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INTERNATIONAL JOURNAL OF PHARMACEUTICAL AND CHEMICAL SCIENCES ISSN: 2277 5005

Vol. 1 (3) Jul-Sep 2012 www.ijpcsonline.com 645

Review Article

An Overview: Aqueous Film Coating Technology onTablets

Parmar Krutin D1*

, Bhatt Nayana M1

, Pathak Nimish L1

, Patel Laxmanbhai D1

Kela Amit N1, Nathani Hitesh S1 and Chauhan Vijay V2

1C. U. Shah College of Pharmacy and Research, Wadhwan city, Surendranagar, Gujarat,India.

2 Astron Research Ltd., Ahmedabad, Gujarat, India._________________________________________________________________________ABSTRACTTablet coating is perhaps one of the oldest pharmaceutical processes still in existence. Earlier, Sugarcoating was adopted for pharmaceutical as from confectionary industry. But as it was tedious processand required skilled manipulation, film coating was started to be preferred over sugar coating.Development of film coating was mainly based on solutions of different polymers in various organicsolvents. All these solvents are toxic in nature. Nobody ever was concerned about the problems likematerial cost, toxic effects due to coating or pollution etc. In today's competitive business

environment, any cost saved will improve the market viability and success of any product. Therefore,left with no other choice but to eliminate the use of organic solvents and to start using water as thesolvent system for tablet coating. The main focus of this review is, to study various aspects of aqueous

based film coating.

Keywords:Aqueous film coating, Film formers, Opadry II, Coating Process parameters.

INTRODUCTIONAll drugs have their own chracteristic, likesome drugs are bitter in taste or have anunpleasant odor, some are sensitive to light oroxides, some are hygroscopic in nature

1, 2, 3.

Because of this reasons, tablet coating is thechoice of option to solve such problems in

conventional dosage form. During the middleof 19

thcentury sugar-coating process was a

skilled manipulative operation and could lastfor even five days. The operator must behighly skilled for such coating. In the last 25years, tablet coating has undergone severalfundamental changes. Many modificationswere advocated to improve the basic processand film coating chosen in place of sugarcoating and after that, the researchers weremoved towards to the organic solvent basedcoating to overcome problem occurred withsugar coating.Tablet film coating is performed by two types,

one is aqueous film coating (generally water isused as a solvent) and non aqueous filmcoating (generally organic solvents are used).Some problems are associated with the nonaqueous film coating like safety of employees(as most of the solvents are dangerous, smell,and they are not good to breathe),atmospheric pollution etc. But key problem iswith the approval of the regulatory authority

4.

High quality aqueous film coating must be

smooth, uniform and adhere satisfactorily tothe tablet surface and ensure chemicalstability of a drug.

Tablet as a dosage formA tablet is a pharmaceutical dosage form. Itcomprises a mixture of active substances and

excipients, usually in powder form, pressed orcompacted into a solid. Tablets Dosage formis one of a most preferred dosage form all overthe world. Almost all drug molecules can beformulated in a tablet and process ofmanufacturing of tablets is very simple and isvery flexible. One can administer 0.01 mg to 1gm of a drug dose by oral route ofadministration, by formulating as a tablet.

Tablet CoatingCoating is a process by which an essentiallydry, outer layer of coating material is applied tothe surface of a dosage form in order to confer

specific benefits that broadly ranges fromfacilitating product identification to modifyingdrug release from the dosage form. Aftermaking a good tablet, one must often coatit

4,5,6.

Coating may be applied to a wide range of oralsolid dosage forms, including tablets,capsules, multiparticulates and drug crystals.When coating composition is applied to abatch of tablets in a coating pan, the tablet

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surfaces become covered with a tackypolymeric film. Before the tablet surface dries,the applied coating changes from a stickyliquid to tacky semisolid and eventually to anonsticky dry surface. The entire coatingprocess is conducted in a series ofmechanically operated acorn-shaped coating

pans of galvanized iron stainless steel orcopper. The smaller pans are used forexperimental, developmental, and pilot plantoperations, while the larger pans for industrialproduction

5,6.

Necessity of Tablet Coating 1, 2, 3

A number of reasons can besuggested, like:

The core contains a material whichhas a bitter taste in the mouth or hasan unpleasant odour.

Coating will protect the drug from thesurroundings with a view to improve

its stability. Coating will increase the ease by

which a tablet can be ingested by thepatient.

Coating will develop the mechanicalintegrity; means coated products aremore resistant to mishandling(abrasion, attrition, etc.)

The core contains a substance whichis incompatible in the presence of lightand subject to atmospheric oxidation,i.e. a coating is added to improvestability.

The core alone is inelegant. The active substance is colored and

migrates easily to stain hands andclothes.

The coated tablets are packed onhigh-speed packaging machine.Coating reduces friction and increasespackaging rate.

Coating can modify the drug releaseprofile, e.g., enteric coating, osmoticpump, pulsatile delivery.

Basic principles involved in tablet coatingTablet coating is the application of coatingcomposition to moving bed of tablets with

concurrent use of heated air to facilitateevaporation of solvent. Coating may be a typewhich influences the release pattern of thedrug as little as possible and does notmarkedly change the appearance of theformulation. Coating may modify release ofdrug with specific requirement and releasemechanism adapted to body function in thedigestive tract. It may be color coating whichprovides insulation. Incorporation of another

drug or formula adjuvant in the coating toavoid chemical incompatibilities or to providesequential drug release is also possible.Through coating, improvement of thepharmaceutical elegance by use of specialcolors and contrasting printing is also possible.Primary components involved in tablet coating

are tablet properties, coating process, coatingequipments, parameters of the coatingprocess, facility and ancillary equipments, andautomation in coating processes, etc

5,6.

Coating Process Design and ControlIn most of the coating methods, the coatingsolutions are sprayed onto the tablets as thetablets are being agitated in a pan, fluid bed,etc. As the solution is being sprayed, a thinfilm is formed that adheres directly to eachtablet. The coating may be formed by a singleapplication or may be built up in layers throughthe use of multiple spraying cycles. Rotating

coating pans are often used in thepharmaceutical industry. Uncoated tablets areplaced in the pan, which is typically tilted at anangle from the horizontal, and the liquidcoating solution is introduced into the panwhile the tablets are tumbling. The liquidportion of the coating solution is thenevaporated by passing air over the surface ofthe tumbling tablets. In contrast, a fluid bedcoater operates by passing air through a bedof tablets at a velocity sufficient to support andseparate the tablets as individual units. Onceseparated, the tablets are sprayed with thecoating composition

4,7.

The coating process is usually a batch driventask consisting of the phases like, batchidentification and recipe selection (film orsugar coating), loading/dispensing (accuratedosing of all required raw materials), warming,spraying (application and rolling are carriedout simultaneously), drying, cooling, andunloading.A modern tablet coating system combinesseveral components like a coating pan, aspraying system, an air handling unit and adust collector.

Fundamentals of Film Formation

In the pharmaceutical industry, polymeric filmsare generally applied to solid dosage formsusing a spray-atomization technique. Thepolymer is dissolved or dispersed in aqueousor organic solvents prior to spraying. The solidcores are often preheated in the coatingequipment prior to initiation of the coatingprocess. This pre-warming stage is especiallyimportant in the coating of soft gelatincapsules. The coating solution or dispersion isatomized with air into small droplets, which are

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then delivered to the surface of the substrate.Upon contact, the atomized droplets spreadacross the substrate surface.The solvent may penetrate into the core,causing surface dissolution and physicalmixing at the filmtablet interface. As thesolvent begins to evaporate, the polymer

particles densely pack on the surface of thesolid. Upon further solvent evaporation, theparticles flow together due to the cohesiveforces between the polymer spheres, aprocess known as coalescence. Heat isgenerally added to the coating equipment tofacilitate solvent evaporation and filmformation. Immediately following thecompletion of the coating process, coatedsolids are generally stored at temperaturesabove the glass transition temperature of thepolymer to further promote coalescence of thefilm and ensure a homogeneous distribution ofthe plasticizer

8.

Aqueous film coatingAqueous film coating is applied as a thinpolymeric film to the surface of a tablet. Filmcoating can protect the tablet from light,temperature and moisture; mask undesirabletaste or odor; improve the appearance;provide tablet identity; facilitate swallowing andcontrol or modify the release of the drug.Aqueous coating of oral solid dosage formshas rapidly replaced solvent-based coating forsafety, environmental and economic reasons.Film-coating of tablets is a multivariateprocess, with many different factors,

such as coating equipment, processconditions, composition of the core tablets,shape of tablets, coating liquid, etc. whichaffect the pharmaceutical quality of the finalproduct. High quality aqueous filmcoating must be smooth, uniform andadhere satisfactorily to the tablet surfaceand ensure chemical stability of a drug

9.

Mechanisms of Film FormationAqueous film coating applications are eithersolutions or dispersions, depending on thewater solubility of the film former polymers.Film formation from the polymer solution

occurs through a series of phases. When thepolymer solution is applied to the surface ofthe tablet, cohesion forces form a bondbetween the coating polymer molecules. Toobtain high cohesion, the cohesive strength ofthe polymer molecules must be relatively highand continuous surface of the film materialmust coalesce. Coalescence of adjacentpolymer molecular layers or surfaces occursthrough diffusion. When most of the waterevaporates, the viscosity of the solution

increases (gelation) and leaves the polymerchain in close proximity to each other anddeposit over a previous polymer layer. If thereis adequate cohesive attraction between themolecules and sufficient diffusion andcoalescence upon the more completeevaporation of the residual water, the

individual polymer chains align themselves toform a cohesive film

10.

However, the mechanism of film formation isfundamentally different when using aqueouspolymer dispersions instead of organicpolymer solutions: Once the latter are sprayedonto a surface, the organic solventevaporates, the polymer chains approacheach other and finally form a continuoushomogeneous film. In contrast, upon sprayingaqueous polymer dispersions onto the dosageform's surface, water evaporates, the polymerparticles approach each other and underappropriate conditions (in particular

temperature, presence of sufficient amounts ofwater and/or other plasticizers) coalesce toform a homogeneous polymeric film. Inpractice, it is often difficult to assure completefilm formation during coating. That is whygenerally a thermal after-treatment (curing) isperformed, in order to complete polymerparticle coalescence

11.

Film formation from dispersion occur whenpolymeric particle coalesce to form acontinuous film (Fig.1), making it a morecomplex mechanism compared to filmformation from solution

7.

Fig 1: Film formation from aqueouspolymeric dispersion7

The coalescence of aqueous polymerdispersion deposited on the surface of thetablet into continuous film initiated by waterevaporation. As water evaporates, dispersedpolymer particle are pushed into a closelypacked, ordered array with water filling thevoids. After the polymer particles come intocontact with each other, they must deform andfuse in order to coalesce into a film.Coalescence will occur when the promoting

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forces are greater than the resistive forces ofthe particles. The forces promoting particlecoalescence include capillary pressure (water-air interfacial tension), as well as particle-airand particle-water interfacial tension. Finally,the coalescence of the polymer particles isfurther complemented by inter-diffusion of

polymer chains (autohesion) occurringthrough particle interfaces, making the filmmore homogeneous.Film formation, i.e. coalescence, is a complex

process and dependent on coating andstorage conditions, coating polymer, polymermolecular weight and particle size, coatingliquid constituents and properties like viscosityand surface

12.

Since coalescence only occurs above aminimum film formation temperature (MFT) ofcoating polymer, temperature and waterevaporation are considered to be majorprocess-related factors affecting the

properties of coatings7

.

Coating equipment14

Before few years different types of coatingpans are used for coating like conventionalcoating pans, manesty accelacota, driam (driacoater ), butterfly coater etc. Now a daysthe side-vented, perforated pan-coater is themost commonly used coating device of tablets.In equipment, spray nozzle, number of spraynozzle, pan size, etc may also affect thequality of final product. Its air flow systemthrough a perforated pan ensures rapid andcontinuous drying conditions. The low

evaporation capacity of water requires highdrying efficiency of aqueous film-coatingequipment.

Process parametersSpray rateThe spray rate is an significant parametersince it impacts the moisture content of theformed coating and, subsequently, the qualityand uniformity of the film

7,15,16. A low coating

liquid spray rate causes incompletecoalescence of polymer due to insufficientwetting, which could effect in brittle films

7. A

high coating liquid spray rate may result in

over wetting of the tablet surface andsubsequent problems such as picking andsticking

7,15. If the spray rate is high and the

tablet surface temperature is low, films are notformed during the spraying but the post dryingphase, and rapid drying often produces cracksin the films

7.

Atomizing air pressureIn general, increasing the spraying airpressure decreases the surface roughness of

coated tablets and produces denser andthinner films

17,18,19. If spraying air pressure is

excessive, the spray loss is great, the formeddroplets are very fine and could spray-drybefore reaching the tablet bed, resulting ininadequate droplet spreading andcoalescence

19. If spraying air pressure is

inadequate, the film thickness and thicknessvariation are greater possibly due to change inthe film density and spray loss. In addition,with low spraying air pressure big, dropletscould locally over wet the tablet surface andcause tablets to stick to each other.

Inlet air temperatureThe inlet air temperature affects the dryingefficiency (i.e. water evaporation) of thecoating pan and the uniformity of coatings

17.

High inlet air temperature increases the dryingefficiency of the aqueous film coating processand a decrease in the water penetration into

the tablet core, decreases the core tabletporosity, tensile strength and residual moisturecontent of coated tablets

17,20. Too much air

temperature increases the premature drying ofthe spray during application and,subsequently, decreases the coatingefficiency

16,21. Measuring the pan air

temperature helps to manage the optimumconditions during the coating process and,consequently, enables predicting possibledrying or over wetting problems which mayresult in poor appearance of the film or mayhave unfavorable effects on the moisture andheat sensitive tablet cores

22.

Rotating speed of panIt is well documented that increasing therotating speed of the pan improves the mixingof tablets

21,22,23,24,25. The pan speed affects the

time the tablets spend on the spraying zoneand, subsequently, the homogeneousdistribution of the coating solution on thesurface of each tablet throughout the batch.Increasing the pan speed decreases thethickness variation and increase the uniformityof coatings

7,21,24. Too much rotating speed of

the pan will cause the tablet to undergounnecessary attrition and breakage.

History of Aqueous Film CoatingInitially, aqueous processes were met withskepticism because of the longer process timeand the inferior appearance of the coatedproduct. A few desired release functions wereobtainable only with organic solventsolublefilms. However, the development andintroduction of latex and pseudo latexmaterials as well as improvements inequipment design have broadened the

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spectrum of aqueous coating. With correctlyselected equipment and processingconditions, it is now possible to apply water-based films to small particles withoutagglomeration or to tablets containingsuperdisintegrants without core penetrationand dissolution of the tablet surface.

Until about 1950, sugar was the first choice asa coating agent for pharmaceuticalpreparations and much time and efforts werespent in perfecting the sugar coatingtechniques. Nobody ever was concernedabout the problems like material cost, toxiceffects due to coating or pollution, etc.because the solvent used was always water.However, sugar coating technique was timeconsuming; affecting the productivity and thequality of finished product was dependent onthe skills of operator. Many a times thecompanies had to reschedule their productionplans due to the non-availability of skillful

coating operator. These problems led to thedevelopment of film coating technique whichwas mainly based on solutions of differentpolymers in various organic solvents. Almostall of these solvents are toxic in nature

26.

Aqueous film coating systems have beenwidely used for many years and offersubstantial advantages over organic solvents.Traditionally, organic solvents had been usedto dissolve the polymer but modern techniquesrely on water because of significantdrawbacks. Below lists some of the problemsassociated with organic solvents, are relatedto environment, safety, financial, solvent

residues, etc.Venting of untreated organic solvent vapourinto the atmosphere is ecologicallyunacceptable and removal of gaseous effluentis expensive. Organic solvents are also asafety hazard as they are toxic, explosive andfire hazard is also involved.The hazards associated with organic solventsnecessitate the need for building flame andexplosive- proof facilities. In addition, the costof their storage and ingredients are relativelyexpensive. The organic solvents areexpensive and are likely to become moreexpensive in future. For a given process, the

amount of residual organic solvent in the filmmust be investigated. Thus, stringentregulatory controls are existing

8. The cost of

insurance of the manufacturing facility is nowvery high if one uses the organic solvents. Thebest alternative to overcome these adversesituations is to revert back to water as thesolvent medium for tablet coating

26.

As the level of understanding regarding thetoxic effects of organic solvents is increasing,industrial hygiene rules and Food and Drug

Administration (FDA) regulations are beingtightened world over, limiting the use of thesesolvents and exposure of workers to thesesolvents.Although these formulated aqueous entericcoating systems were advancement fromtraditional solvent systems, they required the

separate addition of plasticizers, detackifiers,pigments, and other process aids.Another area of concern is the cost of thesesolvents, which can only be expected toincrease in coming times. In todayscompetitive business environments any costsaved will improve the market viability andsuccess of any product

27.

We are, therefore, left with no other choice butto eliminate the use of organic solvents andstart using water as the solvent system fortablet coating. Like any other system, aqueousfilm coating has some disadvantages.The main reason for using organic solvents

was to avoid possible decomposition of activeingredients and many other process relatedproblems such as over wetting, picking andsticking, etc. which may occur with aqueouscoating systems. However, research andexperience of industry has indicated that thedecomposition of active ingredients andpossible coating difficulties are not so seriousissues in actual applications and all suchproblems can be sorted out by scientificevaluation of the reasons for these problems.Most of these problems could be categorizedas material related problems, coatinginstrument related problems and coating

process related problems.

Effect of Residual MoistureOne should keep in mind that water is lessvolatile than organic solvents, and will requiremuch better dryingcapacity resulting in higherenergy cost to the coating process. However,exceptions do exist in optimized film coatingformulations which have a very low affinity forwater, and therefore, can be run at lowertemperatures, higher spray rates. Ideal CuresPvt. Ltd. has developed few such products(under INSTACOAT range of products)whichdries faster and the whole coating process can

be completed in the same or sometimes littleless time as compared to organic solventbased coatings. The use of organic solventsraises the possibility of residual solvent in thefinish product which is increasingly becominga concern to the regulatory agencies due totheir adverse effects on the consumer health

26.

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Current trend of aqueous film coating inpharmaceutical oral solid dosage formsAqueous coating technology remains the mainoption for film coating of oral solid dosageforms. This is irrespective of the purpose ofthe film-coating applications, i.e. forconventional and modified-release film

coatings. The main reasons for its continuedpopularity are the environmental limitations oforganic solvents used, recent advances in theformulation of aqueous f ilmcoating materials,as well as major improvements made in thecoating machines and their ancillaries.Aqueous coatingsystems are widely used forconventional film-coating systems (immediaterelease), enteric film-coating systems (delayedrelease), and barrier membrane controlledrelease film-coating systems (extendedrelease)

28.

Opadry formulations provided numerousadvantages versus the use of individual raw

materials including the reduction of thenumber of raw materials for QC testing,reduced dispersion preparation time,consistent color-matched formulations, goodprocessibility, excellent appearance on tablets,and good mechanical film properties. Opadryformulations have enjoyed widespread,successful use globally and are still found on agreat variety of marketed products. However,one drawback of the Opadry formulations isthat dispersion solids must be kept in therange of 10% to 15% by weight in water toachieve processible dispersion viscosities of300 to 600 centipoises. In order to increase

productivity, by decreasing coating time and/orincreasing spray rate, the Opadry II family ofproducts comprising HPMC andpolysaccharides, was introduced in the 1980s.With Opadry II, processible dispersions can beobtained at 20% rather than 10% to 15%solids, which allows for the productivityincrease as well as increased adhesion

29.

The most significant recent advances in thedevelopment of fully formulated aqueous filmcoatings have been the introduction of newfilm coatings based on polyvinyl alcohol (PVA)and sodium Carboxymethylcellulose(NaCMC). Film coatings comprising these

polymers offer the formulator the same orgreater production conveniences afforded tothem when using Opadry formulationscontaining Hydroxypropoxymethylcellulose(HPMC) and also provide functionalitypreviously unrealized. PVA-based films areknown to have relatively low moisture vaporand oxygen permeability. On the other hand,NaCMC-based films have low oxygenpermeability but relatively high water vaporpermeability. Another important feature of

NaCMC-based films is that, when formulatedand applied properly, they are very glossy.NaCMC-based film coatings therefore offer thepossibility of both enhanced functionality andaesthetics

29.

Opadry aqueous moisture barrier (AMB) andOpadry II 85 series are two proprietary families

of PVA-based products that werecommercialized in the mid to-late 1990s. TheOpadry AMB formulation was optimized toprovide the lowest moisture vaportransmission rate (MVTR) possible while stillaffording all the conveniences of fullyformulated film-coating systems. It is suppliedas a color-matched system and can be readilydispersed into water at the 20% solids level.Owing to the inherent tackiness of the PVApolymer, the maximum achievable spray ratesobtained with Opadry AMB are not as high asthose of HPMC-based Opadry II film coatings.The Opadry II 85 series family of products was

developed to address this. Opadry II 85 seriesproducts offer MVTR almost as low as OpadryAMB but can be applied at significantly higherspray rates

30.

Film coatings based on PVA and NaCMC offerthe formulator new functional benefits. It isnow possible to coat moisture-sensitive cores,using aqueous coating processes, andpreserve them through the use of PVA-basedcoatings. NaCMC based coatings providedemonstrable oxygen barrier properties andalso excellent aesthetic characteristics.

CONCLUSION

On the basis of the studies carried out ti ll date,it is focusing that aqueous film coatingtechnology is now a days very important in thefield of pharmacy particularly in formulationdevelopment and the aqueous based coatingand its various aspects which are giving themore benefits over the organic coating, whichleads to non-toxicity, cost effectiveness andnon hazardous to environment.

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