastric fluid and many polar organic solvents. HPC is extremely tacky as it dries fromsolution system. It is used for sub coatand not for colour or glass coat. Itgives very flexible film. v. PovidoneDegree ofpolymerization decides molecular weight of material. It is available in four viscosity grades i.e.K-15, K-30, K-60 and K-90. Averagemolecular weight of these grades is 10000, 40000, 160000 and 360000respectively. K-30 is widely used astablet binder and in tablet coating. Ithas excellent solubility in wide variety of organic solvents, water, gastricand intestinal fluids. Povidone can becross-linked with other materials to produce films with entericproperties. It is used to improvedispersion of colourants in coating solution.vi. Sodium carboxy methylcelluloseIt is available in medium, high andextra high viscosity grades. It iseasily dispersed in water to form colloidal solutions but it is insoluble inmost organic solvents and hence not a material of choice for coating solutionbased on organic solvents. Filmsprepared by it are brittle but adhere well to tablets. Partially dried films of are tacky. So coating compositions must be modified withadditives. viii. Polyethyleneglycols (PEG)Lower molecular weightsPEG (200-600) are liquid at room temperature and are used as plasticizers. High molecular weights PEG (900-8000series)are white, waxy solids at room temperature.Combination of PEG waxes with CAP gives films that are soluble ingastric fluids.ix. Acrylate polymersIt is marketed underthe name of Eudragit. EudragitE is cationic co-polymer. OnlyEudragitE is freely soluble in gastric fluid up to pH 5 andexpandable and permeable above pH 5.This material is available as organic solution (12.5% inisopropanol/acetone), solid material or 30% aqueous dispersion. EudragitRL & RS are co-polymers with low content ofquaternary ammonium groups. These areavailable only as organic solutions and solid materials. They produce films for delayed action (pHdependent).II. Solvents(1)Solvents are used to dissolve ordisperse the polymers and other additives and convey them to substratesurface. Idealrequirement are summarized below:i) Should be eitherdissolve/disperse polymer systemii) Should easilydisperse other additives into solvent systemiii) Small concentration of polymers (2-10%) should not inan extremely viscous solution systemcreating processing problemsiv) Should be colourless, tasteless, odorless, inexpensive,inert, nontoxic and nonflammablev) Rapid dryingratevi) No environmental pollutionMostly solvents are used either alone or incombination with water, ethanol, methanol, isopropanol, chloroform, acetone,methylene chloride, etc. Water is moreused because no environmental and economic considerations. For drugs that readily hydrolyze in presenceof water, non aqueous solvents are used. III. Plasticizers(1)As solvent is removed, most polymeric materials tend topack together in 3-D honey comb arrangement.Internal or External plasticizing technique is used to modifyquality of film. Combination ofplasticizer may be used to get desired effect.Concentration of plasticizer is expressed in relation to the polymerbeing plasticized. Recommended levels ofplasticizers range from 1-50 % by weight of the film former. Commonly used plasticizers are castor oil,PG, glycerin, lower molecular weight (200-400 series), PEG, surfactants,etc. For aqueous coating PEG and PG aremore used while castor oil and spans are primarily used for organic-solventbased coating solution. Externalplasticizer should be soluble in the solvent system used for dissolving thefilm former and plasticizer. Theplasticizer and the film former must be at least partially soluble or misciblein each other. IV. Colourants(1)Colourants can be used in solution form or in suspensionform. To achieve proper distribution ofsuspended colourants in the coating solution requires the use of the powderedcolourants (